evolutionary biology || a developmental model for evolution of the vertebrate exoskeleton and teeth

10

A Developmental Model for Evolution of the Vertebrate

Exoskeleton and Teeth The Role of Cranial and Trunk Neural Crest

MOY A M. SMITH and BRIAN K. HALL

EXOSKELETAL DEVELOPMENT AND EVOLUTION

An exoskeleton is extensive in the head, trunk, and tail of agnathan and gnathostome fishes, where it forms a thick, rigid armor in most fossil fishes, although many only have a covering of separate denticles. This armor consists of three basic layers of mineralized tissue surrounding soft tissue components. There is a superficial odontode layer (see pp. 391 and 393 for discussion of odontodes) with enamel, dentine, attachment bone, and pulp chambers; a middle layer with trabeculae of cellular or acellular bone surrounding soft tissue vascular spaces; and a basal laminated layer of compact cellular or acellular bone, with zones of attachment fibers. Among extant osteichthyan fishes are those that retain some of these odontode components in their dermal scales as a mineralized exoskeleton. These are the so-called living fossils, the

MOY A M. SMITH • Division of Anatomy and Cell Biology, United Medical and Dental Schools of Guy's and St Thomas's Hospitals, London, SEI 9RT, England. BRIAN K. HALL • Department of Biology, Dalhousie University, Halifax, Nova Scotia, B34 HJl, Canada.

Evolutionary Biology, Volume 27, edited by Max K. Hecht et al. Plenum Press, New York, 1993.

387

388 M. M. Smith and B. K. Hall

Polypteridae and the extant coelacanth, as well as catfishes. Odontode components are retained as placoid scales in all chondrichthyans (see Figs. 1 and 2).

In addition to this general body covering of scales, part of the fin skeleton-the terminal mineralized portion of the fin rays, or lepidotrichia-is considered to be exoskeleton, because these skeletal elements are often associated with enamel and dentine layers and do not form first in cartilage, but directly as membrane bone. Knowledge about the developmental embryonic origin of these different skeletal dermal components is fragmentary and sometimes at variance. The skeletogenic cells of the paired and median fins are said to have different embryonic origins: those forming lepidotrichia of the paired fins being derived from dermatomal or splanchnic mesenchyme, those forming lepidotrichia ofthe median fin being derived from neural crestderived mesenchyme (Schaeffer, 1987).

The developmental origin of the exoskeleton is mostly assumed to be from neural crest (M. M. Smith and Hall, 1990). Conclusions extrapolated to fishes from much experimental data gathered from mammals, birds, and amphibians are that both the odontogenic and skeletogenic components of the exoskeleton are cranial neural crest derived. Lack of developmental evidence for a skeletogenic role of trunk neural crest stands in stark contrast to the vast amount of data showing cranial neural crest to be both skeletogenic and odontogenic.

A conclusion that neural crest forms all of the competent skeletogenic mesenchyme contributing to the dermal skeleton (lepidotrichia and scales) of fishes requires either a considerable extension of our existing knowledge on distances and rates of migration of cranial neural crest, or an acceptance that trunk neural crest must be skeleto-odontogenic in lower vertebrates. A skeleto-odontogenic trunk neural crest challenges the assumption that cranial neural crest alone accounts for the initiation and differentiation of all skeletal cells in the extensive exoskeleton in lower vertebrates.

In this review an explanation is sought for the extensive distribution, diverse morphology, and histology of the exoskeleton in extant and fossil fishes, whether that exoskeleton has its developmental and evolutionary origin in the neural crest, and, if so, whether in cranial or trunk neural crest. This is an attempt to provide a more logical explanation for the developmental mechanism that allows an extensive exoskeleton to develop in the trunks and tails of the majority of extinct fishes and in the trunk and tail skeleton in some extant fishes. We compile and evaluate information from exoskeletal evolution, morphology, histology, and experimental and molecular studies of skeletal development. We propose a model of skeletal origin and diversity in the exoskeletons of "lower" vertebrates that also explains loss or reduction of the exoskeleton within most "higher" vertebrates (amphibians, reptiles,

Vertebrate Exoskeleton and Neural Crest 389

birds, and mammals). A central question considered is adequacy of the evidence for lack of skeletogenic potential in trunk neural crest.

Five possible cellular origins of scleroblasts (cells capable of making mineralized connective tissues) to form the trunk exoskeleton are considered:

1. Scleroblasts differentiate from cranial neural crest after caudad migration into the trunk and without involvement of other mesenchymal populations.

2. Scleroblasts form from trunk neural crest and make the exoskeleton without involvement of other mesenchymal cell populations.

3. Scleroblasts differentiate from trunk neural crest, but are dependent for their differentiation on somitic mesoderm (dermomyotome).

4. Scleroblasts differentiate from mesenchyme derived from somatic lateral plate mesoderm.

5. Scleroblasts differentiate from mesenchyme derived from somatic lateral plate mesoderm, but are dependent for their differentiation on trunk neural crest.

Principles of Skeletal Development and Evolution

Neural Crest versus Mesodermal Cellular Origins

Throughout this review it is assumed that homologous parts of the skeleton in extant and fossil vertebrates share the same embryonic cellular origins: neural crest for teeth, visceral arch skeleton, some ofthe neurocranium, and all dermal bones except a few of the dermatocranium (see p. 390; Couly et al., 1993); mesoderm for cartilage bones of most of the neurocranium, and all of the axial and appendicular skeleton. Cellular origins are therefore divisible into ectodermal or mesodermal skeletal lines. Within each are two histogenetic modes of development, intramembranous and endochondral. For example, perichondral bone of the endoskeleton is a bone tissue forming in membrane (the same developmental mechanism seen in dermal bone, but specifically here in the connective tissue sheath on the surface of the cartilage), but the majority of the bone in the endoskeleton is formed by endochondral ossification of the cartilaginous models of those bones. Again, visceral cartilage elements (endoskeleton from cranial neural crest) are replaced by bone endochondrally after the perichondral collar surrounding them has been made in membrane, as in the mesodermally-derived endoskeleton. Perichondral bone is developmentally and phylogenetically earlier than endochondral bone (Patterson, 1977; Maisey, 1988; M. M. Smith and Hall, 1990) [see Fig. 1 of


M. M. Smith and Hall (1990) for an illustration of the components of exoand endoskeletons].

As a basis for evaluating the role of developmental mechanisms of constraint or plasticity in evolutionary change, we use previous postulates established by M. M. Smith and Hall (1990), in particular those built upon Patterson (1977), concerning the separate origins in development and evolution of the exo- and endoskeleton.

Prior to Patterson's conclusions, many statements had been made about the presumed interchangeability of dermal and cartilage bones, such as "there cannot be any fundamental difference between the endoskeleton and the exoskeleton" (Jarvik, 1959). Although Patterson stated that the endoskeleton and exoskeleton are distinct and have separate phylogenetic and ontogenetic origins, little was said about the embryonic origin of the participating cells. This was because little was then known, although in a paper in the same volume Schaeffer (1977) postulated that "ectomesenchyme is present in the dermis throughout the body and. . . can form dermal bone as well as dental organs." Schaeffer was aware of the association of enamel and dentine with the dermal skull, scales, and lepidotrichia in all fossil fishes and some extant ones, such as Polypterus, and of the dependent link with mesenchymal neural crestderived cells for tooth development. These concepts of Schaeffer are central to this review.

The information from developmental biology on the origin of cells that contribute to the dermal bones of the skull in birds has provided a picture in which most of the cranial vault is mesodermal in origin (frontal, parietal, supraoccipital, exoccipital, and orbitosphenoid) but the facial skeleton is neural crest in origin (squamosal, supraorbital, lacrimal, sclerotic, premaxilla, maxilla, dentary, splenial, parasphenoid, palatine), a distribution illustrated by Le Douarin (1982, Fig. 3.6). Recent publications by eouly et al. (1992) have further elaborated on the contribution of the paraxial mesoderm and the prechordal plate to the head skeleton and produced many interesting additions to the accepted story. They found that neural crest cells were in all of the dermis prior to membrane bone formation, with the implication that all dermal bones of the skull come from neural crest (eouly et al., 1992, Fig. 9, and Table 2) with only the chondrocranial bones, basisphenoid, orbitosphenoid, and otic capsule arising from paraxial mesoderm. In their subsequent paper (eouly et al., 1993), they have further refined the information by taking chimeric embryos to the stage where frontal and parietal bones can be identified. They have found that, as with the facial skeleton, these bones also originate in neural crest. Of great importance, they have also found that all the 'prechordal' chondrocranium originates from neural crest. This new information considerably alters our concept of the development of the head skeleton (facial and skull roof) and the chondrocranium anterior to the notochord. As eouly


et al. concluded, "The anterior limit of the notochord then corresponds to the limit where the mesodermal skeleton ends and the neural crest derived skeleton begins." This new information from studies on the chick indicates a more restricted contribution of the paraxial mesoderm to the skull than previously shown, and may be vital to our understanding of the evolution of the cranial and trunk dermal skeleton in lower vertebrates. It now seems that all the dermal bones of the head originate in neural crest except parts of the occipital bone; these have contributions from the sclerotomal mesenchyme of the first four somites. One of the postulates of a fundamental difference between the exoskeleton (cranial and trunk) and the axial and appendicular endoskeleton, to which this new data has a direct bearing, is that they have different embryonic cell origins, and that this is evolutionarily conserved.

Postulates and Potential Mechanisms

First, and fundamental to the assumptions discussed in this review, is the postulate that all dentoskeletal components of the exoskeleton require neural crest, both for induction and for participation in skeletogenesis, and that inductive interactions occur with epithelia. We consider it crucial for understanding evolutionary origins and for comparing exoskeletons with endoskeletons to look for evidence of compatibility or discordance of the initial signal for specification of the type of skeletal tissue in these epitheliomesenchymal interactions. But as M. M. Smith and Hall (1990) commented, the scarcity of data on mechanisms of induction of skeletogenic and odontogenic potential in fishes prevents such a comparison until experimental data are provided. Data for the identification of the origin of cell types are also not available for the dermal skeleton in fishes, although some are available for the neural crest origin of the visceral skeleton in lampreys and teleost fishes (Langille and Hall, 1988a,b).

Although teeth and skin denticles in extant and fossil forms have been assumed to be similar structures since the publications of Williamson (1849), Hertwig (1879), and Goodrich (1907), it was Schaeffer's (1977) review of the dermal skeleton in fishes that first clearly linked common structure with shared morphogenetic mechanisms. Incorporated in this hypothesis was the concept that "the dermal skeleton develops from a single, modifiable morphogenetic system." He had envisaged regulation of morphogenetic change through this interactive system as significant for evolution of the dermal skeleton. We now propose that part of the "morphogenetic system" is the dental primordium of interactive dental cap ectoderm and dental papillary and follicular ectomesenchyme (Fig. 1). This system forms the developmental basis of the odontode, the primitive exoskeletal building block (see Fig. 2). In addition to the odontode, a skeletal primordium would be involved, i.e., a subpopulation of


suc. prim.

i. en. epith.

o. en. epith.

epith.

dent. cap

dent. foil.

dent. pap.

osteo. condo

boo

c

FIG. 1. Three stages of development of the epithelioectomesenchymal interactions involved in skeletoodontogenesis, showing the three sets of condensations that are proposed as subpopulations of the neural crest involved in induction and morphohistogenesis of the vertebrate dentoskeletal tissues in development of the entire exoskeleton. This is based on the classic stages of morphogenesis of mammalian tooth germs and skeletal support tissues of the jaws: (a) bud stage, primordial tissue of the odontogenic epithelium and ectomesenchyme, with adjacent condensations of osteogenic and chondrogenic ectomesenchyme; (b) bell stage, morphogenesis within the enamel organ and dental papilla, and histodifferentiation of these cells to produce the tooth shape and tissues, enamel and dentine; more extensive membrane bone and established cartilage; (c) cap stage, three


ectomesenchymal cells dependent upon specific epithelium for induction and competent to produce dermal bone to support the odontodes (see Fig. 1) and, in the head, cartilage of the jaws and the pharyngeal arches.

Much earlier than Schaeffer (1977), however, Moss (1960, 1968a,b) had argued that "the earliest ossified vertebrates possessed the intrinsic capacity to produce the entire spectrum of vertebrate skeletal tissues," and that this required the acquisition in evolution of two essential developmental processes, (1) "inductive interaction between ectoderm and neural crest derivatives" and (2) "epidermal co-participation," implied to occur between epithelial cells and neural-crest-derived mesenchymal cells, both in morphogenesis and in histogenesis. We specifically propose that such intrinsic capacity would have extended from head to tail in those early vertebrates possessing an extensive bony armor of separate or fused denticles, i.e., in the majority of early vertebrates. In fact, Moss (1968a) concluded that "Dentine did not evolve from bone. Dentine could arise as soon as the neural crest had evolved and with it the topographic relationships and their inductive sequences of tooth formation." The proposals in the next section accept this dichotomy between odontogenic and skeletogenic development and evolution and discuss the role of the odontode as a basis for the exoskeleton.

The Odontode as Dermal Exoskeleton

The unit structure that develops from the interactive morphogenetic system proposed by Moss and elaborated by Schaeffer is the odontode.

The term odontode was first proposed by 0rvig (1967), and later expanded (0rvig, 1977) to include all hard tissue units of the dermal skeleton, composed of enamel, dentine, and pulp. Reif( 1982) subsequently expanded this concept to include oral denticles and teeth. Two assumptions are that teeth and dermal denticles are homologous, constrained by common developmental patterns, and that dermal denticles in the oral cavity gave rise to teeth during the

cellular aggregates of the odontode/tooth, epithelial dental cap, ectomesenchymal dental papilla, and ectomesenchymal dental follicle; with the osteogenic condensation and early bone, and chondrogenic condensation and early cartilage; the developmental origins of four differentiated cell types are indicated. Abbreviations: amelo., ameloblasts; bo., bone; ca., cartilage; chondro., chondrocyte; chond. cond., chondrogenic condensation; chond. ecto., chondrogenic ectomesenchyme; de., dentine; dent. cap, dental cap; dent. foil., dental follicle; dent. pap., dental papila; dent. lam., dental lamina; en., enamel; en. epith., enamel epithelium; enam. org., enamel organ; epith., epithelium; odonto., odontoblast; odonto. ecto., odontogenic ectomesenchyme; odonto. epith., odontogenic epithelium; osteo., osteocyte; osteo. cond., osteogenic condensation; osteo. ecto., osteogenic ectomesenchyme.


evolution of jawed vertebrates (Fig. 2). This axiom may be seriously challenged by new information on the relationship of conodonts to vertebrates (see p. 416). These postulated chordates are now proposed as the sister group of all vertebrates. They possess arrays of teeth in an oral/pharyngeal cavity, but show no evidence of mineralized dermal armor.

The odontode sensu 0rvig did not include the tooth-integrated basal tissue "bone of attachment" developed after but in topographic relationship with dentine of the exoskeleton. We include bone of attachment as part of the fundamental odontogenetic unit. All three tissues [enamel, dentine (pulp),

denticle I tooth

':' f .<j< ·x .·,".·.···· ·}{"" ,"', ! L,

odontogeniC cells

denticle I tooth

odontogenic primordium

a

skin denticles teeth dental lamina

oral denticles

FIG. 2. Model redrawn from Reif (1982) to show his "odontode regulation theory" and teeth developing from a dental lamina. It assumes that a zone of inhibition exists around the developing primordium (extent indicated by the arrows) and further that it is still effective in the fully formed tooth/denticle; evidence for the molecular/cellular control has not been presented. All assume that the primitive craniate condition is microsquamous and that skin denticles gave rise to teeth through the development of a dental lamina as shown in (b), in which new denticles form in the inhibition-free space developed with growth of the tissues, and new teeth develop from the dental lamina, at the inhibition-free end. In (a) three stages show initiation, functional position, and loss by shedding of an odontode. This provides the possibility that a new, larger denticle can develop in the inhibition-free space; also the assumption is that shedding of denticles is the primitive craniate condition.


and bone] are present and anyone could have evolved earlier than the others and be present in the fossil record independently of the others. This could happen in development if part of the odontode differentiation program was suppressed through a heterochronic shift, or if one cell type had not acquired competence in the odontode system. Whether dermal bone or dentine or bone in the odontode evolved first is contentious [see M. M. Smith and Hall (1990) for a review, and M. M. Smith (1991)] and has been made more so by the proposal that conodonts are vertebrates, a contention based on recent histological evaluation of mineralized conodont elements (serving as teeth) in which dentine is absent but cellular bone present (Sansom et aI., 1992) (see p. 417) and on new descriptions and a reassessment of the anatomy of conodonts (Aldridge et aI., 1993).

Reif(1982) based his "odontode regulation theory" of the dermal skeleton on Schaeffer's proposal, but extended it to propose that regulatory processes (we suggest, involving heterochrony, and cell or matrix-based specific regulatory molecules) are the means of varying differentiation programs of odontodes. Such variation would produce different unit products in different combinations from a single morphogenetic system, which would be the developmental basis for evolutionary change of the dermal skeleton.

Reifproposed a model for evolution of the dermal skeleton starting from a microsquamous condition in which early vertebrates possessed an exoskeleton of non growing odontodes covering the whole body uniformly [examples are thelodont scales, and placoid scales of sharks (M. M. Smith and Hall, 1990, Fig. 5)]. Implicit in this proposal is that odontodes evolved before dermal bone, and this was absent from these exoskeletons. Each odontode was easily shed from its functional site by soft tissue resorption, as there was not an ankylosis to underlying bone. Regrowth was from a new primordium developed in the soft tissue space (Fig. 2). Growth was from new morphogenetic units developing as soon as inhibition-free space was created between existing odontodes. This model depends on the concept of alternate fields of induction and inhibition. Variability within these units could occur by altering such parameters as spacing, size, shape, tissue proportions, or tissue types.

We examine possible developmental mechanisms that may regulate and modify this system, and will suggest certain developmental constraints that apply. The ultimate basis for understanding control mechanisms will be through molecular control of genetic expression in the phenotype, but, as suggested by M. M. Smith and Hall (1990), this will operate within the developmental framework of causal cascades of dependent, sequential steps, on a controlled spatial and temporal basis. There are some very exciting new developments in the understanding of the regulatory role of hom eo box genes in an individual's development. That is, those that apply to pattern formation, as discussed by Hanken and Thorogood (1993) and Thorogood (1993) with


regard to the evolution and development of the vertebrate skull, "mechanisms of pattern formation-the events and processes by which cells are organized into predictable spatial arrangements (morphologies) oftissues in their proper locations." We discuss this below (p. 406).

In one early vertebrate with good preservation of histology and separate odontodes ["the enigmatic third vertebrate" (Denison, 1967)], M. M. Smith (1991) described enamel, cellular dentine, and cellular bone. There is good evidence that an exoskeleton, with abundant odontodes, was extensive in the trunk, tail, and head of the other earliest vertebrates [see M. M. Smith and Hall (1990) for a description of these], and was much more developed than the endoskeleton in the same regions. It is not possible to comment on uncalcified cartilage; it may either not have been present or have been present but not preserved, an issue also discussed by M. M. Smith and Hall (1990).

In summary, odontodes are products ofa morphogenetic unit (the odontode primordium) developing through epitheliomesenchymal cooperative interactions, in constrained, causal, and temporal sequences. Modifications of the differentiation program result in different phenotypes in the dermal skeleton in fishes, including teeth of the oral cavity.

Lepidotrichia as Dermal Fin Rays

One aspect of the skeletal tissues of the dermal skeleton that should be addressed in this review is the development and embryonic origin of the dermallepidotrichia, the distal elements of the fin rays formed in association with the actinotrichia and which develop without passing through a cartilaginous precursor. Frequently in many fossil fishes, and in some extant fishes such as Polypterus, dentine with an enamel covering is fused to the underlying lepidotrichia. Any developmental model for skeletal tissue evolution and variation should take into account this part of the dermal skeleton, especially because it is suggested that neural crest mesenchyme is implicated as one source of cells.

Both Hall (1991) and Thorogood (1991) considered the embryonic source of skeletal tissues when evaluating the developmental basis of limb evolution. Hall discussed the origin of the dermal fin skeleton as probably neural crest, and the loss of these elements in the transition from fin to limb. Thorogood commented that the neural crest origin of the lepidotrichia remains to be demonstrated. Prior to this, Schaeffer (1977) had reviewed the origins of cells that contribute to the induction and skeletogenesis of the median and lateral paired fins, concluding that induction of median fins is by neural crest-derived cells, but that the lateral fins are different, being induced by lateral plate mesoderm. Probably the most significant statement for this review is that by Schaeffer (1977), who commented that the histological resemblance between


lepidotrichia and scales does not indicate that one evolved from the other, but that "scales and lepidotrichia are. . . manifestations of the same morphogenetic system." This statement captures the basis for our model of development of the dermal component of the fin skeleton.

Both Schaeffer (1977) and Patterson (1977) rejected the delamination theory for derivation from epitheliodermal morphogenetic units of superficial and deeper separate elements ofthe dermal skeleton [conceived by Holmgren (1940) and applied to the dermal fin rays by Jarvik (1959)] as having any value as an explanatory principle for development or evolution of the fin skeleton. But it had, for some time previously, been accepted, leading to the implication that "there is some sort of interchangeability between the dermal skeleton and endoskeleton" (Patterson, 1977, p. 110). This is a concept accepted neither by Patterson nor by the authors of this review (see section below on the experimental embryology of fin rays p. 431).

Odontogenic Dependence on Neural Crest and Extent a/Competence

An implication of the postulate that all dentoskeletal elements of the exoskeleton require neural crest as a component and are based on the odontode as the developmental system is that neural crest is necessary for induction, morphogenesis, and differentiation of odontodes wherever they occur on the body and in whichever vertebrates they occur. In mammals, reptiles, and amphibians, odontodes are found in the oral tissues, where they function as teeth, whereas in extant fishes they occur throughout the exoskeleton, on scales (Bhatti, 1938; M. M. Smith, 1979), fin spines (Maisey, 1979), and lepidotrichia (Geraudie, 1988). In all documented cases neural crest is required for induction and development of teeth (p. 428) (M. M. Smith and Hall, 1990). No data are available, however, concerning neural crest involvement in the development of teeth or denticles of fishes.

If we consider that most of the earliest recorded fossil vertebrates had an extensive exoskeleton composed of either thick dentoskeletal mineralized tissues or separate denticles inserted via their basal bone into a soft connective tissue, then, following our postulate of neural crest origin of odontodes, neural crest would have provided dentoskeletogenic competence over the entire vertebrate body. Presumably this competence was acquired before mesodermallyderived mesenchyme had evolved competence to develop perichondral membrane bone or endochondral bone. We do not know if dermal bone was ever derived from a mesodermal mesenchymal population (see p. 390).

Gans and Northcutt (1983, 1985) and Northcutt and Gans (1983) proposed that neural crest was a vertebrate innovation, from which the first vertebrate skeletal tissues arose. They did not address the question of whether it was initially cranial neural crest that was skeletogenic or both cranial and


trunk together. If we extend the concept that neural crest and the tissues derived from this source account for all vertebrate synapomorphies, one of which is the dentoskeletal tissue, to all fossil fishes, then the following is also true. In the earliest recorded vertebrates with isolated scales (odontodes) in their skin, without extensive sheets of dermal bone (M. M. Smith, 1991), then, at least in some early basal vertebrates, neural crest was probably the only skeletogenic population in the dermis. 0rvig, T, (1951), Obruchev (1964), Karatajute-Talimaa (1978), Reif (1982), Maisey (1988), and most recently Blieck (1992) concluded that micromeric dermal armor (scales from head to tail, and absence of plates of bone), as in thelodonts, was the primitive vertebrate condition. Thelodonts are therefore one basal vertebrate group in which neural crest would have been the only skeletogenic population of both head and trunk, because no sheets of bone developed in their exoskeleton, and there is no evidence of either calcified or noncalcified cartilage.

Evidence for Skeletogenic/Odontogenic Cranial versus Trunk Neural Crest

In a review of the development and evolutionary origin of skeletal tissues in vertebrates, M. M. Smith and Hall (1990) distinguished between skeletogenic and odontogenic potentials in cranial neural crest populations and argued for the existence of subpopulations of neural crest cells morphogenetically specified and with restricted differentiative potential. These were cells which after migration and superficial location beneath epithelia would, as an odontogenic subpopulation, differentiate into tooth buds and produce dentine and bone of attachment. The other skeletogenic subpopulations of cranial neural crest would differentiate into cartilage, dermal bone, perichondrium and periosteum, and periosteal and endochondral bone, for cartilages in the visceral skeleton and bone in the facial skeleton.

These suggestions of separate skeletogenic and odontogenic cell populations were expanded further by Atchley and Hall (1991) in a review using the mouse mandible as a model for development of complex morphological structures (see p. 402).

Evidence for a substantial role of cranial neural crest in induction and skeletogenesis of the head exoskeleton and endoskeleton in tetrapods began to accumulate as data was provided from experimental embryology. By contrast, trunk neural crest has been consistently demonstrated not to share this skeletogenic potential. In this context, several references to dermal armor of fossil fishes were made. These were put on one side for later investigation by de Beer (1947) and explained by Wilde (1955) as dermal bone and a denticle system developed from head ectomesenchyme with a protective function, assumed to later extend into the trunk. Gaunt and Miles (1967) realized that


there was a fundamental question to be answered in considering the possible neural crest origin of these exoskeletal tissues, "the apparent common origin in ontogeny of membrane bone and dentine from neural crest is of great interest and raises fundamental questions which cannot be answered decisively."

Although much ingenious and delicate experimental embryology has been carried out to address the question of trunk neural crest competence, the vital set of experiments creating temporal and spatial conditions in which competence (potential) of trunk neural crest to express odontogenic and skeletogenic ability can be expected has only just begun to be undertaken in amphibian embryos (Graveson, 1993) (see following section) and has yet to be initiated in fish embryos.

Evolutionary considerations would suggest that dentine and/or boneforming capabilities of trunk neural crest have either been lost or are latent, but suppressed in amphibians, reptiles, birds, and mammals. (We have excluded consideration of the embryonic origin of osteoderms in amphibians and reptiles in this review, due to lack of both data and understanding.) Several reasons can be put forward to explain why this potential is not expressed when amphibian, avian, or mammalian trunk neural crest is grafted into the head in place of cranial neural crest, an experimental design frequently used to test the potential of trunk neural crest. The first reason may be spatial, reflecting lack of migration of trunk neural crest in the cranial site (Graveson, 1993). The second reason may be the lack of suitable extracellular substrates (fibronectin, etc.) required to maintain trunk neural crest cells. The third reason is temporal; trunk neural crest may not confront inductive epithelia at the appropriate time for required epithelial-mesenchymal interactions to occur.

One set of experiments has been carried out by Lumsden (1987) in postconception-aged mice from embryonic day 8 (E8) to day 15 (EI5). Heterochronic explants of odontogenic mandibular epithelium (E 10) combined with premigratory trunk neural crest from (E8) resulted in the formation of teeth and adjacent bone, but no cartilage. The segment of neural crest taken in Lumsden's experiments was from the level of the open posterior neuropore, or future cervical somites (S5-1O); in this restricted location (caudad to S5), the most rostral trunk neural crest could form dentine and bone of attachment in response to mandibular ectoderm [this has been discussed in detail by M. M. Smith and Hall (1990, p. 333)]. This result alone suggests that postcranial ectomesenchyme can be induced to participate in odontogenesis and osteogenesis, but not chondrogenesis. Recent work by Graveson (1993) and A. Graveson, M. M. Smith, and B. K. Hall (in preparation) has also revealed that the segment of axolotl cranial neural crest immediately caudad to the


chondrogenic region will make teeth but not cartilage, confirming in amphibians Lumsden's (1987) conclusions that some mammalian postcranial ectomesenchyme is odontogenic.

The experimental demonstration of odontogenic capability in mammalian trunk neural crest and axolotl caudad cranial neural crest suggests that the potential to make odontogenic tissues should be present in lower vertebrates. As suggested previously, "because postcranial skeletal tissues develop from mesoderm and not from neural crest, developmental biologists have drawn an absolute dichotomy between a cranial skeletogenic/odontogenic and a trunk non-skeletogenic/non-odontogenic neural crest." (Smith and Hall, 1990, p. 332). The region oftrunk neural crest with odontoskeletogenic ability might be expected to be more extensive in those extant fishes with an exoskeleton than so far shown for amphibians.

Dependence of Neural Crest on Epithelium for Acquisition of Morphogenetic Specificity

Before discussing the role of neural crest in patterning skeletal morphogenesis, we should consider the dependence on, and role of, epithelia in this interaction. Lumsden (1988) concluded that "tooth initiation involves an inductive interaction between region specific epithelium and competent but unspecified mesenchyme cells, normally derived from the cranial neural crest" and also "a prepatterned distribution of inductive potency may exist in the epithelium. . . thus controlling the spatial organization of the dentition as a whole." This model could also provide a patterning mechanism for scales/ denticles in the entire body exoskeleton of fishes, with an inductive signal from the epithelium, perhaps only effective outside zones of inhibition. Reif (1980) proposed a model of morphogenesis in the dermal skeleton of sharks to explain regulation of new scale development using the inhibitory field model for teeth of Osborn (1978) (see Fig. 2).

Lumsden (1987) had previously cautioned about conclusions that ectomesenchyme produces the initial signal for differentiation of teeth, although it may initiate tooth morphogenesis. Many experiments addressing tooth morphogenesis involve ectomesenchyme that has been primed to make teeth, before histological recognition of tooth sites-"the patterning process involved in tooth initiation, controlling its timing and positioning, are critical events that must precede the morphological appearance of the tooth germ." In a precisely chosen series of times and sites, Lumsden (1987) demonstrated that tooth development in mammals is initiated via a permissive signal of specific oral ectoderm on competent ectomesenchyme.

Graveson (1993, and personal communication) has found in isochronic and heterochronic recombinations of neural crest with ecto/endoderm as ex-


plants from amphibian neurulae that ectoderm alone will not induce teeth from neural crest, but endoderm will. In mammals endoderm also could be a prerequisite for tooth initiation, because the precise boundaries of ectoderm and endoderm in the oral cavity are not known (A. Lumsden, personal communication).

It may be significant that there appears to be a difference between initiation of tooth and bone morphogenesis, although both share dependence on epithelium for the inductive signal. Hall (1983) showed that foreign epithelia can permit mandibular ectomesenchyme to differentiate into membrane bone in the chick (i.e., the tissue generating the permissive signal is not site specific), but that the shape of the bone is specified by the ectomesenchyme. Experiments by Noden (1983) also led to the suggestion that some populations of cranial neural crest, those that are destined to make the visceral cartilages, are morphogenetically specified prior to emigration. Such divergent mechanisms would effectively uncouple the patterning mechanism of the two components of the exoskeleton, "odontodes and membrane bone," and those of the viscerocranium. The potential patterning mechanisms in the vertebrate skull have been recently reviewed by Hanken and Thorogood (1993), who suggest that one fundamental topic that remains to be adequately addressed is "the relation of initial embryonic patterning to adult skull morphology, and especially to the dermatocranium." These issues are further discussed in the following section.

Exoskeletal Diversity and a Developmental Model

The greatest diversity of skeletal tissues is apparent among the exoskeletons of lower vertebrates. Variation occurs in tissue type (enamel, enameloid; mesodentine, semidentine, orthodentine); pulp (open, closed, or infilled with dentine or bone); bone (cellular or acellular); arrangement of bone tissues (spongy or compact); bone composition (from surface parallel lamellae, or lamellae concentric with vessels); whether bone tissue has inserted attachment fiber bundles; whether tubercles (odontodes) on the dermal armor are resorbed, or nonresorbed and superimposed with new growth; whether skin denticles are free (odontodes), or ankylosed to bone; whether dermal bone occurs as extensive, continuous sheets, or as small plates called jointed tesserae. A model offering a precise understanding of development, in terms of spatial and temporal patterning, as well as specific molecular data on control systems, would be powerful aids to explaining the evolution and variation of such skeletal types and their arrangement (see p. 406).

A developmental model from which skeletal evolution and diversity in the dermal armor can be explained is based on the concept of both a single


T ABLE I. The Cranial Neural Crest-Derived Populations of Cells Associated with the Dentary of the Mammalian Mandible

Cell population

Chondroblastic/first arch Skeletogenic/ramus

Skeletogenic/processes

Odontogenic/papilla Skeletogenic/follicle

Cell types derived from the population

Chondroblasts of Meckel's cartilage Osteoblasts of ramal bone of dentary, chondroblasts of

callus cartilage in fractured mandibles, and secondary chondroblasts of the mandibular symphysis

Osteoblasts of bone of the condylar, coronoid, and angular processes; chondroblasts of secondary cartilage of the bony processes

Odontoblasts of the teeth, fibroblasts of the dental pulp Osteoblasts of alveolar bone, fibroblasts of the

periodontal ligament, cementoblasts of cementum

morphohistogenetic unit, the odontode (utilizing a sequence of epitheliomesenchymal interactions), and a bone morphohistogenetic unit (blastema or condensation), both with intrinsic capability of inducing cell differentiation, establishing and controlling shape (morphogenesis), and producing variation in tissue type and tissue arrangement. Development of the three tissues of the odontode (enamel, dentine, and bone of attachment) is linked in a causal sequence, independent of, although coordinated with, the development of deeper basal bone.

Cell Populations of the Mammalian Mandible

A model for coordination of odontode patterning and association with a dermal bone in the exoskeleton may be developed from one produced for a restricted part of the dermal skeleton in mammals, the mandible and teeth. Atchley and Hall (1991) proposed that the population of migratory neural crest cells that reach the maxillary and mandibular arches to contribute to the teeth, cartilage, and the basal dermal bone are restricted in their potential and establish three subpopulations, one only osteogenic, one only chondrogenic, and one only odontogenic. A further restriction of the odontogenic population into two subpopulations is discussed below, dental papilla (dentine and pulp-forming) and dental follicle (cementum, periodontal ligament, and lining alveolar bone-forming, the latter assumed to be homologous with bone of attachment in lower vertebrates). Evidence for such restrictions is provided by Palmer and Lumsden (1987) and Vaahtokari et al. (1991) and discussed by M. M. Smith and Hall (1990, p. 320). Atchley and Hall (1991) proposed


a classification ofthese cells which we now modify and expand. (Tables I and II). The following cell populations can be identified.

1. A chondrogenic population differentiating into Meckel's cartilage, the cartilaginous precursor of the lower jaw, and the skeletal element directing growth of the mandible.

2. A skeletogenic population forming bone of the body (ramus) of the dentary, chondroblasts of the symphyseal cartilages, and chondroblasts in repair of fractures of the body of the mandible. Symphyseal cartilage arises after bone formation; they are both secondary cartilages. The skeletogenic population therefore produces osteoblasts of ramal bone, callus cartilage in fractures, and secondary cartilage of the symphysis (Table II).

3. A skeletogenic population forming the bone and secondary cartilage of the three bony processes of the dentary, the condylar, coronoid, and angular processes.

4. Populations associated with the teeth, called odontogenic by Atchley and Hall (1991) to signify that they produce odontoblasts and deposit dentine at a particular stage in their development. Initially, these are the cells of the condensed dental mesenchyme associated with the developing enamel organ (Fig. la). This population of cells subdivides into two, one population becoming the dental papilla and the other the dental follicle (Fig. 1 b). Cells in the dental papilla differentiate into odontoblasts and fibroblasts of the pulp. These cells are not osteogenic. They are the population labeled odontogenic/papilla in Table II. Cells in the dental follicle differentiate into osteoblasts of the alveolar bone, fibroblasts of the periodontal ligament, and cementoblasts of

TABLE II. A Further Summary of Cell Types Formed from the Cell Populations Summarized in Table I

Cell typea

Cell population CB OB CC SC OD PFb PDLFb C

Chondroblastic/Meckel's + Skeletogenic/ramus + + + Skeletogenic/processes + + Odontogenic/papilla + + Skeletogenic/follicie + + +

a CB, chondroblasts; OB, osteoblasts; CC, callus cartilage chondroblasts; SC, secondary chondroblasts of secondary cartilage; OD, odontoblasts; PFb, pulpal fibroblasts; PDLFb, periodontal ligament fibroblasts; C, cementoblasts.


the cementum, three tissues anchoring the tooth into the ramal bone of the mandible. These are the cells labeled skeletogenic/follicle in Table II.

The tissue types formed by these different cell populations, all of which arise from the cranial neural crest, are further summarized in Table II.

Models for morphogenesis of such a complex structure as the exoskeleton will require knowledge of such subpopulations of ectomesenchymal cells.

Evidence that neural crest may have such population heterogeneity comes from in vitro studies using quail and mouse premigratory neural crest (Heath et al., 1992). These showed at least four separate populations within individual crest cultures, each with a unique antibody reactivity pattern. These results using both mesencephalic and trunk premigratory neural crest revealed different epitope expression 15 hr after explantation of premigratory neural crest at a stage when all cells were still morphologically identical. Essentially these in vitro studies are compatible with previous cell lineage studies using dye injection to mark emigrating neural crest cells (Fraser and Bronner-Fraser, 1991). These data demonstrated that restriction of neural crest cell fate must occur relatively late in migration, and that migrating trunk neural crest cells can be multipotent.

In a review of neural crest lineages, Marusich and Weston (1991) discussed how developmentally restricted intermediate subpopulations might arise sequentially by differential responses to developmental cues on their migration pathways. The molecular cues to determine phenotypic expression are part of the control mechanisms that select odontogenic and skeletogenic populations from intermediate populations of partially committed neural crest cells.

An explanation of skeletal diversity in the exoskeleton of vertebrates, using developmental and molecular data on the epitheliomesenchymal interactions of teeth and supporting dermal bone, will be proposed from data now available from the development of mammalian teeth (see next subsection). We propose that the same set of sequential restrictions would operate to produce subpopulations of neural crest cells in the trunk and head dermal skeleton.

Odontogenic and Skeletogenic Subpopulations of the Cranial Neural Crest

Odontogenic and skeletogenic populations of cranial neural crest cells are clearly a fundamental part of mammalian tooth development. The range of cell types arising from these populations localized, or at least recognizable, in the dental papilla and dental follicle is enormous. They produce odontoblasts of the teeth, osteoblasts of alveolar bone, cementoblasts of cementum, and fibroblasts of both the pulp and the periodontal ligament.

Clearly, the odontogenic lineage of the dental papilla separates out from the skeletogenic lineage of the dental follicle at an early stage in morphodif-


ferentiation ofthe tooth germ. Exactly when is not clear. This is an important subdivision, separating as it does an odontoblast-fibroblast lineage from a cementoblast-osteoblast-fibroblast lineage. Presumably a common progenitor capable of forming all these cell types exists at the pre-dental papilla/follicle stage (as shown in Fig. la, odontogenic ectomesenchyme). Experiments of Palmer and Lumsden (1987) show a choice in differentiation dependent upon location of the ectomesenchyme. Contact with inner epithelium produces odontoblasts, whereas contact with outer epithelium produces cern en to blasts (see Fig. 1c).

Experimental evidence for the existence of such separate odontogenic and skeletogenic lineages comes from experiments in which isolated dental follicles have been shown capable of forming these tissues (Ten Cate and Mills, 1972; Ten Cate, 1975; Osborn, 1984; Osborn and Price, 1988; Hall, 1992a,b) and from the physiological individuality of alveolar from ramal bone. Alveolar bone has a much higher turnover rate than does ramal bone and is dependent on biomechanical factors for its continued existence (Stutzman and Petrovic, 1989; Petrovic et aI., 1990; King et al., 1992). Alveolar bone is rapidly resorbed when teeth are extracted; ramal bone is not. Ramal bone develops in relative independence from any extrinsic influences from either the teeth or from muscles inserting on the dentary.

Cells of the skeletogenic lineage of the dermal bone can form cartilagethe secondary chondroblasts found in alveolar bone. Cells of the odontogenic lineage are not chondrogenic. Secondary cartilage only forms after osteogenesis has been initiated and arises as a localized response to functional need. The ability of skeletogenic cells to form secondary cartilage is a property not shared with cells of the follicle, but is confined to osteogenic cells of the ramus (which form symphyseal cartilage) and with osteogenic cells of the bony processes (which form secondary condylar, coronoid, and angular cartilages).

Several independent criteria therefore can be used to distinguish between and among skeletogenic and odontogenic cell populations. These include the forming primary tissues, the metabolic nature of the forming bone, dependence of the bone on mechanical and extrinsic factors such as muscle or tooth action, and whether and what type of cartilage forms.

Murine cranial neural crest contains both odontogenic and skeletogenic lineages. Rostral murine trunk neural crest contains only the odontogenic lineage, including its osteogenic subpopulation, but probably not the skeletogenic lineage (Lumsden, 1985, 1987, 1988). Amphibian cranial neural crest contains both odontogenic and skeletogenic lineages. Rostral amphibian trunk neural crest contains odontogenic lineages [see the studies of Graves on (1993)].

Thus, studies on murine and amphibian neural crest indicate odontogenic potential existing outside the cranial neural crest. Potential to form teeth extends beyond the cell populations normally forming teeth. Odontogenic


and skeletogenic cell populations can be distinguished and separated from one another along the neural axis from rostral (cranial) to caudal (trunk), odontogenic populations extending further caudal than skeletogenic populations.

Given these studies on mouse and amphibian embryos and given that a dermal exoskeleton of dentine and bone exists in many fishes, we would expect trunk neural crest of such fishes to possess odontogenic cell lineages. Indeed, in two teleost fishes, the goldfish and the Nibe croaker (Nibea mitsukurii), dentine and dermal bone can be obtained from cell lines derived from tumors of neural crest origin (Matsumoto et aI., 1983).

On the other hand, we would expect that vertebrates, such as birds, having lost teeth during their evolution, would have lost the odontogenic lineages from both cranial and trunk neural crest cell populations. Cross-species tissue recombination experiments of Kollar and Fisher (1980) indicate this to be the case, at least for mesenchyme derived from cranial neural crest (they did not test trunk neural crest). Teeth develop when murine tooth mesenchyme is combined with chick mandibular oral epithelium, a finding indicating chick epithelium to have retained the ability to induce mouse dental mesenchyme to differentiate into odontoblasts and deposit dentine, and to be induced by murine mesenchyme to form enamel in these chimaeric teeth. Teeth failed to form, however, when chick mandibular mesenchyme was recombined with mouse dental epithelium. This again emphasizes the primacy of the epithelium in initiation of the sequence of inductive interactions that typifies this epitheliomesenchymal model. Chick neural crest-derived mesenchyme has lost dentine-forming capabilities, even though oral epithelium retains enamelforming potential, albeit unrealized other than under experimental conditions.

Molecular Control Mechanisms

In the first two parts of this section we sought to elaborate a cellular basis for exoskeletal diversity using, as model systems, cell populations in the mammalian mandible and the discordance between skeletogenic and odontogenic cell populations within and between cranial and trunk neural crest. Eventually, we will require an explanation of exoskeletal diversity that includes molecular control mechanisms. Some signposts on the road to that explanation are provided in this section.

Molecular control mechanisms will have to be sought for at least the following developmental processes or events: (1) the origin of neural crest and mesodermally-derived scleroblast-forming cells; (2) the developmental origins of the neural crest; (3) segregation of the neural crest into cranial and trunk domains; (4) segregation of subpopulations of cells within neural crest and mesoderm; (5) onset and timing of differentiation of skeletogenic and


odontogenic cell types; (6) patterning of cell populations into skeletogenic and odontogenic tissues and organs; (7) regulation of turnover, replacement and growth of skeletogenic and odontogenic tissues and organs; (8) coupling or uncoupling of formation, rates, and timing of development of tissues such as dentine and bone within morphogenetic units such as odontodes and their accompanying osteogenic condensations. Needless to say, we are a long way from understanding the entirety, subtlety, or complexity of even one of these processes or events.

Analysis of molecular control mechanisms will require an understanding of whether control resides within individual cell populations, i.e., is intrinsic, or whether it resides, in whole or in part, within adjacent cell populations, i.e., is extrinsic or epigenetic (Hall, 1992a,b). In reality, exoskeletal and dental development is a subtle mix of intrinsic cellular properties acquired during ontogeny (and sometimes lost later in ontogeny) that are only realized following interactions with adjacent cell populations and/or their extracellular products, also subject to their own temporal and spatial regulation during ontogeny.

Furthermore, differentiation, morphogenesis, and growth of individual cell populations, tissues, or organs may be controlled by different mixes of intrinsic and epigenetic controls, mixes that can change throughout ontogeny. In this way, differentiation, morphogenesis and growth may be coupled or uncoupled, co- or independently regulated, or cause or effect. Such flexibility provides a powerful basis for developmental/ontogenetic, and evolutionary / phylogenetic modulation of the exoskeleton and teeth (M. M. Smith and Hall, 1990; Atchley and Hall, 1991; Hall, 1992a,b).

Patterning of the Neural Crest. Given the important role assigned to the neural crest in the development and evolution of exoskeletal and dental tissues, we begin our analysis by assessing the types of molecular control mechanisms responsible for patterning the neural crest. Such mechanisms are likely to have been how cranial and trunk neural crests were delineated and skeletogenic and odontogenic neural crest cells established. Langille and Hall (1993) have recently analyzed patterning of the neural crest, indeed of the entire head, in some detail. As the issue of whether the head or its component elements is segmented is discussed in that review and in some depth in three recent volumes devoted to the vertebrate skull (Hanken and Hall, 1993) segmentation per se will not be considered here. Hanken and Thorogood (1993), in their analysis of the role of pattern formation in the vertebrate skull, compare two different models, one in which pattern is specified intrinsically and the other in which an extrinsic, molecular prepattern is expressed by the neuroepithelium, to which skeletogenic mesenchyme conforms. It may be very significant that the first model applies generally to the viscerocranium (the morphogenetic specificity of neural crest referred to earlier in Noden's experiments); the sec-


ond, applies primarily to the neurocranium where gross spatial patterning is established by the matrix molecule, type II collagen, expressed transiently by the neuroepithelium and sensory epithelium as a prepattern, to which the mesenchyme responds. Experimental research is offering a molecular basis for intrinsic differences in the pattern-forming ability of neural crest cells, based on homeobox genes, as discussed later (see also Hanken and Thorogood, 1993). These gene products bind to other genes and regulate their expression, and in this way are candidates for determining the development of visceral arch-specific morphological patterns.

An important problem when considering patterning of the neural crest is the establishment of boundaries. What mechanisms determine where brain stops and spinal cord begins, where forebrain ends and midbrain starts, where cranial neural crest ends and trunk neural crest begins, where the posterior boundary of the skull lies? Since the pioneering scenario of the neural crest origin of the vertebrate head provided by Northcutt and Gans a decade ago, the skull has been seen as a composite of a neural crest anterior (rostral) portion fused onto a more ancient posterior (caudal) mesodermal portion.

As the neural crest arises in the developing neural tube, it is to the latter that we turn for molecular clues of patterns that might be imposed onto the neural crest while it is still within the neural ectoderm. The most obvious pattern, long suspected but only recently directly demonstrated, is organization of the developing brain into segmentally arranged rhombomeres (Lumsden and Keynes, 1989; Lumsden, 1990; Fraser et al., 1990). This neuromeric organization and the boundaries that it establishes are reflected in the patterning of the migration of neural crest cells away from the brain (Hunt et al., 1991; Hunt and Krumlauf, 1991), i.e., primary patterning of the brain is imposed onto neural crest-derived mesenchyme.

The molecular basis of this pattern has been shown to reside in the pattern of expression of homeobox genes, which are either restricted to individual neuromeres or whose rostral or caudal limits of expression coincide with neuromeric boundaries (Holland, 1988; Wilkinson et aI., 1989; Kessel and Gruss, 1990; Lonai and Urtreger, 1990; McGinnis and Krumlauf, 1992). Hox genes continue to be expressed in spatially regulated patterns in mesenchyme derived from neural crest cells, as is discussed in the following section.

Direct evidence for the role of homeotic and other patterning genes in establishing boundaries comes from such recent studies as that by Krauss et al. (1992) on the role of the gene pax[ b] in determining the midbrain-hindbrain boundary in the zebrafish, Brachidanio rerio, pax[b] is expressed in the midbrain and future midbrain region of the neural tube from gastrulation onward. Injection of antibodies against pax[b] into fertilized eggs resulted in specification of abnormalities at the midbrain-hindbrain boundary. These boundary anomalies were associated with failure of expression of endogenous pax[b] in


the posterior midbrain, and the alteration of such other patterning genes as wnt-l and en-2. Thus, we see that individual genes and their downstream effects play specific roles in establishment of boundaries within individual organ systems.

Evidence for a patterning role of Hox genes comes from experiments in which animals (usually mice) are made transgenic by insertion of a homeobox gene such that patterns of spatial expression of the genes are altered, the gene being expressed either more rostrally or more caudally than normal. Such ectopic expression of Hox genes repatterns mesenchymal cell populations and skeletal elements.

Lufkin et al. (1991) disrupted Hox-I.6 in embryonic stem cells using homologous recombination and then introduced the transformed cells into the germline of mice. The resulting embryos exhibited defective hindbrain development, lacked some cranial nerves and ganglia, and had abnormal inner ears and malformed skull bones. Hox-I.6 has a rostral boundary of expression; its disruption altered structures associated with the rostral hindbrain.

Chisaka and Capecchi (1991) used gene targeting to disrupt Hox-I.5. The resulting homozygous embryos displayed a syndrome-absence of thymus and parathyroid glands, diminished thyroids, heart defects, and craniofacial malformations-strikingly reminiscent of DiGeorge syndrome in humans. DiGeorge syndrome is thought to result from defective neural crest development (Hall and Horstadius, 1988). Hox-I.5 is one of the earliest Hox genes expressed in the mouse embryo. Disruption of Hox-I.5 therefore produces defects in many tissues.

Other Hox genes have more specific actions. Insertion of Hox-l.l into mouse embryos produces an extra vertebra, a proatlas (Kessel et al., 1990). Perturbation of Hox genes by retinoic acid causes anterior shifts in Hox gene expression and posterior transformations of vertebral type (Kessel and Gruss, 1991).

Hox-4.2 has a pattern of expression in the mouse such that the most rostral (anterior) boundary of expression in mesoderm is at the level of the somites forming the first cervical vertebra. If the pattern of expression of H ox-4.2 is experimentally manipulated so that the gene is expressed more rostrally, the occipital bones are transformed homeotically, into the next most posterior skeletal element, cervical vertebrae (Lufkin et aI., 1992). These authors discuss their results in the context of evolutionary transformations, with variation in expression boundaries of Hox genes as a genetic mechanism responsible for evolutionary modification of the skeleton.

Because homeobox genes provide a positional code utilized by both neural crest and mesodermally-derived skeletal elements, Hox gene transformations are particularly relevant to our analysis of exoskeletal and tooth patterning.


Patterning of the tissue producing exoskeletal scleroblasts (the neural crest) by homeobox genes provides a molecular mechanism for patterning the exoskeletal or tooth derivatives of the neural crest along the body axis. Example of tooth patterning involving Hox-7.1 and Hox-8 are discussed in the next section.

Whether the embryonic ectoderm is also patterned is more difficult to say. Couly and LeDouarin (1990) demonstrated an organization of anterior embryonic ectoderm into regions they called ectomeres, but it is not yet accepted that they represent true cell-restriction areas. Ectomeric boundaries do not correspond to neuromeric boundaries; indeed, they cut across neuromeric boundaries, so that a simple one-to-one correspondence between ectomeres and neuromeres does not exist. Nevertheless, if this ectodermal regionalization demonstrated in avian embryos exists in toothed vertebrates such as fishes and mammals, it would provide a mechanism for regionalizing ectoderm that will subsequently participate in formation of exoskeleton and/ or teeth. The molecular basis of ectomeric organization does not exist.

Odontodes and Condensations. In this section we assess molecular control mechanisms by evaluating current knowledge concerning the two fundamental units of the exoskeleton: the odontode, responsible for forming the odontogenic tissues (p. 393) and the osteogenic condensation, responsible for forming bone of attachment or alveolar bone (p. 402). There are sufficient parameters in common between basic mechanisms that molecular control of odontodes and condensations will be considered together. The cellular parameters of these two fundamental units were discussed earlier.

Few studies on exoskeletal or dental development in fishes are available, Nakajima's studies on pharyngeal teeth in larval and adult Japanese fishes (Nakajima, 1984, 1987, 1990; Nakajima and Vue, 1989) and Huysseune and Sire's (1992a) study on mandibular teeth being recent notable exceptions. Given that we lack molecular information on odontodes and condensations in fishes but do have information on molecular aspects of tooth and alveolar bone formation in mammals (primarily derived from studies on mice), the developing mammalian tooth will serve as our guide to molecular control of odontode formation. We must bear in mind, of course, that mammalian teeth are derived structures whose evolutionary history is removed from odontodes of fishes. Nevertheless, given the parsimony of developmental mechanisms that pervades vertebrate embryos in general (Langille and Hall, 1989; Hall, 1992a,b) and the common developmental bases of differentiation and morphogenesis of teeth and skeletal tissues (M. M. Smith and Hall, 1990) we feel justified in taking molecular control of mammalian teeth as a realistic signpost pointing the way to the types of information required to understand exoskeletal and tooth development/evolution in fishes. A similar argument can be made for utilizing knowledge of preosteogenic condensations acquired from avian


or mammalian embryos as the basis for exploring exoskeletal development and evolution in fishes.

All the available literature on molecular control of mammalian tooth development cannot be exhaustively reviewed in this chapter; recent overviews of molecular control of tooth development are Mina et al. (1990), Slavkin (1990, 1991), and Thesleff (1991). Rather, and to indicate the general categories of molecular control involved, we provide a summary of the type of molecular information that has accumulated concerning mammalian tooth development.

Vertebrate teeth are composite organs formed by two mineralized extracellular matrices: dentine, a mesenchymal (neural crest) product, and enamel, an epithelial (ectodermal) product. Consequently, the history of tooth development is the history of interactions between epithelial cells, from which enamel-depositing ameloblasts develop, and neural crest-derived mesenchymal cells, from which dentine-depositing odontoblasts develop. Epithelialmesenchymal interactions therefore epitomize, characterize, and constrain tooth development (Kollar, 1986; Slavkin, 1990, 1991; Thesleff, 1991). Intrinsic potential in each of these two cell layers is only realized following activation by the other cell layer.

Mammalian odontodes, as they proceed from bud, to cap, to bell stages, are classic representatives of the interplay between intrinsic and extrinsic control. It is at the cap stage that condensed tooth mesenchyme can first be subdivided into odontoblast-forming cells of the dental papilla on one hand, and the bone, cementum, and periodontal ligament-forming cells of the dental follicle on the other (see p. 393). Tooth morphogenesis is established at the bell stage; secretion of dentine and enamel quickly follows (see Fig. 1).

Essentially, two data sets pertaining to molecular control of tooth development are available: one is based on localization or visualization of molecules at particular stages, and the other derives from studies of the perturbation of molecules.

Syndecan, a proteoglycan on cell surfaces, and tenascin, a component of extracellular matrices that binds to syndecan (Salmivirta et ai., 1991), are present throughout tooth mesenchyme, initially at the bud, but in abundance at the cap stages (Thesleff et ai., 1987, 1988). Vainio et al. (1989) demonstrated that both syndecan and tenascin are induced because of interaction between epithelial and mesenchymal components of developing teeth. Syndecan, tenascin, and probably fibronectin are required for initiation of the important condensation stage of tooth development (Vainio et ai., 1992; Vainio and Thesleff, 1992).

Although fewer data are available, a similar scenario is emerging for control of preosteogenic condensations (Hall and Miyake, 1992). The initiating action of an epithelial-mesenchymal interaction and subsequent interplay between extracellular molecules such as fibronectin, cell-surface and cell-


adhesion molecules, and growth factors such as TGF-iJ (see below) facilitate the formation of condensations and the selective gene activation that accompanies condensation formation.

Coregulation of these molecules therefore facilitates formation of the fundamental anlage for teeth and for bone of attachment (alveolar bone). The importance of the condensation at the onset of selective gene activation during tooth or skeletal development cannot be overemphasized; see Hall and Miyake (1992) for a recent review.

Once cells have accumulated in the odontogenic or preosteogenic condensation, growth factors increasingly regulate tooth and bone development. Growth factors are small peptides. Originally thought only to act on transformed cells, growth factors have now been shown to be key players in regulating embryonic development; see Hall and Ekanayake (1991) for an analysis of the role of growth factors in regulating development of the neural crest and of neural crest derivatives. Growth factors have been visualized in developing murine tooth primordia and experimental manipulation of individual growth factors has begun.

. Once tooth development has been initiated, transforming growth factor (TQF)-iJl can be visualized in the enamel organ, in the stellate reticulum and in tieveloping tooth mesenchyme at both bud and cap stages of both mice and rats (Cam et aI., 1990; D'Souza et at., 1990). TGF-iJl, 2, and 3 have been vi!jualized at the epithelial-mesenchymal junction at these stages (Pelton et at, 1990; Millan et al., 1991).

The most convincing demonstration for an association between TGFiJl and epithelial-mesenchymal interactions during tooth development comes from the elegant study ofVaahtokari et al. (1991), who documented the temporal sequence of expression of TGF-iJl mRNA in dental epithelium and demonstrated its regulation by dental mesenchyme. TGF-iJ mRNA was shown to be present in dental epithelium at the bud stage and to increase rapidly as tooth primordia progressed from bud to cap stages, at which time mRNA was intense in the dental epithelium and initiated in dental mesenchyme. Message was not seen in dental mesenchyme until odontoblasts began to differentiate at 15 days of gestation. At the bell stage when the inner enamel epithelium begins to form (18 days of gestation in the mouse), TGF-iJ mRNA can no longer be visualized, but expression reappears in ameloblasts at 1 day posthatching.

Significantly, Vaahtokari and colleagues demonstrated regulation by dental mesenchyme of TGF-iJ mRNA synthesis in dental epithelium. This demonstration was achieved by separating and recombining epithelial and mesenchymal components of developing teeth. No TGF-iJ mRNA was expressed in dental epithelium combined with nondental mesenchyme. When dental epithelium was recombined with dental mesenchyme, however, TGF-


{3 mRNA was expressed in the epithelium. It is clear that dental mesenchyme regulates the synthesis ofTGF-{3 mRNA in dental epithelium. These workers noted that TGF-{3 synthesis was controlled by the product of the Hox-7.1 gene, providing a potential link between epithelial-mesenchymal interactions, homeobox genes, and growth factor expression; see page 414 for homeobox genes.

Nerve growth factor (NGF) and NGF receptor (NGFR) have been localized in preameloblasts and odontoblasts and temporal regulation of NGFR suggested; NGFR is only transiently expressed in inner dental epithelium and in odontoblasts (Mitsiadis et aI., 1992). The temporally and spatially complex patterns of expression displayed by NGF and NGFR are consistent with changing roles in regulation of both differentiation and morphogenesis.

Epidermal growth factor (EGF) receptors are present in the enamel organ at the outset of tooth development, but are lost from both inner and outer enamel epithelia as they differentiate from the enamel organ (Cam et aI., 1990) [see Partanen (1990) for a review]. EGF is found in both epithelial and mesenchymal components of developing murine teeth (Slavkin et aI., 1990).

EGF within odontodes is temporally regulated (Partanen and Thesleff, 1987; Topham et aI., 1987). At the bud stage, EGF is found in the dental epithelium but not in the dental mesenchyme, a pattern that is reversed at the cap stage. EGF persists in the dental follicle, the precursor of alveolar bone, cementum, and periodontal ligament. EGF regulates gene expression in both epithelial and mesenchymal components of developing teeth; EGF inhibits synthesis of type I collagen by odontoblasts and synthesis of enamel protein by ameloblasts when tooth buds are cultured in the presence of EGF (Hata et aI., 1990).

A role for EGF has been forcefully demonstrated by Kronmiller et al. (1991), who showed that an antisense oligodeoxynucleotide to EGF blocked odontogenesis in tooth germs from E9 embryos explanted in vitro, apparently by blocking initiation of odontoblast differentiation. EGF itself is transcriptionally regulated by retinol (vitamin A). Mandibles from 9-day-old embryos exposed to 1-5 JLg retinol/ml produce supernumerary teeth associated with enhanced epithelial proliferation and increased expression of EGF mRNA (Kronmiller et aI., 1992). Therefore tooth patterning, including specification of the number of teeth primordia, can be modulated by agents such as retinoids which act on growth factors such as EGP. As discussed above when considering patterning of the neural crest, retinoids do act as pattern-generating molecules by modifying the patterns of expression of hom eo box genes. Retinoids have now been shown to specify patterns in a number of embryonic organ systems


(Maden et al., 1992; Murphy et al., 1992). As discussed below, a link between retinoids, growth factors, and homeobox genes has also been established.

Homeobox genes, increasingly being recognized as important components of axis, polarity, and segmentation specification in invertebrates and vertebrates, have already been described as key molecular players in specification of the neural crest and its regionalization into cranial and trunk neural crest. They also appear to play a role in specification of tooth morphogenesis.

Mackenzie et al. (1991) found Hox-7.1 expression to be maximal in dental mesenchyme at the cap stage when dental mesenchyme condenses and tooth morphogenesis is initiated. It may be of morphogenetic significance that H ox-7.1 can be activated by retinoids, and that retinoic acid receptor and retinoic acid-binding proteins have both been localized in developing teeth. Theslef et al. (1990) represented Hox-7.1 as active early in the cascade of molecular changes leading to expression of syndecan, tenascin, growth factors, and ultimately to differentiation of dental mesenchymal cells as odontoblasts and deposition of dentine. Epithelial-mesenchymal signaling regulates this molecular cascade.

Another homeobox gene, Hox-B, has also been shown by Mackenzie et al. (1992) to be localized at sites and times oftooth morphogenesis, including localization in a proximodistal gradient in dental mesenchyme at the sites of future tooth formation. Hox-B is found in the oral epithelium at the dental placode stage, a spatial and temporal localization consistent with a potential role in specification of tooth position.

A direct link between epitheliomesenchymal interactions, Hox gene expression, and osteogenesis of neural crest-derived mesenchyme has been demonstrated by Takahashi et al. (1991). Quox-7 (the quail homologue of mouse Hox-B, now msx 2) is only expressed in mandibular mesenchyme of the embryonic quail when epithelium is present. Osteogenesis of mandibular mesenchyme depends upon interaction with mandibular epithelium (Hall, 1983). It is obviously tempting to link these two observations causally and to speculate that the mandibular epitheliomesenchymal interaction initiates osteogenesis after activation of Quox-7 (msx 2): msx 1 (Hox-7) in the chick is known to regulate muscle differentiation. Also in the chick msx 1 is expressed in the mesenchyme of all the branchial arches (A. Graham, personal communication).

So we see that cell surface molecules (syndecan), components of extracellular matrices (tenascin, fibronectin), growth factors (EGF, TGF-,B), homeobox genes (Hox-7.1, B.1), and retinoids represent five classes of inter dependent molecules involved in molecular control of odontode formation. The potential for developmental and evolutionary modulation of such pentapartite control is enormous.


Origins of Skeletal Tissues and an Evolutionary Model

Since the review of evolutionary origins of vertebrate skeletogenic and odontogenic tissues by M. M. Smith and Hall (1990), a new interpretation of the presence of osteocytic bone for one of the Harding Sandstone early vertebrates has been published (M. M. Smith, 1991), confirming previous suggestions that cellular tissue was present (Denison, 1967); also, a new report appeared on the histology of the phosphatic mineralized elements known for over a century as conodonts (Sansom et aI., 1992) in which a new interpretation is made of the inner core "white matter" in which the minute spaces are interpreted as evidence of osteocytic bone with integral cell spaces and interconnecting cannaliculi. This cellular tissue and the basal tissue, which is compared with calcified cartilage of Eriptychius, one of the two named vertebrates of the Harding Sandstone, are used as synapomorphic vertebrate characters for conodonts (Sansom et ai., 1992; Briggs, 1992; Aldridge et ai., 1993), because cellular bone is certainly present in dermal denticles of the third vertebrate of the Harding Sandstone, and calcified cartilage is represented as a mineralized tissue in close association with the acellular bone of Eriptychius.

Previous work on conodont histology (Gross, 1954) had looked for a resemblance to acellular bone (aspidin) because this was the tissue character ofheterostracans, the primitive agnathan group. This phylogeny predicts that aspidin would be present in a more primitive outgroup (acellular bone assumed to precede cellular bone). The new interpretations that odontode-related cellular bone is present in conodonts and primitive vertebrates have major implications for the evolution of vertebrate skeletal tissues, namely, that cellular bone is more primitive than acellular bone, and that it may be more primitive than orthodentine (see Fig. 5). This would suggest that the latter type of vertebrate tissue, with only the cell processes in spaces without the inclusion of the cell body, is a later evolutionary development, and also that conodonts are the primitive sister group of the higher craniates, excluding myxinoids (Briggs, 1992; Aldridge et ai., 1993) (see cladogram in Fig. 4b).

In this new phylogeny conodont phosphatic tissues present many problems for discussion of the evolution of odontoskeletogenic tissues in the exoskeleton in vertebrates, and we propose that these should be considered in this review. Among these are (I) the presence of oral mineralized tissues before any in the dermal skeleton, (2) the apparent absence of dentine, and (3) the presence of cellular bone in conodonts and osteostracans, but its absence in heterostracans. As Briggs (1992) concluded from the new evidence on conodont histology, "The absence of dentine and the presence of cellular bone in a highly adapted feeding apparatus in the earliest vertebrates leave current hypotheses of the early evolution of their skeletal tissues in some disarray."


We therefore evaluate conodonts in our evolutionary model, and reassess the conclusions drawn previously (M. M. Smith and Hall, 1990) from the available fossil evidence on the sequence of evolution of vertebrate skeletal tissues, with particular reference to the evolution of cranial neural crest as a vertebrate innovation. Several major questions are raised by accepting conodonts as vertebrates: (1) Are conodont elements (teeth or tooth plates) independently evolved from those of gnathostomes, or are they the earliest examples of teeth and homologous with those in gnathostomes? Also, did these conodont teeth arise before a dermal skeleton? A consequence of an affirmative answer to the latter question would be that loss of teeth in all other agnathans was secondary, and that teeth had been reacquired in gnathostomes. (2) Does the presence of enamel, tooth-related bone, and tooth-related cartilage suggest a cranial neural crest odontoskeletogenic origin of the visceral skeleton, before neural crest made dermal bone as a body armor? (3) The absence of dermal skeletal denticles in conodonts must question the proposed primitiveness of such denticles in the vertebrate exoskeleton. The reverse view-"odontodes ofthe dermal skeleton being already present in the dermal skeleton before any skeletal structures in the oral cavity" (M. M. Smith and Hall, 1990)-has been long held.

No previously postulated evolutionary scenario envisages mineralized pharyngeal teeth before a body armor. Gans and Northcutt (1983), however, argued that a cartilage-supported muscular pharynx was an early prerequisite of vertebrates before gas exchange through the skin could be sacrificed for a protective, sensory dental skeletal armor. Acceptance of this argument prompted Hall and Horstadius (1988) to comment that "early association of neural-crest-derived skeletal tissues (visceral cartilages) with the pharynx establishes the dichotomy between a skeletogenic cranial and non-skeletogenic trunk neural crest." A calcified cartilage support and tooth elements at the rostral end of a pharynx are now known for conodonts (Sansom et al., 1992; Aldridge et al., 1993), but no cartilaginous supporting elements remain in the fossils of the whole animal.

Can we therefore use the evolutionary record of conodont tissues in an evolutionary model to explain the order in which skeletal tissue components arose? Did these tissues arise because of evolving developmental competence of various populations of neural crest or mesodermal mesenchyme to make the whole variety of skeletal tissues? Such questions are central to this review.

Conodonts as Vertebrates

The debate on conodont affinities (a separate phylum, nemerteans, mollusks, chaetognaths, cephalochordates, or vertebrates) has been readdressed in a paper by Aldridge et al. (1993) in which six new specimens of the conodont


animal are described, others reevaluated, and details given of newly discovered element assemblages. These new anatomical characters, plus the histology of the conodont elements described as being vertebrate tissues (Sansom et aI., 1992), together with details of how these worked as a feeding apparatus (Purnell and von Bitter, 1992; Purnell, 1993), make acceptance of these phosphatic mineralized mouth parts as vertebrate teeth much more compelling than previously thought.

Conventionally, conodonts had been studied as one type of phosphatic microfossil, with any of the affinities listed above. Sansom et al. (1992), in an initial report of euconodont histology, assigned them to vertebrates on the basis of both their tissue composition (enamel homologues, cellular bone, and calcified cartilage) and their tissue growth, as if in apposition from a dermal-epidermal interface in a toothlike structure (conodont elements). As discussed by Sansom et al. (1992), other workers had previously claimed the presence of most examples of extant and fossil vertebrate hard tissues, as well as similarities with the un mineralized keratinous teeth of myxinoids. There has been little agreement, however, on definitive evidence of a vertebrate affinity for conodonts.

One remarkable feature is that conodonts are found over a very wide stratigraphic range, from long before the previously accepted earliest vertebrates are recorded (at least 30 million years prior to this), until all major vertebrate groups, except the mammals, had evolved (350 million years ago), a range from the Cambrian to the Triassic.

Although isolated conodont elements have been known as fossils for 130 years, it was not until relatively recently that soft-body impressions of the whole animal were discovered with these elements in situ (Briggs et al., 1983). Sansom (1992) has documented the earliest notes on conodonts and their monographic description in 1856 by Pander. From their location in the cephalic region as a spatially preserved assemblage, conodont elements were interpreted as a feeding apparatus within an oral, or at least a pharyngeal, cavity. There is no evidence of any paired branchial cartilages as functional skeletal supports for a moveable pharyngeal apparatus, but their presence is not precluded; the soft tissues are extremely poorly preserved (M. P. Smith, personal communication).

Subsequent to the whole-animal descriptions, a more reliable basis for restoring the in vivo arrangement of "conodont elements" was discovered. This was a lower Silurian conodont animal, Panderodus, with a bedding plane assemblage of elements preserved as arranged in the living animal (Mikulic et al., 1985). M. P. Smith et al. (1987) reconstructed the three-dimensional arrangement of this assemblage with the bases of the paired elements arranged on an arch and the tips in an overlapping occlusion. From this reconstruction they discussed the probability of a contiguous cartilaginous basal support for


each half set of the elements, noting that basal filling material of several Ordovician taxa resembled globular calcified cartilage of the earliest vertebrates. They concluded that the 14 bilaterally opposed conodont elements were a feeding apparatus. Sansom (1992) has reevaluated this specimen and reconstructed the apparatus as two arched bilateral blades of eight pairs, and an additional, 17th, symmetrical element along the midline. Using an analysis based on comparison between two functional hypotheses, Purnell (1993) has concluded that conodonts were not suspension feeders, but used the elements of the feeding apparatus in toothlike function. His reconstruction of this apparatus is reproduced in Fig. 3 to show their position relative to the whole animal, and also how the posterior P-elements might slice against each other, with overlapping occlusion.

Two sets of coniform elements set in rows parallel to the trunk axis suggested a similarity of function with the feeding apparatus of myxinoids, and led to the proposal that the jaw musculature may be homologous, but associated with differently derived elements. In fact, the latest report on the histology of conodont elements (Sansom et aI., 1992) shows many differences between these and keratinous teeth of myxinoids. Indeed, as M. P. Smith (1990) pointed out, the elements of all three agnathan groups (myxinoids, petromyzontids, and heterostracans) are histologically different. M. M. Smith and Hall (1990) also did not support homology of hagfish teeth with those of other vertebrates. In the light of new histological data, however, we do have to reconsider the status of conodont elements as potentially representing the earliest form of craniate skeletal tissues.

Evidence from conodont whole bodies and intact assemblages reinforced the hypothesis that conodonts are more closely related to jawless craniates than to cephalochordates (Aldridge et al., 1986; M. P. Smith, 1990). The cladogram then proposed, however, contained an unresolved dichotomy between myxinoids and heterostracans (Fig. 4). Blieck (1992) identified the central problem of conodont relationships as the diversity of opinion on the histological types present, and concluded that features used to determine chordate affinity were not convincing. He presented a cladogram with conodonts in an unresolved position between cephalochordates and myxinoids, as reproduced in Fig. 4c. The relationships of conodonts within the vertebrates will remain a source of debate until the histological characters are further refined. This is a view endorsed by Forey and Janvier (1993) in their review of agnathan relationships and early vertebrate history, who "await more research on a wide variety of conodonts before histological structure may be used as evidence of vertebrate affinities." On the basis of current information, however, they can be placed closer to the heterostracans than the myxinoids, as in the cladogram (Fig. 4b) suggested by Briggs (1992). Sansom (1992) and Aldridge et al. (1993) have reassessed the phylogenetic position of conodonts

Vertebrate Exoskeleton and Neural Crest

a

b

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.' ~ .' ~ .. :,. .... . '~ .;.'/

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FIG. 3. The conodont elements arranged in !$itu as a projected reconstruction from their anteroventral position in the conodont animal, based on a partially disarticulated bedding plane assemblage of a single apparatus of Vogelgnathus campbelli from the lower Carboniferous, as illustrated by Purnell and von Bitter (1992, Figs. 1 and 3). They concluded that "the anterior elements grasped the food, which was then cut and ground by the Pb and Pa elements." The whole apparatus functioned as an integrated feeding structure, grasping, cutting and grinding to process the food. Part (b) shows the cutting function in operation with the Pa elements interacting as in "serrated scissors." [By permission of the authors and Nature. Copyright 1992, Macmillan Magazines Ltd.]


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FIG. 4. Different cladograms taken from the publications of (a) Gans (1989), (b) Briggs (1992), and (c) Blieck (1992), illustrating the different views on the relationships of conodonts to chordates and to the agnathan vertebrates, the Heterostraci.

within the vertebrates and produced a cladogram for pregnathostome vertebrates based on the consensus of that of Janvier (1981). They have used the presence of cellular dermal bone, enamel homologues, and calcified cartilage to place conodonts crownward of myxinoids and a sister group of higher vertebrates.

Chordate/Vertebrate Origins and the Primitive Skeletal Tissues

Conodonts may represent an initial approach to skeletization of the feeding apparatus and an early experiment of neural crest in making a set of mineralized tissues. This interactive developmental program, initiated by ectodermal cells, was probably of a type that was later modified and extended in evolution to make odontodes and teeth with recognizable dentine that later covered the whole body. In a recent paper M. M. Smith (1992) has argued for the primitiveness of enamel in vertebrate odontodes/teeth and that heterochrony can explain the early or late appearance of this ectodermal secretion product in teeth of different types, an explanation that would allow conodonts


to develop enamel perhaps before dentine. Observation that conodont elements and some associated elements belonging to a species of Pseudooneotodus possessed a distinctly recognized enamel layer and possibly also dentine (Sansom, 1992, and personal communication) emphasizes the primacy of ectoderm in the odontogenic morphogenic system and its ability to induce or produce phosphatic mineralized tissues.

In our view, enamel and dentine of vertebrate teeth are always associated with cellular bone, or alternative attachment tissue, and cellular dentine may be more primitive than dentine without cell bodies, but with cell processes, as evidenced by the early vertebrate tissues (M. M. Smith, 1991). There is a problem with the allocation of the euconodont tissues to bone or an early type of dentine based solely on the size and arrangement of the cell and cell process spaces, and this will need to be resolved by examination of many examples from further species and by using other parameters. Primitive dentines are generally described as belonging to one of four patterns as shown in Fig. 5, based on those suggested by 0rvig (1967) and figured by Baume (1980, Fig. 4.1), and Reif (1982, Fig. 2). The implication has been that this represents an evolutionary series with a progression from primitive mesodentine to the

a. Mesodentine b. Semidentine c. Metadentine d. Orthodentine

FIG. 5. Four types of dentine all occurring in early agnathan vertebrates, proposed by 0rvig (1967) as sequential stages in an evolutionary series in which progressive changes in development of the primitive (a) mesodentine and (b) semidentine released the odontoblast body from the mineralized matrix, leaving only a branching network of cell process spaces leading from a central polarized cell process as in (c) metadentine and (d) the more regular orthodentine. The number of cell bodies enclosed in mesodentine relative to cell process spaces varies considerably among the fossil agnathan osteostracans.


most advanced orthodentine as found in human teeth, but also present in some heterostracan dermal armor of the lower Silurian forms [examples of these different types of dentine have been illustrated in M. M. Smith and Hall (1990, Figs. 12,13, and 15). As stated previously, "The question of the relative primitiveness of dentine and bone is one that has aroused the most debate" (M. M. Smith and Hall, 1990), and the assumption that bone arose earlier than dentine was challenged with the proposition that "the earliest skeletal tissues were dental." The tissue beneath the enamel-like layer in conodont elements could represent the primitive papillary product, a fine-gauge syncitium of cell process and cell body spaces, without any polarity to them, more bonelike than dentinelike, but with the potential to be transformed into the types recognized as mesodentine, semidentine, and metadentine in the later evolving vertebrates. In this sense the conodont tissues present in elements that function as teeth could be the most primitive type of dentine and be present before any tissue developed in a bonelike way, as in fetal or dermal membrane bone.

Suggestions that cellular bone is more primitive than acellular were discussed by M. M. Smith (1991) with a new evaluation of the earliest vertebrates with well-preserved histology. The presence of cellular bone in conodonts polarizes this character for vertebrate evolution. In both it is bone of attachment that forms in association with the tooth and fixes the tooth/odontode to a support system, either soft connective tissue, calcified cartilage, or bone.

The decision that conodont elements function as teeth or dental plates, is now more universally accepted [see Jeppsson (1979) and Conway Morris (1980, 1989) for discussion of all previous views]. We would suggest that the skeletogenic ability of neural crest evolved first in the head, and that dermal denticles or a body armor of sheets of dentine and bone did not precede oral teeth or dental plates in the fossil record. It is interesting that experimental embryology in amphibians indicates that development of teeth is frequently found to be spatially and temporally associated with cartilage of the visceral skeleton (A. Graveson, M. M. Smith, and B. K. Hall, in preparation). In cichlid fishes, tooth formation and attachment is intimately associated with (and possibly inductively coupled to?) resorption of cartilage (Huysseune and Sire, 1992a,b).

If conodont elements were the first craniate experimentation with mineralized tissues in the mouth/pharynx, the following reconstruction can be proposed as being in accord with some ofthe elements of the scenario discussed by Gans (1987). Predatory, muscularized conodonts with paired sensory organs (derived from embryonic placodal systems) for sight and balance (M. P. Smith, personal communication; Aldridge et al., 1993; Aldridge and Theron, 1993), serving as distance receptors, were armed with two laterally opposed sets of piercing teeth for food trapping, supported by calcified cartilage ofthe visceral


skeleton (derived from cranial neural crest). The success ofthis reconstruction depends on the evolutionary novelty of the cranial neural crest providing migratory cells, as suggested by Gans and Northcutt (1983, 1985), with the potential to induce and differentiate into odontoskeletogenic cells that produce mineralized tooth tissues and cartilage alongside the pharyngeal muscles.

At this stage the trunk neural crest would not have evolved its skeletogenic role. Prepatterning of ectoderm alongside the cranial region of the neural crest, in addition to the placodal tissue, remaining superficial to ectomesenchyme of the branchial region, would provide the initial induction signal. This is suggested by experimental work in the chick that shows regions of superficial ectoderm that correspond with the spatial distribution of neural crest cells (Couly and Le Douarin, 1990). None of this will preclude the possibility that an electroreceptive sensory system would have existed in the soft tissues of the skin as the first true distance-receptive system, as proposed by Gans (1987). As Gans implied, enhancement ofthis system would have been secondary by positional stabilization of the signal receptors/sources, with either dentine or bone deposition associated with this system. This would be acquired when skeletogenic ability of extensively migratory neural crest cells evolved in the dermis. These patterns generated in the phylogeny of vertebrates would be conserved and operate as constraints in a developmental model.

Teeth are not present in the mouths of the other fossil agnathan groups; this poses a far more fundamental question of homology. Again, if not homologous, then teeth were acquired independently several times in vertebrate evolution and the involvement of neural crest in conodont elements cannot be assumed. The search for teeth in other agnathan groups becomes even more imperative (see p. 438). Also, the significance of enamelinlike proteins in hagfish teeth must be reassessed. Although internal pharyngeal teeth are not known, dermal structures at the margins of the mouth in heterostracans, referred to as dental plates by Halstead (1973) and Janvier (1981), are reported to be used in food gathering, but assumed not to be homologous with teeth. Recent discovery of an articulated Silurian agnathan fish representing the oldest and most primitive heterostracan adds further information on the arrangement of oral plates surrounding an anteroventral mouth (Wilson and Soehn, 1990; Soehn and Wilson, 1990). Soehn and Wilson report that "We see no evidence of anterior denticles, sharp ends, or slicing surfaces on the oral plates of Athenaegis, although there is a finely ornamented area on the rostral brim." They further suggest that there was "unified opening and closing movements of a scoop-shaped mouth" and that "they most likely fed upon plankton or detritus." Their histology and mode of growth are unknown, so that new data of this type would almost certainly lead to a new interpretation of the relationship of these structures to other feeding apparatuses. This would


be crucial to theories on the relationship between teeth, tooth elements in conodonts, and dermal armor in other established early vertebrates.

With respect to the great variety of dentine and bone in the early fossil record, Gans (1987), noting the complex armor of heterostracan and osteostracan agnathans, suggested that "each represents a derived condition relative to the animals showing the 'earliest' ossification. The fossils and their 1:UIDour give us a record of an evolutionary experiment that is analogous to the agnathan experiment with paired appendages." It would seem that there is now a possibility of the earliest skeletization being part of a toothed apparatus in the head of a craniate animal.

In the next section we review and compile what some of these developmental patterns might be, as deduced from experimental embryology.

EXPERIMENT AL EMBRYOLOGY OF SKELETOGENESIS IN LOVVER VERTEBRATES

The Role of the Neural Crest

Although a few studies rely on natural markers, most knowledge about the neural crest is from experimentally marking or manipulating embryonic tissues, admitedly from a very small number of species.

Most studies of skeletogenesis, apart from those on mammals, have been in the chick or in amphibians, both anurans and urodeles. Most information available on neural crest in fishes concerns nonskeletogenic tissues. Migration pathways of trunk neural crest cells were traced using tritiated thymidine in a teleost, Barbus conchonius (Lamers et al., 1981). Distribution and migration of HNK-l immunoreactive neural crest in both the head and the trunk of three species of teleost (Xiphorous helleri, X. maculatus, and Orizias latipes) were reported by Sadaghiani and Vielkind (1990). In neither study were embryos studied long enough for any comments to be made on skeletogenesis.

The basis for reviewing details of the involvement of neural crest in skeletogenesis in fishes is the accepted view that all vertebrates develop a neural crest tissue with an underlying common pattern of morphogenesis and migration, which, at least in the cranial region, is skeletogenic. As remarked by Hall and Horstadius (1988), neural crest is "characterized by similarity in site of embryological origin, by similar regionalization within the neural tube, by similar migratory behaviour and by production of a similar range of cell and tissue types." It is axiomatic that this pattern is conserved throughout the vertebrates and is synapomorphic. For instance, to date, no publications


demonstrate an origin of odontogenic tissue other than from neural crestderived mesenchyme. Comments by Kemp (1990) from extirpation studies of the neural crest in the Australian lungfish Neoceratodus that "cells of the neural crest are of limited importance in this animal" and also "that lungfishes are so far from the main stream of vertebrate evolution that their neural crest cells are atypical (in not producing dental or skeletal tissues)" (Kemp, 1993) are hard to reconcile with all other published accounts, in particular with the underlying concept of conserved developmental strategies, which allows the statement that "the neural crest is a quintessential vertebrate characteristic . . . producing a myriad of craniate tissues" (Hall and Horstadius, 1988, p. 20), a major group of which is skeletogenic, i.e., cartilage, bone, and dentine.

The question of similarities between patterns in development of the skeleton from cranial neural crest has been reviewed by Hall, in Hall and Horstadius (1988), in which he acknowledges that little is known about neural crest in fish. He cited the work of Damas (1944, 1951), an early example of experimental work on neural crest skeletal structures in the lamprey, in which defective chondrogenesis in the head cartilage resulted from exposure of developing larvae to light. Interestingly, the conclusions were that these defects resulted from a failure of the interactions between epithelium, neural crest, and pharyngeal endoderm, developmental mechanisms assumed to operate in lampreys as they do in amphibians. The possibility that defective chondrogenesis was due to lack of neural crest cell migration was not discussed.

Extirpation and transplantation experiments ofNewth (1950, 1951, 1956) on Lampetra planeri and L. jluviatalis were very thoroughly discussed by Hall (1987) and Hall and Horstadius (1988) with an assessment of their significance. Despite initial failure by Newth to show that cartilage was produced by cranial neural crest, he continued with further experiments (Newth, 1956) which indicated that although lamprey neural crest was capable of forming cartilage in flank grafts, it no longer received the required inductive influence, whereas in branchial grafts induction was provided and cartilage developed from donor neural crest. Damas (1951) had also speculated that contact with branchial arch epithelium was necessary for chondrogenesis to occur in the lamprey, Lampetra jluviatis. As Hall and Horstadius (1988) summarized, "both the cellular origin of these cartilages and the requirement for activation via inductive interaction with embryonic epithelia corresponded well with previous results in amphibia." The questions raised here are, has the epithelium lost or did it never acquire its inductive ability in the trunk during the evolution of "higher" vertebrates? Also, is migration of neural crest necessary to determine when the epithelial inductive signal will be produced?

A recent study of the sea lamprey, Petromyzon marin us, by Langille (1987) and Langille and Hall (1988a) used very precise extirpations of regions of 250-mm craniocaudad lengths to analyze deletions of the cartilaginous


skeleton. These extirpation studies showed good correlation between the region extirpated and the cartilages that failed to develop and relatively few abnormalities of structure, certainly not massive ones. From these studies, the chondrogenic portion ofthe lamprey neural crest was mapped. It shows good correspondence with maps developed for other vertebrate embryos, absence of chondrogenic potential in the anterior prosencephalon and caudad to the sixth pair of somites. Absence of cartilages following extirpation showed that regulation was not a significant mechanism, and reinforced data that specific regions of cranial neural crest alone contribute to the branchial arches in fishes (see below).

Regional information for neural crest cells is also important. The more posterior branchial arches are derived from the more posterior region of cranial neural crest. Some cartilages (i.e., parachordals, otic capsules) developed normally following deletion of the neural crest, the conclusion being that these are not derived entirely from neural crest mesenchyme. Couley et al. (1993) have now provided very precise evidence in the chick for the cellular origin of the sphenoid (a cartilage bone from the parachordals) from cephalic paraxial mesoderm, and the basipresphenoid from cranial neural crest (see p. 390). Hall and Horstadius (1988) concluded that this mapping of the chondrogenic part of the cranial neural crest in lampreys represents a pattern developed early in vertebrate evolution and conserved among divergent groups.

Langille and Hall (1988b) were also the first to report an experimental study mapping the cranial neural crest of teleost fishes (the Japanese medaka, Orizias latipes) preceded by a baseline study of normal development of the head skeleton (Langille and Hall, 1987). In the experimental study sections of neural crest were removed from stage 18-19 neurula embryos. From an analysis of changes to the head skeleton it was possible to conclude that most of the anterior neurocranium and the entire viscerocranium received neural crest contributions during development. But of even greater interest was the mapping of neural crest cells contributing to individual elements from particular rostrocaudal regions along the neural axis. This pattern was found to be very similar to the regional contributions from the neural crest to the head skeleton in lampreys, urodeles, and birds (Langille and Hall, 1988a; Fig. 7). These patterns are evolutionarily conserved.

An elegant set of experiments in teleost fish (Lamers et al., 1981) uses transplantation of previously labeled grafts to reveal migration pathways of trunk neural crest cells. They used donor neural crest from an embryo prelabeled with [H3]-thymidine and injected labeled neural crest into rhombencephalic and trunk regions of 1- to 16-somite host embryos. A migratory pattern similar to that observed in birds was seen. There was, however, one small difference; the ventral stream did not penetrate mesoderm, but migrated within spaces between the neural tube/notochord and somites. This difference


was explained by Lamers and colleagues as possibly due to a relatively much earlier period in fish development of "explosive growth and differentiation of the somites, to become functional within a few hours after their formation."

Sadaghiani and Vielkind (1990) used HNK-l antibody visualization to mark and follow premigratory and migratory neural crest cells in developmental series of three species of teleost. Their results were similar to those reported in birds and mammals, which they concluded "reflected the close similarity of embryogenesis amongst the vertebrates." In an earlier study of normal development of the neural crest in Xiphophorus teleost fishes (Sadaghiani and Vielkind, 1989) and in their HNK-l labeling study, Sadaghiani and Vielkind commented that one marked difference between these fishes and other vertebrates was segregation of neural crest cells from the dorsolateral aspect of a neural keel, rather than from the dorsal midline as in other vertebrates. They described ridges ofHNK-l-positive cells appearing topographically lateral to, and temporally after, neural keel formation, and identified these as populations of neural crest cells. One route for migration of neural crest cells in the trunk is below the epithelium. These superficially-migrating cells are presumed to give rise to pigment cells. This issue will be addressed in the following section on the potential of trunk neural crest, as will the issue of the dorsal location of many neural crest-derived cells.

An early report (Lopashov, 1944) of extirpation and grafting in several teleost fishes (Misgurnis jossilis, Nemacheilus barbatulus, Perca f/uviatilis) showed that cartilage developed in neural crest cells grafted into yolk sacs, supporting a neural crest origin of cranial cartilage in these fishes. It seems one must assume either that yolk sac epithelium is inductive on neural crest cells for chondrogenesis, or that these neural crest cells were specified prior to transplantation.

The labeling study by Lamers et al. (1981) was not directed at skeletal tissue development. Nevertheless, they showed an accumulation of trunk neural crest-derived cells under sensory organs of the neuromasts. Such a localization can be linked to, and provide indirect evidence with, observations that the first preosteogenic cells aggregate in association with neuromasts. Whether the early aggregating cells observed by Lamers and colleagues would have become neural or connective (skeletal) tissue is unclear.

All the experimental studies just discussed reveal different aspects of the neural crest as a potent source of cells in embryogenesis, giving rise to a great diversity of cells and tissues with a remarkable precision of distribution. In a summary of work on the neural crest and its derivatives since 1950, Weston (1970) emphasized the understanding of developmental problems provided by each different experimental method, and evaluated those conclusions that describe processes and those that allow causal analyses. Experiments involving


the neural crest were grouped into three types as presented above, ablation, explantation, and markers.

The full range of the prospective potential of the neural crest, independent of any environmental developmental cues, is evidenced in explantation studies. The normal fate of crest cells is best determined by labeling and orthotopic grafting, provided that, as Weston pointed out, the region excised from the donor for grafting is very precisely determined and host age controlled to avoid precocious migration of crest cells. Permanent and cell-specific markers allow patterns of early migration and localization to be followed, provided that the chosen markers do not transfer to other cell populations and that they are not toxic to the cells in normal development.

Cell-marking experiments are probably where the most advances are being made today, with the development of very precise methods for injecting specific, spatially and temporally located sites in the embryo, even into single cells. Such experiments allow real cell lineage studies to be performed, and as discussed by Lumsden (1989), begin to allow the distinguishing of subsets of cells and "provide for the first time direct evidence that some neural crest cells are multipotential at the time of their initial migration." As Lumsden (1989) summarized, the elegant studies of Bronner-Fraser and Fraser (1988) have allowed "direct observation of cellular diversification from multipotent progenitors in the emergent crest and paves the way for detailed examination of the process offate restriction by marking cells ... at different points along their paths of migration." This type of investigation will be of special value in the trunk neural crest, and is in progress (M. M. Smith, A. G. S. Lumsden, and P. V. Thorogood) in relation to skeletogenesis in fishes, where somatic lateral plate mesenchymal cells make a major contribution to the skeleton, as will be reported in the subsection following the next one.

Teeth from Cranial Neural Crest

The role of cranial neural crest in inducing teeth and contributing cells to make dentine has been firmly established since the work of Adams (1924) on urodele amphibians. All subsequent work has been expertly reviewed by Gaunt and Miles (1967), Lumsden (1987), and Hall and Horstadius (1988).

It is known with some precision from the work of Chibon (1966) on amphibians that odontogenic potential is very precisely localized in cranial neural folds. These locations are craniorostral regions expressed as a series of 30° segments, starting from the medial axis and progressing through 180°. These segments covering 30-100° correlate with tooth formation. Cells from the 30-70° regions produce teeth of the upper jaws, cells of the 70-100° regions teeth of the lower jaws.


Direct evidence that the role of neural crest in tooth development is not just evocative, but includes the contribution of cells differentiating as tooth germs, was provided by the experiments of Chibon (1966, 1967) in which tritiated thymidine-labeled donor cranial neural crest cells appeared in odontoblasts and pulp cells of larvae with orthotopic grafts.

The significance of tissue interactions in odontogenesis was emphasized by Lumsden (1987), who cited the dual origin of the enamel organ from ectoderm or endoderm as the reason for de Beer (1947) suggesting that ectomesenchyme rather than epithelium provided the initial role in tooth induction. Later work, however, in particular the studies of Wagner (1949, 1955), Henzen (1957), and Lumsden (1987), suggested that stomodeal ectoderm and endoderm provided the first inductive signal for the initiation of tooth development, and that although the cranial neural crest has odontogenic competence in anurans, the signal is lacking from or suppressed in the ectoderm until metamorphosis. An important aspect of metamorphosis relates to the mechanisms controlling when signals are switched on and how such signals are modified in amphibians that have lost the tadpole stage, i.e., those demonstrating direct development (Hanken, 1986).

Some of the most interesting experiments are those of Wagner (1949, 1955) and Henzen (1957), who used reciprocal orthotopic grafts to produce chimaeric tooth germs in anuran and urodele larvae. Wagner (1955) found that urodele ectoderm would allow urodele-type teeth to form from anuran neural crest, the dental papilla forming from frog neural crest-derived cells. He concluded that stomatodeal ectoderm lacks the inductive signal in larval anurans, acquiring it after metamorphosis. In the reciprocal experiment, where urodele cranial neural crest was grafted orthotopically into anuran hosts, no teeth formed, but a urodele-type visceral skeleton developed. As well as supporting the conclusion that anuran cranial neural crest is competent to form teeth but does not receive inductive signals because ectoderm is deficient, this experiment also suggests that the permissive signal is generated by ectoderm, and that cartilage formation does not depend on the same inductive signal as does tooth formation.

Remarkably, it was not until much later that direct evidence for the neural crest contribution to teeth in mammals was provided by Lumsden's (1987) explant experiments. This study also demonstrated that oral epithelium provided the initial signal for tooth development. Because the boundary between ectoderm and endoderm in the oral cavity of mammals has not been precisely established, whether tooth initiation depends on endoderm or ectoderm cannot be established.

There are no data for the involvement of neural crest in odontogenesis in fishes. Langille and Hall (1988b), in their extirpation study of cranial neural crest in the Japanese medaka, demonstrated neural crest involvement in the


cartilaginous skeleton of the branchial region, but their study did not allow comments to be made on the origins of dermal bone or teeth. Sadaghiani and Vielkind (1990) commented on a positive reaction with HNK-l antibodies in the pharyngeal teeth of xiphophorids, but quite inexplicably they found a strong reaction in the epithelium of the dental organ and apparently not in the dental papilla.

The experiments on amphibian tissues using extirpation and transplantation, or recombinations as explants, provide information on the range of potential of cranial neural crest cells. As Lumsden (1987) commented, the use of explants addresses the following questions:

1. Is odontogenic ectomesenchyme committed at mid-neurula stage? 2. Or is it committed at a later time during migration? 3. Or after contact with pharyngeal endoderm? 4. Or after contact with stomodeal ectoderm?

Lumsden (1987) concluded that Avery's (1954) experiment in which cranial neural crest of Ambystoma mexicanum formed teeth in ectopic sites indicated that "a degree of committment had been acquired by the crest before migration," but he suggested that "this committment is not complete"; there is a requirement for interaction with epithelium before odontogenesis is effected. Such a requirement would be a permissive signal from epithelium to competent ectomesenchyme.

Sellman (1946) first explored the potential of cranial neural crest as heterotopic grafts to the trunk region of Ambystoma larvae. He found that teeth are not formed unless three tissues are present-odontogenic cranial neural crest, stomatodeal ectoderm, and pharyngeal endoderm. But the transplantation method leaves open the possibility of contact with mesodermal mesenchyme, either at the future site or with the explant. Wilde (1955) confirmed this tripartite dependence in an explant system using Ambystoma mexicanum, where he could be sure that mesodermally-derived mesenchyme was excluded. As with Sellman's study, teeth were only obtained if all three tissues were present in the explant.

Neither Sellman's nor Wilde's experiment answers the third and fourth questions posed by Lumsden (1987), viz. whether contact with pharyngeal endoderm or stomodeal ectoderm is required to commit ectomesenchyme for odontogenesis. This is because cranial neural crest cells cannot be traced sequentially in their interactions with pharangeal endoderm and stomatodeal ectoderm. Cassin and Capuron (1979) appear to have provided a solution to this problem, although their results differ from Sellman's and Wilde's. As well as obtaining teeth with all three tissues in both explant and blasocoel transplants, they obtained teeth with cranial neural crest and endoderm, but not with cranial neural crest and ectoderm. This would seem to implicate phar-


angeal endoderm rather than stomodeal ectoderm as the epithelium providing the initial signal for commitment of cranial neural crest to odontogenesis. Cassin and Capuron (1979) concluded that endoderm promotes the differentiation of cranial neural crest into both odontoblasts and chondroblasts, and that both can only differentiate from cranial neural crest after contact with pharangeal endoderm. Despite this dependence of cartilage and teeth on pharyngeal endoderm, cartilage and teeth develop independently of one another; development of bone required cartilage to be present.

Results from work in progress (A. Graveson, M. M. Smith, and B. K. Hall, in preparation) testing all combinations of tissues in explants of Ambystoma mexicanum without serum (which was used in all previous studies) confirm that teeth and cartilage will form when cranial neural crest (CNC), endoderm, and ectoderm are present in the explants; when endoderm and CNC are cocultured, but not when ectoderm alone is cocultured with CNC. These workers also confirmed that teeth and cartilage form independently of one another, although there is a close topographic relationship between some cartilages and teeth. Remarkably, spatial positioning seen in vivo was maintained in explants. A symmetry developed whereby rows of teeth opposed each other across an epithelially-lined space, with the oldest teeth on the outside of the explants (labiobuccal) and tooth buds on the inside (lingual). Bone of attachment formed after teeth had formed, but never before. Bone of attachment was in continuity with the dentine cone, linking teeth to one another and to the perichondrium of the cartilage. Membrane bone of the dentary or palate had not formed at this stage.

Trunk Neural Crest or Mesodermal Origins of Trunk Odontogenic and Skeletogenic Tissues

We have argued in previous sections that major questions in the development of the postcranial skeleton in fishes are still to be answered:

1. What is the embryonic origin of odontogenic and skeletogenic tissues in trunk and tail exoskeleton of all major groups of fossil fishes, all chondrichthyan fishes, and several species of extant osteichthyan fishes, including the coelacanth and armored catfishes?

2. Can it be demonstrated from experimental studies on lower vertebrates, including extant fishes, that the cell lineage of scleroblasts producing denticles (tubercles and dermal bone) is from trunk neural crest?

The data produced so far to answer these questions are negligible. There are some fascinating, general, nonexperimental, observations on dermal skel-


etons in fishes requiring explanation. Hall (1991) stated, "there is lack of critical data for key developmental events" and "the embryonic origin of mesenchymal elements offish fins would advance our knowledge of the evolution of connective tissue in higher vertebrates."

Until very recently there was no obvious reason to seek any skeletogenic capability of trunk neural crest cells. Classic studies on the neural crest origin ofskeletogenic and odontogenic tissues, performed largely, if not exclusively, on amphibian and avian embryos, established cranial but not trunk neural crest as skeletogenic in both avian and amphibian embryos [see above, and Le Douarin (1982), Hall and Horstadius (1988), and M. M. Smith and Hall (1990) for reviews]. Trunk neural crest has therefore long been regarded as neither skeletogenic nor odontogenic. Reasons for such dichotomous thinking concerning capabilities of cranial and trunk neural crest are many.

Most fate mapping studies demonstrate cranial neural crest to be skeletogenic (and odontogenic in toothed vertebrates), not that the trunk neural crest is not. As reviewed by Le Douarin (1982) and Hall and Horstadius (1988), many experimental studies in amphibians have shown that trunk neural crest transplanted into the head does not make cranial skeletal bone, cartilage, or teeth. Similarly, cranial neural crest transplanted into the trunk does not express its skeletogenic potential.

Evidence, therefore, for inability of trunk neural crest to form skeletal or dental tissues is negative and by exclusion, rather than positive and from direct demonstration. This remains so, even though several early studies specifically sought to address whether trunk neural crest was skeletogenic or odontogenic. They did so by grafting trunk neural crest into the cranial neural tube and failing to observe chondrogenesis, osteogenesis, or odontogenesis (Horstadius and Sellman, 1946; Sellman, 1946; Chibon, 1966). As has been demonstrated by Graveson (1993), however, failure to observe development of skeletal or dental tissues in such transplantation experiments need not necessarily mean trunk neural crest lacks these potentials. Heterotopic transplantation experiments are performed assuming grafted trunk neural crest cells undergo normal patterns of migration and interactions with cranial inducers. Graveson (1993) has shown this need not, and indeed often is not, the case; see p. 431.

None of the postcranial skeleton has been shown to be of neural crest origin in any vertebrates. An "exception" to this statement is cartilage nodules in the hearts of domestic fowl, shown to arise in mesenchyme derived from the cardiac neural crest. Cardiac neural crest cells migrate into the heartforming region to form some heart mesenchyme, conotruncal region, and cartilage. It is arguable whether this cartilage is part of the skeleton or ectopic. In either case, it is a skeletogenic tissue in the trunk of neural crest, albeit cranial neural crest, origin.


The postcranial endoskeleton arises from somitic or lateral plate mesoderm. Within the postcranial endoskeleton, axial and rib skeletons arise from somitic mesoderm, appendicular and girdle skeletons from lateral plate mesoderm (Hall, 1986, 1991; Hall and Horstadius, 1988). As documented by M. M. Smith and Hall (1990) and Hall (1991), there is little if any likelihood of any trunk endoskeleton being of neural crest origin. Whether trunk exoskeleton in vertebrates such as fishes is of neural crest origin is the issue.

In our earlier review (M. M. Smith and Hall, 1990), we did not discuss the developmental origin of the exoskeleton of either medial or paired fins in fishes because little is known, and what is stated is partly contradictory. Schaeffer (1977), although accepting that there is no experimental evidence for involvement of neural crest in the development of body scales and fin lepidotrichia, proposed, on the basis of similarity of tissue composition, that "ectomesenchyme is involved in the formation of all dermal calcifications." "Similarity of tissue compositions" may not be a good criterion, unless specific combinations of tissues are used, e.g., dentine and bone.

Although trunk endoskeleton is not of neural crest origin, connective tissue of the median fin folds in the amphibians Ambystoma mexicanum and Pleurodeles waltii is derived from trunk neural crest (Raven, 1931, 1936; Du Shane, 1935; Holtfreter, 1935; Detwiler, 1937; Chibon, 1966). Furthermore, median, unpaired fins are induced to form under an inductive influence arising from trunk neural crest (Du Shane, 1935). This lone study needs to be reinforced by further analyses.

Paired fins, on the other hand, arise following inductive influences either from trunk neural crest or lateral plate mesoderm, but do not contain neural crest-derived connective tissues (Balinsky, 1975) [see reviews in Schaeffer (1977), Thomson (1987), Hall (1991), and Table III]. The few studies in these areas also need to be augmented by additional experimental studies on additional species.

Both in his 1977 review on the dermal skeleton in fishes and in his 1987 review comparing early craniate development with other deuterostomes, Schaeffer (1977, 1987) suggested that different cell populations provide the initial inductive signal and form scleroblastic mesenchyme in paired and unpaired median fins. Somatopleural mesenchyme provides the cells in paired

TABLE III. The Relationship between Trunk Neural Crest and Median (Unpaired) and Lateral (Paired) Fins in Amphibians and Fishes

Fin type

Median unpaired Lateral paired

Neural crest induced

++++ ?

Contains trunk neural crest connective tissue

++++


fins, while neural crest-derived mesenchyme provides cells in unpaired, median fins. Schaeffer (1987) stated that "there is substantial evidence that the epithelial unpaired fin-fold is invaded by mesenchyme from the trunk neural crest in fishes," citing Terentiev (1941). The latter author had shown, in urodele amphibians, that trunk neural crest could induce dorsal fin folds to form from both trunk and head ectoderm, and concluded that both positional and morphogenetic information came from trunk neural crest.

Differences in inductive cell populations between median and paired fins are unexpected. All fins are thought to have been derived evolutionarily from continuous fin folds, and to develop using the same sequence of morphogenetic-histogenetic events (Hall, 1991; Thorogood, 1991). In sequence these would be, first, development of an apical epidermal ridge; second, formation of actinotrichia from ectoderm with which they retain contact; third, formation of lepidotrichia from mesenchyme as the first elements of the dermal skeleton, aligned by the actinotrichia (Wood, 1982; Schaeffer, 1987; Thorogood, 1991).

Schaeffer (1987) further believed that "the initial inductive signal for the paired appendages comes from somatopleural mesoderm" and that "the somatopleural mesenchyme interacts with the presumed ectomesenchymal actinotrichial substratum," both activities being necessary for the development of pectoral fins. Evidence for this comes from Wood (1982), who suggested, on the basis of an ultrastructural study, that the cells that migrate along actinotrichia (assumed to form from the ectoderm) in developing teleost pectoral fins arise from adjacent somatopleural mesenchyme. The only experimental work bearing on this is a paper by Lopashov (1950), who showed that lateral plate mesoderm when transplanted to the abdominal wall of teleost fishes induced ectoderm to form fin folds.

There is no evidence that ectomesenchyme makes a contribution to either actinotrichia or lepidotrichia during fin development. Experimental work in early stages ofa project by P. V. Thorogood, M. M. Smith, D. Amanzee, and A. G. S. Lumsden to determine the lineage of scleroblasts in caudal and pectoral fins ofthe zebrafish appears to show that some early-migrating cells, aligned along actinotrichia in the caudal fin, originate from neural crest, but their differentiative fate has yet to be demonstrated.

In a study of the fine structure of the pectoral fin dermoskeleton in two quite primitive osteichthyan fishes, Polypterus and Calamoichthyes, Geraudie (1988) described ganoine-covered lepidotrichia, which she concluded (despite lack of dentine) implied a role for epithelium in producing an enamel-like part to this cover. She further commented that absence of dentine in pectoral fins but its presence in caudal fins "could be related to the disappearance of neural crest cells with skeletogenic fate within the fish dermis during evolution." This comment was based on the presumption that only mesodermal cells formed osteoblasts of lepidotrichia, no data being available for an ec-


tomesenchymal origin. Geraudie (1988) concluded that although dermoskeleton, scales, and lepidotrichia are the result of "dermoepithelial interaction," the component covered by ganoine was either mesodermal or ectomesenchymal in origin.

Le Douarin (1982), in her review of the source of mesenchymal cells from neural crest, commented that, "Morphogenesis of the dorsal fin (in amphibians and fishes) is the result of tissue interactions between ectomesenchyme (trunk origin) and dorsal ectoderm." Citing the work of Twitty and Bodenstein (1941) and Bodenstein (1952), she concluded that both mediodorsal and flank ectoderm in amphibians was competent to participate in fin formation, while admitting that "in fishes, there has been no systematic study of the migration and differentiation of neural crest cells."

Most early transplant work also showed that interaction with pharyngeal endoderm was necessary for skeletal differentiation to occur. This has been confirmed by recent explant studies in which actual cellular contact between ectomesenchyme and pharyngeal endoderm was a prerequisite for transmission of inductive signals (Epperlein and Lehmann, 1975).

Since a study by Lamers et al. (1981) of the migration routes of trunk neural crest cells in fishes, recent papers have documented both timing and pathways of this migration in the chick (Serbedzija et al., 1989) and in teleost fishes (Sadaghiani and Vielkind, 1990). In vital dye studies of these premigratory and migratory trunk neural crest cells in the chick, Bronner-Fraser and Fraser (1988, 1991) and Fraser and Bronner-Fraser (1991) concluded that "trunk neural crest cells are not restricted in developmental potential," but "have the capacity to develop into a wide variety of both neuronal and non-neuronal phenotypes." Their work is encapsulated by their interpretation that "the majority of premigratory and migrating neural crest cells appear to be multipotent," but "there may be minority populations of predetermined cells." There is very little information on when these cells become committed or of the factors influencing phenotypic selection.

Scarcity of data on migration, commitment, and selection of phenotype of trunk neural crest cells leaves the question of their skeletogenic potential in fish entirely open, with the corollary that research should be directed toward migration and differentiation of neural crest cells in fishes, where it could be predicted that induction of, and commitment to, exoskeletal skeletogenesis would have been evolutionarily preserved. Such work is in progress.

SUMMARY OF POSTULATED MECHANISMS OF SKElETOGENESIS

We provide two summaries in this section, a summary of the data and postulates presented and an overview of the developmental model for the


evolution of exoskeleton and teeth. The data discussed can be summarized under 12 headings.

1. The exoskeleton includes in its entirety denticles, scales, armor (dorsal and ventral plates, branchial plates, etc.), fin lepidotrichia, and teeth, in agnathan and gnathostome fishes.

2. The dermal exoskeleton is made up of alternative combinations of three basic layers: enamel, dentine, bone of attachment; overlying cellular or acellular spongy bone; overlying cellular or acellular compact bone.

3. The exoskeleton is a neural crest derivative wherever it forms along the body.

4. Evidence for a skeletogenic and odontogenic cranial neural crest, well established from experimental analyses, is reviewed. Trunk neural crest has in the past been taken as both nonskeletogenic and non odontogenic. This presumption is challenged in this review.

S. The neural crest versus mesodermal origins ofskeletogenic and odontogenic mesenchyme from which exoskeletal and endoskeletal tissues arise is discussed.

6. Activation of skeletogenic and odontogenic mesenchyme by epithelialmesenchymal inductive tissue interactions is discussed, as is the source of the patterning.

7. The odontode is defined as the "single morphogenetic modifiable system" envisaged by Schaeffer and as a fundamental differentiative and morphogenetic unit of the exoskeleton by Reif. Within the odontode develop enamel, dentine, bone of attachment, and pulp.

8. Osteogenic condensations are identified as the second fundamental differentiative and morphogenetic unit of the exoskeleton. From such condensations comes the basal bone to which exoskeletal scales, denticles, or teeth are attached, and from which come the lepidotrichia.

9. Individual cell populations can be recognized within odontogenic condensations and osteogenic populations. These are discussed, using the mammalian mandible as the model of a neural crest-derived dermal bone. Segregation of odontogenic from skeletogenic cell populations is discussed and related to the apparent dichotomy between cranial and trunk neural crest.

10. Molecular control mechanisms responsible for initiation of differentiation and patterning of exoskeletal elements are identified and discussed. The important role played by homeotic genes is emphasised; evidence that homeotic genes pattern skeletal elements along the anterior-posterior (rostrocaudal) body axis is discussed. Extracellular matrix molecules such as tenascin, cell surface molecules such as syndecan, and growth factors are iden-


tified and discussed as three classes of molecules regulating differentiation of cells within odontodes and condensations.

11. The evolutionary origins of exoskeletal tissues are evaluated especially in the light of recent studies on odontogenic and skeletogenic products present within conodonts and the placement of conodonts within the vertebrate lineage.

12. Experimental embryological evidence on the role and patterning of the neural crest in skeletogenesis and odontogenesis in fishes is evaluated and discussed in relation to mesodermal production of mesenchyme, including mesenchymal derivatives in fins.

The developmental model that emerges from this analysis is hierarchical and combinatorial. Exoskeletal tissues arise in populations of neural crest cells that are patterned through nonuniform expression of home otic and patterning genes. Ectoderm and mesoderm are also regionally patterned. Anteroposterior (rostrocaudal) patterning acquired in the neural tube is retained as neural crest-derived cells migrate throughout the body. Further positional information is imposed on neural crest cell populations as they interact with epithelia, which are responsible for initiation of cytodifferentiation. Cell surface, extracellular matrical, and growth factor molecules provide a combinatorial temporally and spatially regulated code utilized by neural crest-derived skeletogenic and odontogenic cells to segregate into subpopulations, differentiate, and initiate patterned morphogenesis. Potential for modulation of this code, especially through heterochrony, is discussed.

Boundary positions are established by homeotic and patterning genes. That these boundaries can be experimentally modified by manipulation of domains of expression of Hox and Pax genes suggests that establishment of boundaries by combinations of such genes is the mechanism that established exoskeletal and tooth patterning during vertebrate evolution.

The most parsimonious interpretation of the fossil skeletal and experimental embryological evidence is that initially in some basal vertebrate group, both cranial and trunk neural crest cells were skeletogenic and odontogenic. With the loss of exoskeletal denticles/scales, this capability has been retained as an odontogenic and skeletogenic cranial neural crest, but lost from the caudal trunk neural crest. Toothless vertebrates have lost the cranial odontogenic capability, but retained the skeletal potential. The separation of odontogenic and skeletogenic potential is further emphasized by the experimental observations on recombined explants that in mice and amphibians the most rostral trunk segment is odontogenic, but not chondrogenic. It is anticipated that fishes with trunk exoskeletal elements, such as dermal denticles, will possess an odontogenic trunk neural crest that is more extensively rostrally


and/or an odontogenic cranial neural crest that is more extensive caudally. In animals such as armored catfishes and all cartilaginous fishes, the entire rostrocaudallength of the trunk neural crest would be expected to be odontogenic as a retention of the primitive vertebrate condition. More fundamentally in cartilaginous fishes, the inability to make exoskeletal bone but the widespread ability to make teeth (denticles) would further reinforce the separation of osteogenic (missing from cranial and trunk neural crest) and odontogenic potential of neural crest.

The evolutionary interpretation of this difference in distribution of potential skeletogenic and odontogenic developmental mechanisms can be either that "odontodes only," is the primitive condition for vertebrates, accepting conodonts without an exoskeleton but with tooth elements as a stem-group vertebrate, or that it is derived, because heterostracans represent the basal vertebrate group with an extensive bony armor.

NOTE ADDED IN PROOF. Van der Bruggen and Janvier (1993) have just published some findings on teeth in agnathans. They report preliminary information on the finding of denticles in the pharyngeal cavity of a thelodont agnathan, Loganellia, and say that they resemble pharyngeal dermal elements of jawed vertebrates and are the first record of these in any jawless vertebrate. These observations provide data to support our suggestion that pharyngeal denticles evolved before marginal teeth on the jaws, and, in conodonts, existed without any dermal denticles covering the body. The dependent link between odontogenic cranial neural crest and endoderm in tooth development in amphibians and mammals may have been an early developmental mechanism in the evolution of teeth, where the pharynx with cartilage bars as support would have been lined by endoderm.

ACKNOWLEDGMENTS

We wish to thank Jim Hanken for critical reading of an early draft, for which we are much indebted; and Peter Thorogood for comments on the final version. M.M.S. thanks in particular the Nuffield Foundation for a Science Travel Grant, NERC for a project grant (GR3/8543) on biomineralization in conodonts and early vertebrates, and the Royal Society for a Travel Grant; for discussions, Andrew Lumsden, Anthony Graham, Paul Smith, Ivan Sansom, Richard Cloutier; and for editorial work with the references, Annabelle Hickman. B.K.H. thanks the I. W. Killam Trust, NSERC of Canada, and, for discussions, Bill Atchley; both B.K.H. and M.M.S. thank Ann Graveson,


Tom Miyake, and Steve Smith for discussions. M.M.S. is most grateful to Mark Purnell for copies of his drawings and for permission to publish them, and to him, Dick Aldridge, and Paul Smith for access to papers in press and permission to quote from them. We thank Carl Gans, whose comments on the "conodont story" were available to us prior to their publication. We have not discussed them in this review and await their eventual publication.

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evolutionary biology || a developmental model for evolution of the vertebrate exoskeleton and teeth

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