inductive interactions in early amphibian development

Inductive interactions in early amphibian development

H.V. New and J.C. Smith

National Institute for Medical Research, London, UK

Current Opinion in Cell Biology 1990, 2969-974

Introduction

In this review, we examine progress made since the previous review in this journal [l] in our understanding of the sequence of inductive interactions responsible for es- tablishing the body plan of the early amphibian embryo. Most studies on inductive Interactions in amphibia have concentrated on the South African clawed frog Xenopus kmvis and so we have restricted almost all discussion to these. We also consider how recent results may shed light on inductive interactions in amniotes, which, until recently, have proved difficult to analyse.

The first Inductive interactions during early development result in the generation of the mesoderm followed by the neurectoderm. In each case, induction involves the formation of characteristic tissue types as well as their regionalization, although these processes are often consid- ered separately. For mesodenn formation, it is still convenient to discuss much of the work in the framework of the ‘three-signal model (see [l], and Fig 2 therein). Brie& it appears that at the early blastula stage the Xeno pus embryo consists of only two cell types: presumptive ectodenn in the so-called ‘animal’ hemisphere and presumptive endoderm in the ‘vegetal’ hemisphere. In addition, there is, in the vegetal hemisphere, a dorsoventral polarity that is dehned 45-90 min after fertilization by the direction of rotation of the cortex of the egg relative to its cytoplasmic core. This polarity results in the side of the embryo on which the sperm entered becoming ventral and the opposite side becoming dorsal (raiewed in Ml.

During blastula stages, signals from the vegetal hemisphere of the embryo act on overlying equatorial cells to induce them to become mesoderm. There seem to be at least two types of mesoderm-inducing signal. One, em- anating from the dorsal vegetal quadrant (opposite the sperm-entry point), induces dorsal mesoderm such as muscle and notochord. The other, derived from a larger ventral vegetal sector, induces predominantly ventral tissue types such as blood and mesothelium.

The large region of ventml mesoderm resulting from the latter signal becomes subdivided by the third signal of the three-signal model. This is produced by the newly induced dorsal mesoderm: Spemann’s famous ‘organizer’. This third interaction has been called ‘dorsalization’, and

results, for example, in some of the ventral mesoderm becoming specified to form kidney while other regions become muscle. Dorsalization begins at the early gastrula stage, when the mesoderm is also believed to start acquir- ing anteroposterior patterning.

Neural induction, the second major inductive interaction, also takes place at gastrula stages. It occurs through the iniluence of dorsal mesoderm on dorsal ectoderm, and involves the respecification of ectoderm to become neurectoderm, together with the imposition of anteroposterior pattern. The traditional view of neural induction has been that the mesodermal signal acts mdially during gastrulation, the involuting dorsal mesodermal cells acting on the overlying layer of ectodermal cells iirst to induce anterior neurectoderm and then to posteriorize it (reviewed by Nieuwkoop et al In i%e Epigenetic Nu- ture of Early chordate Devekpzent. Cambridge Univer- sity Press, 1985, pp 150-179). Recent results, however, in- dicate that neural induction may start even before the onset of gastrulation and that it can occur tangentially within a tissue layer [3] (see [l] for review; but also, see [4]).

The question of regional specification is central to an understanding of inductive interactions in early development. The above summary of the inductive events in early amphibian development referred, for example, to the specification of dorsal and ventral mesoderm through the action of distinct mesoderm-inducing signals. This ls a convenient description of the differentiated cell types that are induced by each signal, but a major difllculty is that these cell types are usually assessed many hours, if not some days, after the initial interaction. In the case of mesoderm induction, this is also after major cell move- ments have taken place. There is thus the opportunity for additional inductive interactions to occur between induced cells so that the linal outcome may reveal little about the result of the initial interaction. Furthermore, although difTerentiated cell types such as muscle and notochord provide useful indications of initial dorsoventral position, they do not define anteroposterior position. It is important, therefore, to discover early markers of position in the embryo, particularly markeis of anteroposterior position, and to establish how their expression is controlled.

Much work this year has concentrated on these ques- tions, making use of known and newly discovered

Abbreviations bFGF--basic FCF; FGHbroblast growth factor; F!W-follicle-stimulating hormone; MMesoderm-inducing factor;

TCMransforming growth factor.

@ Current Biology Ud ISSN 0955674 %!

970 Cell differentiation

‘mesoderm-inducing factors’ as well as the posterioriz- ing effect of retinoic acid previously discussed [l]. Few markers of anteroposterior position in the mesoderm have been described and so our discussion of this axis is largely restricted to the neurectoderm.

Mesoderm induction

Mesoderm-inducing factors A major iinding in the last year has been that activin is a potent mesoderm-inducing factor and that many of the inducing factors previously studied are either activin itself or activin homologues [5-91. These include XTC-mesoderm-inducing factor (MIF) XTCMIF, made by the XenopusXK cell line (Smith, Development 1987, 99:314), and WEHIMIF, secreted by WEHI- monomye- kxytic leukemia cells (Godsave et al, Development 1988, 1023555566). ‘PIF’, a mesoderm-inducing factor made by the macrophage cell line P388Dl [lo] has also been shown to be an activin [9].

Activins are members of the transforming growth factor (TGF)j3 superfamily, originally identified by their effects on the release of follicle-stimulating hormone (FSH) and on erythroid differentiation (for review, see Ling et al, Vit Harm 1988, %:I-46). They consist of dimers of the in- hibii p chain, either pApA (activin A), pApB (activin AB), or j&h (activin B). Although all three combinations of f3 chain are active [ 111, activin B has not yet been isolated from natural sources. The actions of activin in FSH release and erythroid differentiation assays are antago- nixed by the inhibins, heterodimers consisting of one of the above- mentioned j3 chains and a larger a chain. However, inhibin does not inhibit the action of activin in mesoderm induction [ 51.

It is not yet clear whether activin is present in the early Xen@us embryo. Preliminary studies mentioned by van den Eijnden-Van Raaij et al [7] suggest that the Xenopus blastula contains FSH-releasing activity, but recent work by Melton and colleagues [9] indicates that activln B is not expressed until the late blastula stage and activin A is only expressed from the late gastrula stage. The earlier expression of activin B causes these authors to suggest that this may be the natural inducer. However, even this activin B expression is rather late; vegetal tissue is capable of signalling at least 3 h earlier than the late-blastula stage (Jones and Woodland, Devek@aent 1987,101:557-563) and most aspects of mesoderm induction are likely to be complete by the time activin B transcription begins. The important question, therefore, is whether there is mRNA for a novel member of the activin family present in the early embryo or, alternatively, whether there is a store of maternal activin protein synthesized during oogenesis. This is being investigated in our laboratory and in others.

Members of the iibroblast growth factor (FGF) family are also believed to have a role in mesoderm induction (see [ll). Basic FGF (bFGF) is present in the egg and early embryo, although its distribution is not known, and FGF receptor protein is detectable throughout the embryo.

However, the role of bFGF in mesoderm induction is not clear, because the molecule lacks a secretion signal sequence. Nonetheless, it is possible that other members of the FGF family, such as kFGF [ 121, are the natural inducing signals. We can expect more information on this in the coming year.

Mesodermal patterning Until recently, studies of mesodermal patterning have been hampered by a lack of markers. Despite the in- herent difficulties (see above), it has been necessary to use differentiated cell types as markers of dorsoven- u-al position while for the anteroposterior axis, the only marker which is exclusive to mesodenn is X&OX-~, expression of which is limited to posterior regions of the early embryo (Ruiz i Altaba and Melton, Development 1989,106:173183). Because there are so few anteropos- tenor mesodermal markers, and because regionalization of mesoderm and neuroectoderm along this axis appear to be closely linked, anteroposterior patterning will be discussed with neural induction. Here we concentrate on the dorsoventral axis. Previous work, discussed in [ 11, has indicated that the mesoderm-inducing signal produced by dorsovegetal blastomeres may be represented by a factor such as XTC- MIF (now known to be a homologue of activin) and that the ventrovegetal signal may be an FGF family member. This view has been reinforced by the quantitative study of Green et al. [ 131, who have compared the effects of different concentrations of XTC-MIF and bFGF on animal pole tissue. They found that higher concentrations of XTC- MIF tended to induce dorsal mesodermal cell types while lower concentrations resulted in the formation of ventral cell types similar to those seen in response to FGF.

This observation raised the possibility that XTC-MIF, or a similar protein, diffuses away from the vegetal hemisphere to form a concentration gradient. Responding cells in the animal hemisphere would then respond to diiferent concentrations of factor according to a series of thresholds (see [14]). The thresholds revealed in the original study of Green et al, were not sharp, at least partly because the animal caps used in the study were several cell layers thick, and not all the cells experienced the same concentration of factor. In later work, however, animal pole cells were dispersed during a 1 h expo- sure to XTC-MIF, and then reaggregated [ 151. This proce- dure revealed remarkably sharp thresholds. For example, a twofold increase in concentration is sufficient to divert cells from expressing an epidermal keratin gene to maxi- mal expression of muscle-specihc actin. A further twofold increase causes actin expression to be switched off and expression of a neural- inducing factor to be switched on. It is possible that threshold responses of this sort underlie pattern formation in the early embryo.

Muscle-specific actin expression is a rather late marker of muscle differentiation, only switching on during gastrulation (Gurdon et al, Cell 1985, 41:913-922). It will therefore be interesting to investigate threshold responses of

inductive interactions in early amphibian development New and Smith 971

earlier markers of muscle differentiation such as MyoD [161, myogenin (Wright eta& Cefl1989,56:607-617) and Myf-5 (Braun et al, EMBO J 1989, 8:701-709). The prediction might be that these respond to the same thresholds as actin as they are expressed in the same cell type. In addition it will be important to investigate other recently discovered mesodermal markers such as Xenopus twist, which is expressed in the notochord and lateral plate but not in the myotome [ 171, Xenopus snail, which is initially expressed in the dorsal mesoderm but then spreads laterally and ventrally [18], as well as Xbox3, the sole posterior mesodermal marker. It might then be possible to correlate the position of expression of each marker with the factor concentration required for its expression.

Any model in which mesodermal patterning is specified by a gradient of an inducing factor secreted from the vegetal hemisphere must include the recent results of Cooke [19], who used grafting techniques to change the avail- able extent of competent responding tissue in the animal hemisphere. The number of cells allocated to mesoderm in these experiments depended on the number of responsive cells. Such a result would not be predicted by a simple diffusion model. Rather, it is consistent with a system in which size regulation is achieved by interaction between an ‘activator’ and an ‘inhibitor’, such as that described by Meinbardt [ 201. Interestingly, there is some evidence for the existence of an inhibitor of mesoderm induction in the Xenopus embryo (Cooke et al, Devel- qment 1987, 101:89%908; Cooke and Smith, Da, Biol 1989, 131:383-400).

Neural induction

Little is known about the identity of neural-inducing factors. One ditficulty in designing an assay for such molecules, as previously discussed [ 11, is that more than one factor may be required. Another is that neural tissue is frequently formed by animal pole explants in response to MIPS. This is almost certainly the result of neural-inducing signals being formed by newly induced mesodermal cells, which then act on residual ectodermal blastomeres, rather than the direct effect of the MIFs themselves [ 151. The most dramatic example of this phenomenon may be the demonstration by Melton and colleagues [9,10] that activin (both as the pure molecule and as ‘PIF’) is able to induce abundant neural tissue, including eyes and neural tubes, from animal caps in vitro. The so-called ‘embry- aids’ thus formed even appear to have an anteroposterior axis. Interestingly, XTC-MIF, although capable of inducing abundant neural tissue from responding animal caps, has not been observed to induce ‘embryoids’ [ 131. The reason for this difference is not clear, but it may reflect relatively trivial differences in experimental tech- nique, such as the size of animal caps dissected.

Although little is known about the identity of the neural- inducing factors themselves, their mechanism of action, via second messengers, is being examined in some detail _. -

(reviewed in [ 11). Recent results from Otte et al [21] have demonstrated that several proteins become strongly phosphorylated after neural induction and that levels of inositol phosphates also become elevated. Their results also suggest that there are different isozymes of protein kinase C in the animal and vegetal hemispheres of the embryo. Classic work indicated that neural induction occurred ex- clusively through the radial route, with an initial induction of anterior neurectoderm followed by progressive posteriorization under the influence of the underlying mesoderm (reviewed by Nieuwkoop et al, 1985 and in [l]). This hypothesis has been supported by use of regional markers of neural development such as MF3 and XlEibox G [ 41. However, as discussed in the Introduction and previously [ 11, there is now evidence for tangential signalling in neural Induction, as well as radial signalling. Ruiz i Altaba [ 31 has confirmed the existence of tangential signalling by showing expression of xhox3 in embryos which have exogastrulated and thus have no radial contact between mesoderm and neurectoderm (at the stages studied, X%x3 is no longer expressed in the posterior mesoderm and instead acts as a marker of anterior neural tissue). The most important aspect of these results is that xhox3 expression was restricted to prospective anterior neurectoderm, suggesting for the first time that regionalization as well as induction can occur through the tangential route. However, these results contrast with those obtained by Sharpe and Gurdon [4] using explants of organizer tissue in lateral contact with dorsal ectoderm. Using XrF3 and XlHboxGas markers, these authors were unable to find evidence of sign&ant tangential induction. It is not clear why these results should differ from those of Ruiz i Altaba and the earlier report of Dixon and Kint- ner (Development 1989, 106:749- 757; reviewed in [l]). One possibility is that there are subtle differences ln dis- section; another is that different neural markers give different results. For example, tangential neural induction may activate NCAM and A%ax3 but not XIF3 or Xm Overall, although it seems likely that signals for neural In- duction can pass in both radial and tangential directions, the relative importance of the two routes remains unclear.

Anteroposterior regionalization

We will now consider anteroposterior axis formation, comprising regionallzation of both the mesodenn and the neurectoderm. It is convenient to consider these together because there are few anteroposterlor markers exclusive to the mesoderm, and patterning of the neurectoderm is believed to reflect that ln the mesoderm. Indeed, many markers of anteroposterlor position label both mesodenn and neural tissue. For example, the Xen@us homeobox gene x1;Hboxl labels neatly aligned bands of mesoderm, neural tube, and neural crest in the anterior of tailbud stage embryos (see 1221). ‘C. We have previously suggested, on the basis of their abil- ities to induce the posterior marker Xbax3, that XTC-


MIF and bFGF might more accurately be described as inducing anterodorsal and posteroventral mesoderm, re- spectively [ 11. This proposal has been supported and extended by the work of Cho and de Robertis [ 231 in which they studied the expression of two additional homeobox genes, XlHbaxl and XlHbox6 XlHboxl is expressed in anterior mesoderm and neural tissues [22], and is preferentially activated by XTC-MIF, whereas XiHboxG is expressed in posterior neural tube and lateral plate mesoderm (Sharpe et al, cell 1987, 50:749758) [24] and is preferentially induced by bFGF. Interestingly, there is evidence that as well as being a useful regional marker of induction, X&box2 may be centrally involved in setting up regional pattern. When antibodies to the XlHboxl protein are injected into fertilized Xenqpuseggs, the resulting embryos show a transformation of anterior spinal cord into a hindbrain-like structure, a change reminiscent of the homeotic mutations seen in Draqbih [25].

In an alternative approach to studying anteroposterior pattern formation, mRNA encoding murine i&l, a proto- oncogene related to the Dmsophih segment polarity gene wingless (Rijsewijk et al, Cell 1987, 50:64%57), has been microinjected into fertilized Xenoptcs eggs [ 261. This resulted in the duplication of the anterior embryonic axis, a phenotype not dissimilar to that observed when mRNA for activin is injected into ventral blastomeres [9]. Although Xenopus int-1 has been detected at the early neurula stages of normal embryos (Nordermeer et al, Nucleic Acids Res 1989, 17:11-18), its distribution is not known. At present, therefore, it is difficult to assess the significance of this result in terms of the role of i&l in normal embryonic axis formation.

Retinoic acid, which has been postulated to be a mor- phogen in the chick limb bud (Thaller and Eichele, Nu- ture 1987, 327:625-628), may have a role in specifying the anteroposterior axis of amphibian embryos. Xeno pus embryos cultured in the presence of retinoic acid show morphological transformation of anterior neural structures to posterior, without a reduction in the to- tal amount of neural tissue; furthermore, retinoic acid is known to be present in the early Xenopus embryo (see [l]). Recent experiments have extended this work to study the expression of various neural markers. These include the hindbrain marker mF3 and the more posterior marker xZ~, both of whose expression is in- creased by retinoic acid [ 23,271. Expression of engruik~ which marks the midbmin/hindbrain boundary, is lost at high concentrations of retinoic acid. However, at lower concentrations, where posteriorization is less dramatic, its expression persists despite the loss of structures anterior to its expression domain [27].

The above studies are consistent with the suggestion that retinoic acid is the signal responsible for the progressive posteriorization of the neural plam that occurs during gastrulation. It is also possible, however, that retinoic acid affects patterning in the anteroposterior axis of the mesoderm, and that its e&t on neural tissue is indirect. In this context, it is intriguing that Sive et al [27] lind that heart formation is inhibited by retinoic acid, and it

will be interesting to see what the effect of retinoic acid on expression of X%& is. Further work is obviously required to discover the primary target of retinoic acid in the control of anteroposterior pattern formation and to study the roles of retinoic acid receptors and cytoplasmic binding proteins.

Induction in amniotes

One prediction made in the previous review in this journal [l] was that the lessons learnt from amphibia might begin to be applied to amniotes. This has been the case. Albano et al. [8] purified a factor from the mouse WEHI- 3 cell line that was capable of inducing mesodem-r from Xenqm.s animal pole tissue. This proved to be activin A, and the study of the expression of the activins during early mouse development is now in progress.

In the chick, Mitrani et al. [28,29] have shown that activin, but not bFGF, can induce dorsal axial structures such as notochord and muscle from isolated epiblasts. They also Iind that activin B is transcribed at the appro- priate stages of chick development, with higher levels in the hypoblast. Cooke [30] has also studied mesoderm induction in birds. Using three ditferent assays, he finds that bFGF, as well as activin, is a potent inducer. As his as says do not distinguish between induction of dorsal axial and non-axial mesoderm, it may be that, as in [email protected], activin induces the former and bFGF the latter.

These results must now be combined with those of Stem and Canning [31], who have shown that the epiblast of the chick embryo contains two randomly mixed cell populations during the period when mesoderm induction is assumed to occur. One population is recognized by the monoclonal antibody HNK-1 and it is these cells which eventually form the mesodenn and endoderm of the embryo. How might this randomly distributed population of presumptive mesoderm cells arise through induction, as suggested by the results of Mitrani [28,29] and Cooke [30]? And how do the HNK-l-positive cells sort out from their neighbours to form the mimitive streak? The next year should be even more exciting than the last.

Acknowledgement

We are grateful to J Green for his helpful comments.

Annotated references and recommended reading

l Of interest l e Of outstanding interest

1. Sm C: Induction and early amphibii development. Cw-r l e @in Cell Bid 1989, 1:&l-1070. Review of early amphibian development preceding the present one.

Inductive interactions in early amphibian development New and Smith 973

2. GERHART J, D~HIK M, DONL+CH T, ROBERTS S, RO~NING l B, STRVART R: Cortical rotation of the Xenopus egg: con-

sequences for the anteroposterior pattern of embryonic dorsal development. Develqbmenr 1989, (suppl):37-51.

Review of events that set up the dorsoventrai axis following sperm entry, and of the effect of inhibiting gastruiation on anteroposterior regional- h&on.

3. RUZ I ALTAEL~ A Neural expression of the Xenopus home- *. obox gene Khox3: evidence for a patterning neural sig

nal that spreads through the ectoderm. Deuelqment 1990, 108:595604.

The expression of the Xenqpuci homeobox gene xbax3 was used to look for evidence of tangential spread of a neural regionalizing inilu- ewe. In exogasttulated embryos, X%ox.3wa.s expressed in neural tissues at an equivalent position to that in controls, suggesting that a tangential spread of neural induction could have occurred.

4. SHARPE CR, GUIDON JB: The induction of anterior and pos l terior neural genes in Xenopus laevis Development 1990,

109:76%774. Regional markers of neural induction were used to show that different regions of mesoderm can induce diiferent regions of neurectoderm. No evidence was found of tangentiai spread of inducing factor from dorsal iip mesodenn through neurectoderm.

5. ASASH~MA M. NAKANO H, Stnht~~~ K, KINOSW~A K. BHII K, SHIJLU l e H. UENO N: Mescdermal induction in early amphibii em-

bryos by activin A (erythroid differentiation factor). Routs Arch Dev Bid 1990, 198:33&335.

Activin A was shown to have a mesoderm-inducing effect on explants of Xerrqpus animai pole ceils.

6. Shuru JC, PRICE BMJ. VAN NIMMEN K, HUYLEBROECK D: Iden- l e tilication of a potent Xenopus mesoderm inducing factor

as a homologue of activin A Nature 1990, 345:72?+731. XTC-MIF was identified as a homologue of activin A by virtue of ten identical N-terminal amino acids and its FSH release and elythroid ti ferentiation activities.

7. VAN DEN E~JNDEN-VAN RLUJ AJM. VAN ZOEIENT En, VAN NAN l e K, KOSIZR CH, SNOEK GT, DURSTON AJ, HIJ~IEEIROECK D:

Activin-like factor from a Xen0pu.s l0euf.s cell he responsible for mesoderm induction. Nature 1998, 347:732- 734.

XTC medium and act&in A were compared for mesoderm-inducing, FSH-releasing, and erythroid diiferentiation activities, and were found to have simiiar eifects. In addition, activin-iike activity was found in blast& embiyos.

8. AIBANO RM, GODSAVE SF, HUY~EBROECK D. VAN NIMMEN K, l BAAED HV, SIACK JMW, Shtn~ JC: A mesoderm-inducing factor

produced by WEHl-3 murine myelocytic leukaetnia cells is aetivin A. DeveQnnent 1990, 110:435+3.

WEHI-MlF was purified and found to be a homologue of mammalian activin A by virtue of the first seventeen amino acids being identicai. It was also found to have FSH- releasing and erythroid diiyerentiation activities.

9. THOMSEN G, Woolf T, Wm M, SOKOL S, VAUGHAN J, VALE l e W, MELTON DA Activins are expressed early in Xenopus em-

bryogenesis and can induce axial mesoderm and anterior structures. CelJ 1990,63:485493.

PIF (see [lo]) was identified as a homologue of activin A by antibody ctossreacdvity. Expression of the activin &, chain was seen from late gastrula stages, and the 8A chain from late biastub stages. In addition, injections of 8A or & chain mRN.4 into a ventrai blastomere of a 32.celi embryo resulted in formation of a second body axis.

10. SOKOL S. WONG GG. MELTON DA: A mouse macrophage fat- l tor induces head structures and organizes a body axis in

Xenofw.s Science 1990, 249561-564. A mesodenn-inducing factor, PIF, was identified and found to be capable of inducing ‘embryoids’ with eyes, brains, and axial structures.

11. M&KIN AJ, BERKEMEIER LM, S~HMEIZER CH, Scuw,ut RH: Activin l B: precursor sequences, genotnic structure and in ufttu

act.ivities. Md a&&mind 1989, 31352-1358.

Mammalian ceiis were transfected with an act&in &, expression piasmid and secreted an activin B homodimer with similar activity to activin A

12. PATERNO GD, GIUESPIE U, DIJCON MS, SUCK JMW, HEATH l JK Mesodenn-inducing properties of INT-2 and kFGF: two

oncogene-encoded growth factors related to FGF. Develqb merit 1989, H&79-83.

The kFGF and EVf-2 proteins were found to have mescderm-inducing activities.

13. GREEN JBA, HOGS G, SVMES K. COOKE J, Sham J: The bi- l ological effects of XI%-MIF: quantitative comparison with

Xenopus bFGF. Dtwekpment 1990, 108:173183. The mesoderm-inducing elfects of KTC-MIF and bFGF were compared: depending on the concentration, XTC-MIF induced more dorsal tissue, whereas bFGF was oniy able to induce ventrai tissue. In addition, there was a diiference between the factors in the timing of loss of competence of tissue to respond to their effect.

14. WO~PERT L Positional information revisited Lkzve@nzent l 1989, (suppl):312. A review including discussion of possible mechanisms of positional sig- nauing.

15. GREEN JBA, Sumi JC: Graded changes in dose of a Xenopus l e activin A homologue elicit stepwise transitions in embry-

onic cell fate. Nutire 1990, 347:391-3%. Dissociated animai pole celis were exposed to diiferent concentrations of XTCMIF. It was found that actin expression was related to sharp concentration thresholds, with two dilferent thresholds deKning three ceii states. In addition, high concentrations of factor induced a population of ceils themselves capable of inducing neural tissue.

16. HOPWOOD ND, PLUCK A, GURDON JB: MyoD expression in l the forming sotnites is an early response to mesoderm in-

duction in Xenopus embryos. EMBO J 1989, 8:340%3417. The Xenopur homologue of mouse MyoD was cloned. It was shown to require mesoderm induction for its expression, which was restricted to mesodenn in gastmiae and to somites at later stages.

17. HOF?!QOD ND, PLLICK A, GURWN JB: A Xenopus mRNA l related to Drosopbilu twist is expressed in response to

induction in the mesoderm and neural crest. Cell 1989, 59:893 903.

The Xeurhomologue of DmsqWh lwistwas cloned. It is expressed from the eariy gastrula stage in notochotd and in ventral and lateral mesoderm but not in the myotome. This early phase of expression re- quires mesodenn induction. At later stages Xenapus hu& is expressed in the neural ctest in response to neurai induction.

18. SA%ENT MG. BENNF~T Ml? Identification of a structural ho- e mologue of the Dmsopbika gene SnaiL Dew-t 1990,

109967-973. A Xenc$%r cDNA related to Dmrc@&a snuil was cloned and found to be expressed, in response to mesodenn-inducing factors, predominantly in the mesoderm-forming equatorial zone.

19. COOKE J: Xenopw mesoderm induction: evidence tbr early l e size control and partial autonomy for pattern development

by onset of gastrulation. Devemt 1989, 106:519529. Dietent-sized pieces of animai pole tissue were g&ed to host embryos to change the extent of cells competent to respond to mesodenn induction. The amount of mesoderm formed depended on the size of the field of responsive cells, indicating the existence of a mechanism to Emit induction, perhaps invohring a reaction- ditfusion system. Gastrulae stage embryos were also shown to be capable of some nonnai patterns of development in the absence of most endodenn.

20. MENMRDT H: Mod& for positional signaktg with ap l plication to the dorsoventral patterning of insects and

segregation into di&rent cell types. Development 1989, (suppl):16+180.

Discwxs models of pattern formation in the context of recent experimental results.

21. ma-w MN~~%E M, IAMBRECHIS C, DUR~TQN l AJ: Characterization of protein kinase C hi early Xertopus

embryogenesis. Develapment 1990, 110461470.


Neural induction is shown to increase the level of inositol phosphates and to result in the phosphorylation of several proteins.

22. DE ROBERTB EM, OWER G, WRIGHT CVE: Determination of l axial polarity in the vertebrate embryo: homeodomain pro-

teins and homeogenetic induction. Cell 1989, 57:189191. Review of anteroposterior development, with particular reference to XI- Hbaxl expression.

23. CHO KWY, DE ROBERTIS EM: Differential activation of Xen@ aa pus homeobox genes by mesoderm- Inducing factors and

retinoic acid. Genes Dev 1990, 4:1910-1916. The expression of anterior (X&ftil) and posterior (XIHbaro homeobox genes was studied. XlHboxZ was preferentialfy expressed in response to XTC-MIF, and XlHbarG to bFGF. In addition, retinoic acid was shown to potentiate XIHbarG expression.

24. Wrua-rr CE, MORITA EA, WIIMN DJ, DE ROBERTIS EM: The l Xenopus XIHbox 6 homeo protein, a marker of posterior

neuraI induction. Is expressed in proliferating neurons. De velpnent 1990, 109:225-234.

Using antibody staining, the distribution of XB&ox6was shown to be in the posterior of the embryo, in the spinal cord, particularly in immature neurons, and in lateral plate mesodemt.

25. WRIGHT CE, CHO KWY, HARDWCKE J, COIUNS RH, DE ROBERIIS .a EM: Interference with function of a homeobox gene in

Xenopus embryos produces malformations of the anterior spinal cord. Cell 1989, 59:81-93.

Antibodies against XlHboxl were injected into embryos at the one ceU stage, resulting in the transformation of the anterior spinal cord region that normally expresses X-1 into a hindbrainlike structure.

26. MCMAHON A, Moor RT: Ectopic expression of the proto- a* oncogene int-1 In Xenopus embryos leads to duplication

of the embryonic axis. CeN 1989, 58:10751084. An anterior duplication of the anterior embryonic axis was generated by injection of fenilized Xer2qw.r eggs with ml-1 RNA

27. Sm HL. DRAPER BW, GARLAND RM, WENRNIB H: Identifica- a tion of a retinoic acid-sensitive period during primary axis

formation In Xenopus laevis Germ Dev 1990, 4:932- 942.

Retinoic acid was found to cause a progressive truncation of anterior structures, with loss of neural tissue and of mesodemtal sttuctures such as heart. This sensitivity to retinoic acid was lost during gastrula and earty neurula stages.

28. MKRANI E, SHIMONI Y: Induction by soluble factors of or- e ganixed axial structures in chick epiblasts. Science 1990,

247:1032-1094. Conditioned medium from the XTC cell line, but not other growth factors such as FGF or TGFgl and TGFgZ, can induce axial structures from isolated chick embryo epiblasts.

29. M~XANI E, 2,~ T, THOMSEN G, SHIMONI Y, MELTON DA, BRIL 0 A Activin can induce the formation of axial structures

and is expressed in the hypoblast of the chick. Cell 1990, 63:495501.

Activin was found to induce the formation of chick axial structures (see !28] ). In addition. there was aidence of transcription of the activin pB chain in the chick blastoderm. with higher levels in the hypoblast.

30. COOKE J. WONG A Growth-factor-related proteins that are l inducers in early amphibian development may mediate sim-

ilar steps in amniote (bird) embryogenesis. Development, in press.

Factors with mesoderm-inducing activity in amphibian embryos cause isolated chick epiblast cells to spread on fibronectin. ln addition, epiblast cells treated with such factors were shown to disrupt development when seeded onto host chick blastodemq and incubation of whole early blastodemts with these inducing factors also disturbs normal development.

31. STERN CD, CANNING DR Origin of cells giving rise to l * mesoderm and endoderm in chick embryo. N&we 1990,

343:27%275. The epiblast of the chick embryo contains two populations of cells. one which labels with the antibody HNKI and one which does not. The HNKl-positive cells, which are mixed randomfy with the negative cells, go on to form mesoderm and endoderm. If they are killed, the resulting embryo lacks these germ layers; implantation of a piece of untreated primitive streak restores normal development,

inductive interactions in early amphibian development

Documents