basic body plan of young mammalian embryos

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BASIC BODY PLAN OF YOUNG MAMMALIAN EMBRYOSJOHN JEFFERSON COLLERA

It

is important to recognize that the basic body plan of essentially all the vertebrate embryos during the period of early organogenesis is essentially a carryover of that is seen in the aquatic, gill-breathing ancestral form of vertebrates. characteristics include a circulatory system based on a simple tubular heart emptying into ventral aorta, the breaking pump of the ventral aorta into branches that supply gills as they pass around the pharynx and the collecting of dorsal branches of these arteries into dorsal aorta.

This

The

overall subdivision of pharyngeal region into system of gill (branchial) arches is retained, although with numerous modifications, throughout the embryos of vertebrate classes. the segmental organmization of the body (an adaptation fro sinusoidal swimming movements) is prominent in the somites and spinal nerves and persists into adult life in the jointed vertebral column.

Likewise,

There

is striking similarity in general organization and external appearance of various amniote embryos. if to anticipate its ultimate cerebral dominance, the human embryo however, soon begin to show larger neural plate in its future forebrain region. cephalic part of the neural plateof young human embryo is so large and is expanding laterally so rapidly that its closure is delayed compared with the early closure in other embryos.

As

The

In

the mid-body region, however, the general arrangement of structures and layers is essentially similar among amniote embryos. third week and the fourth week, the human embryo loses its originally straight body axis and begins to show a marked flexure in the cranial region.

Between

Three week human embryo

There

is a basic similarity in the body plan of 4 to 4 day chick embryos , 5 mm pig embryos, and month old human embryo.. differences consist of the larger size of larger size of eyes and optic regions of avian embryos and the relatively more advanced heart, liver and mesonephron of mammalian embryos.

Minor

In the cephalic region the primordia of developing sensory organs are prominent features. The nasal (olfactory) placodes appear as local ectodermal thickenings on either side of the head, and in more advanced embryos, the nasal placodes become depressed to form the nasal pits. The primordia of the eyes, located far laterally in the head ,are prominent landmarks, and the auditory vesicles and contours of brain walls are sharply outlined on clear embryos. The face does not exist as a recognizable region.

EXTERNAL FEATURES

The region of throat is dominated by a system of branchial arches and clefts, a region homologous to the gills of primitive fishes. The branchial arches are best viewed from the ventral aspects. The first on either side is subdivided into two components: the maxillary processes, which form the lateral parts of the upper jaw, and the mandibular elevations, which merge with each other in the midventral line to form arch of the lower jaw (mandibular arch)

EXTERNAL FEATURES

Posterior

to the mandibular arch are three similar areches, the hyoid and the unnamed third and the fourth sycher, all of which appear clearly in lateral views.. hyoid arch is homologous to the operculum (gill cover) of fishes, and even in the human embryo it slightly overhangs the third and fourth arches.

The

EXTERNAL FEATURES

EXTERNAL FEATURES

Between the branchial arches are deep furrows which mark the position of the ancestral gill clefts. Although in mammalian embryos, these furrows do not ordinarily break through into the pharynx, they are commonly called clefts because of their phylogenetic significance. Only the most cephalic of these clefts is named (hyomandibular cleft..); they others are designated by their positional numbers.

EXTERNAL FEATURES

The entire region about the third and the fourth postoral clefts becomes especifically deeply depressed and is known as the cervical sinus.

EXTERNAL FEATURES

The upper body of the early embryo is dominated by the precociously large heart, which form the surface bulge known as the cardiac prominence. Adding the dorsal side of the trunk, the paired somites extend from just caudal to auditory vesicle into the tail. Conspicuouc in the midventral region is the body stalk which in time becomes more dicrete and elongated as the umbilical cord.

EXTERNAL FEATURES

The

appendage buds first appear as flangelike projections from the body wall . The arm bud leads to the leg bud in developmental progress.

EXTERNAL FEATURES

At this stage the human embryo has every bit as well developed a tail as does a pig embryo. Later in development the human tail normally undergoes regressive changes that leave the human tail only a symbolic coccyx. Regression of the human tail is apparently brought about by intrinsic cell death. Occasionally this regression fails to occur, and a human infant is born with a sizable and unmistakable tail.

EXTERNAL FEATURES

The

central nervous system has progressed from a simple neural tube to one which fundamentalbsubdicisions of the brain taking shape.

At first, three divisions are apparent . These are the forebrain (prosencephalon),

midbrain (mesencephalon), and hindbrain (rhombencephalon).

THE NERVOUS SYSTEM

At the stage under consideration, the brain is in the phase of transition from the three to the five-vesicle condition. There is a clear indication of the separation of forebrain into a rostral telencephalon and a diencephalon and the thickening of the walls of the more rostral part of the hindbrain presages the differentiation of the metencephalon from the mylenecephalon. The mesencephalon remains undivided.

THE NERVOUS SYSTEM

Projection from the walls of diencephalon are the optic vesicles, which are beginning to invaginate to form the optic cups.. In response to an inductive signal from the optic vesicle, the ectoderm overlying the optic cup has thicken and has begun to invaginate to form Lens vesicle. From 4 to 5 weeks in the human embryo, many of the 12 cranial nerves begin to appear.

THE NERVOUS SYSTEM

Caudal During

to myelencephalon, the neural tube is more slender and gives more rise the spinal cord. these early stages, it is relatively simple appearing the tube with a slitlike central canal and walls of closely packed cells of ectodermal origin. the spinal cord the ribbonlike neural crest has just broken up and form the spinal (and also the eranial) ganglia.

Alongside

THE NERVOUS SYSTEM

The

primitive gut tube is formed by the endoderm as the body itself folds into a tube. the stage of 5mm pig and 1-month human embryo, it has been deliminated into a tubular foregut and hindgut and a midgut, which has open ventral region leading to the yolk stalk.

By

DIGESTIVE AND RESPIRATORYSYSTEMS

The

intermembryonic gut-tract of young mammalian embryos at first ends blindly at both its cephalic and caudal ends. external depression called the stomodeum marks the future oral opening. the stomodeum deepens, the ectoderm form this floor comes to direct contact with the endoderm of the foregut to form the stomodeal, or oral plate.

And As

DIGESTIVE AND RESPIRATORYSYSTEMS

As

is common with the ectoderm and endoderm abut directly upon each other, the oral plate breaks through and establishes the oral opening into the foregut.

DIGESTIVE AND RESPIRATORYSYSTEMS

Arriving

medially as a slender ectodermal diverticulum from the ventral part of the stomodeum is Rathkes pocket. its first appearance, Rathkes pocket is in close relationship to the infundibular process from the floor of diencephalon. these two structures will form the hypophysis.

From

Together,

DIGESTIVE AND RESPIRATORYSYSTEMS

On the endodermal process of the oral plate, the extreme cephalic end of the foregut remains as the preoral gut, or Seesels pocket. This structure serves only as anatomical landmark and gives rise to no adult structure.

DIGESTIVE AND RESPIRATORYSYSTEMS

The

cephalic end of the foregut is mainly invoplved in the formation of the pharynx, fro which four pouches extend on either side toward the corresponding external gill furrow.. the thin membrane that remains between the pharyngeal pouch and the external gill furrow in mammals does not ordinarily breakdown to form an open gill cleft such as that found in our water living ancestors, the similarity in relationships is obvious.

Although

DIGESTIVE AND RESPIRATORYSYSTEMS

The significance of the structural relations in this regions is further emphasize by the location of the aortic arches, which lie in closely packed mesenchymal tissue between the gill clefts. In the embryos of birds and mammals, the aortic arches do not form capillary beds as they do in gill-breathing animals. Nevertheless, the basic ancestral pattern can be seen in the way the aortic arches pass from the ventrally located heart to the dorsal aorta by the way of the gill arches flanking the pharynx..

DIGESTIVE AND RESPIRATORYSYSTEMS

In

older embryos, a number of important structures arise from the endodermal lining of the pharynx. these, the only small cluster of cells which constitute the primordium og the thyroid gland has emerged from the floor of the pharynx at this stage.

Of

DIGESTIVE AND RESPIRATORYSYSTEMS

Also,

from the floor of the posterior pharynx, a median ventral groove is rapidly converted into a tubular outgrowth parallel to