Book - Embryology of the Pig 5

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Patten BM. Embryology of the Pig. (1951) The Blakiston Company, Toronto.

Patten 1951: 1 Foreword to the Student | 2 Reproductive Organs - Gametogenesis | 3 Sexual Cycle | 4 Cleavage and Germ Layers | 5 Body Form and Organs | 6 Extra-Embryonic Membranes | 7 Embryos 9-12 mm | 8 Nervous System | 9 Digestive - Respiratory and Body Cavities | 10 Urogenital | 11 Circulatory System | 12 Bone and Skeletal System | 13 Face and Jaws | Bibliography
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This historic 1951 embryology of the pig textbook by Patten was designed as an introduction to the topic. Currently only the text has been made available online, figures will be added at a later date. My thanks to the Internet Archive for making the original scanned book available.

By the same author: Patten BM. The Early Embryology of the Chick. (1920) Philadelphia: P. Blakiston's Son and Co.

Patten BM. Developmental defects at the foramen ovale. (1938) Am J Pathol. 14(2):135-162. PMID 19970381

Modern Notes


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Pages where the terms "Historic" (textbooks, papers, people, recommendations) appear on this site, and sections within pages where this disclaimer appears, indicate that the content and scientific understanding are specific to the time of publication. This means that while some scientific descriptions are still accurate, the terminology and interpretation of the developmental mechanisms reflect the understanding at the time of original publication and those of the preceding periods, these terms, interpretations and recommendations may not reflect our current scientific understanding.     (More? Embryology History | Historic Embryology Papers)

Chapter 5. The Early Development of the Body Form and the Establishment of the Organ Systems

I. Body Form

The formation of the body is initiated by the same growth processes which establish the germ layers. But even after the germ layers have been laid down and have begun to show considerable differentiation, the configuration of the young embryo is so unlike that of the adult that except to one familiar with embryology there are no readily identified landmarks. There is no distinct head, no neck, no trunk; there are no appendages ; — in short there are none of the conspicuous structural features by which we are accustomed to orient ourselves in dealing with adult anatomy (Figs. 23-25). It therefore seems wise to interrupt tracing in detail the processes which give rise to the internal organs of the embryo and follow very briefly the changes in the outer form of its body. Such a preliminary survey should make clear the significance of the unfamiliar topography of the young embryo and give us a perspective from which to follow out the internal changes occurring in the various regions.

The Embryonic Disk. Very early in development we noted the differentiation of a region of the blastocyst which was designated as the embryonic disk (Figs. 16-19). Although the embryonic disk has no semblance of adult anatomical form it is, nevertheless, the beginning of the body of the embryo. The remaining regions of the blastocyst will take part only in the formation of membranes accessory to the body.

Establishment of Body Axis. In the preceding chapter it was stated that with the establishment of the primitive streak in the embryonic disk the embryo had definitely differentiated its future body axis. As far as any anatomical features then exhibited by the embryo were concerned, this statement had to be taken on faith. If, however, the position of the priniitive streak is followed in a series of progessively older embryos (Figs. 23, 24, 25, and 28) and if then the place of its final disappearance in the floor of the sinus rhomboidalis is observed (Fig. 29), one readily comprehends why the primitive streak is said to mark at once the longitudinal axis, and the caudal end of the body.

Shortly after the primitive streak becomes clearly defined, the thickened ectoderm of the neural plate is folded to form the neural groove (Figs. 24, 25, and 26). This folding coincides in direction with the primitive streak and further emphasizes the longitudinal axis of the growing embryo.

Fig. 28. Drawing (X 15) of six-somite pig embryo. (Carnegie Collection, Cl 95-1 and C162.) i^ge approximately 14^^ days.

Differentiation of the Cephalic Region. With the establishment of the neural groove, body landmarks begin to appear with rapidly increasing clearness. The neural folds in the cephalic region are of much greater size than they are farther caudally (Figs. 25 and 28). This condition foreshadows the differentiation of the neural tube into a conspicuously enlarged cephalic portion, the brain, and a more attenuated caudal portion, the spinal cord. The cephalic region of the embryo is, therefore, already indicated by this enlargement of the fore part of the neural plate (Fig. 28).

At first the topography of the head is vaguely defined, but it soon becomes more precisely marked out by the appearance of characteristic local structures. On either side of the head, in the future oral and cervical region, a series of circumscribed elevations appear. These are gill arches {branchial arches)^ homologous with the gill arches of ancestral water-living forms. In the mammalian embryo the most cephalic of these gill arches appears just caudal to the primitive mouth opening. Because it is involved in the formation of the lower jaw, it is called the mandibular arch. The arches caudal to the mandibular arch become less conspicuous and are incorporated into the neck (Figs. 30-34).

Coincidently the primordia of both the ear and the eye become

Fig. 29. Drawing (X 15) of 10-somite pig embryo. (Carnegie Collection, Cl 76-6.) Age approximately 15 days.

recognizable. The ears arise as a pair of locally thickened areas {auditory placodes) in the superficial ectoderm at the level of the more posterior part of the brain. The auditory placodes soon sink below the surface to form the auditory pits. As the pits are deejjened and enlarged they are designated as the auditory or otic vesicles. For a time they maintain an opening to the surface (Fig. 30), but this opening soon closes and the vesicles are not prominent externally (Figs. 31, and 32). They can, however, be seen readily in cleared specimens (Fig. 39) or in sections (Figs. 41, A, and 47). The auditory vesicles are destined to form the inner portion of the auditory mechanism. Somewhat later the beginnings of the external ear can be made out not far from the site of the original invagination of the internal ear primordium (Figs. 33 and 34).

The eyes arise as local outgrowths from the lateral walls of the rostral part of the brain. Long before the optic vesicles, as the early outgrowths are called, bear any resemblance to adult eyes, their position can be seen because of the prominence they make in the overlying ectoderm (Optic vesicle, Fig. 30). Specializations of the super

Fig. 30. Drawing (X 15) of 17-somite pig embryo. (Modified from Keibcl.) Age approximately 16 days.

ficial tissues about the optic vesicles soon make the developing eye readily identifiable (Figs. 32—34). With the establishment of the mouth parts, the ear, and the eye there is no longer any difficulty in recognizing the general topography of the cephalic region.

The Trunk. Like their invertebrate ancestors, all vertebrates have a segmentally organized body. In adult mammals the underlying metamerism is largely masked by local fusions and specializations. But even so, unmistakable evidences of this fundamental plan of Structure persist in the segruentally arranged spinal nerves and ganglia, in the series of vertebrae constituting the ‘'back bone,” and in the arrangement of the ribs and of the intercostal musculature. In the young mammalian embryo metamerism is much more obvious. One of its most conspicuous superficial markings is the series of paired prominences which indicate the location of the mesodermic somites (Figs. 25 and 26, C). These masses of mesodermal tissue are clearly metameric in arrangement. In fact, it is through them that we trace the origin of the segmental arrangement of the axial skeleton and the

Fig. 31. Photograph (X 12) of 5 mm. pig embryo. (C’arnegic Collection, C'266-1.) Age approximately 17-18 days.

thoracic musculature just alluded to as one of the characteristic* evidences of metamerism in adult mammalian anatomy.

The external prominence made by the developing heart appears in mammals at a strikingly early stage of development. The heart at first lies far toward the head from its definitive position. Bearing in mind that the mandibular arch will form the lower jaw, we can say that the heart originates ^‘under the chin” (Fig. 30). As growth proceeds there is rapid elongation of the embryo between its head and trunk, which results in the establishment of the neck region. In this process the heart is carried caudad to lie in its characteristic position in the anterior part of the trunk (Figs. 30-34). A slight depression may be seen between the external prominences due to the heart and

Fir;. 32. Drawiiii^ (X 8) of 7.5 nun. pija^ embryo. (Modified from Minot.) A^c approximately 18-19 days.

the liver. This depression indicates the position at which the diaphragm develops, and with its appearance we can differentiate the thoracic from the abdominal region of the trunk (Figs. 32-34).

Caudal to the hepatic prominence is the conspicuous belly-stalk. Over this stalk the tissues of the embryo are continuous with the extra-embryonic membranes. In it are embedded the large blood vessels by way of which the embryo receives its food and oxygen supply from the uterus of the mother.

The Appendages. From the sides of the body two pairs of outgrowths are formed. The first pair makes its appearance at the level of the heart. These bud-like tissue masses (Fig. 32) become differentiated into the anterior appendages. A second pair, located caudal to the belly-stalk, appears somewhat later and gives rise to the posterior appendages. The bulk of the tissue which goes to make up the appendage buds arises by outgrowth from the mesodermic somites at that level.

Flexion. The embryos of all the higher vertebrates develop within a confined space. The growing body must conform it^self to the limitations imposed by the egg shell, as in birds and reptiles, or the uterine cavity, as in mammals. It is not at all surprising, therefore, that )^oung embryos show a marked tendency to become curled, head to tail. This process by which an embryo at first straight (Fig. 29) becomes bent into more or less the shape of a letter C (Fig. 32) is called flexion. Flexion becomes apparent first in the cephalic region (Fig. 30) but soon thereafter involves the entire body (Fig. 32). At certain points flexion is especially strongly marked. This has led to speaking, for convenience in description, of the cranial flexure, the cervical flexure, the dorsal flexure, and the lumbo-sacral flexure. These so-called regional flexures in reality grade into one another and are nothing more than local accentuations of a process which involves the entire body.

Torsion. Preceding or accompanying flexion the body tends to show some twisting about its long axis (Fig. 30). This torsion, as it is called, is by no means as conspicuous or constant in mammalian embryos as it is in certain other forms, as for example the chick. In embryos with a large yolk, the body at first lies prone on the surface of the yolk sphere. Flexion would be impeded by the yolk except for the fact that the embryo, preceding flexion, twists its body through 90 degrees so that instead of lying prone on the yolk it lies on its side. The torsion which occurs in mammalian embryos is probably to be

Fig. 33. Drawing (X 6) of 15 mm. pig embryo. (Modified from Minot.) Age approximately 24 days.

Fig. 34. Drawin^^: (X 5.5) of 20 mm. pig embryo. (After Minot.) Age, about 4 weeks, i.e., approximately one-fourth of the total period of gestation.

regarded as la. tendency inherited from their large-yolked ancestors. Like certain vestigial structures this change in body shape tends to make its appearance even after the conditions in connection with which it was developed have ceased to exist. Like vestigial structures, also, it is strikingly variable in the degree to which it develops. It may be hardly noticeable (Fig. 31) or it may be very strongly marked (Fig. 50). In either case it is but transitory, and the body soon loses all evidence of its torsion (Figs. 32-34).

II. The Establishment of the Organ Systems

Tke Nervous System

Formation of the Neural Tube

In dealing with the early differentiation of the germ layers, attention was called to a thickened area of the ectoderm, known as the neural plate, which constituted the

Fig. 35. Drawing (X 135) showing closure of the neural tube and formation of the neural crest. From pig embryos of: A, 8 somites; B, 10 somites; (<, 11 somites; D, 13 somites.

primordium of the central nervous system. The first step in the formation of the brain and cord from this primordial mass of cells is its transformation from a superficial plate into a tubular mass of cells lying beneath the ectoderm.

As is the case with so many early embryological phenomena, the formation of the neural tube from the neural plate is brought about by a process which can most conveniently be described as ‘Tolding.” Due to differential growth the neural plate is depressed centrally and elevated laterally. It is said to have become folded to form the neural groove (cf. Figs. 24 and 25 for its appearance in dorsal views of entire embryos and Figs. 22, C, and 26, C, for its configuration in transverse sections). The elevated lateral margins of the neural plate are now called the neural folds (Fig. 26, C).

As the neural folds become more elevated they grow toward each other, tending to close over the neural groove (Figs. 35, A, B). Up to this time the neural plate has remained directly continuous laterally with the superficial ectoderm, but when the neural folds meet in the mid-dorsal line this continuity ceases. A double fusion takes place. The mesial or neural plate components of the two folds fuse with each other and the lateral limbs consisting of unmodified ectoderm also fuse with each other (Fig. 35, C, D). Thus in the same process the original neural plate becomes the wall of the neural tube and the superficial ectoderm closes over the place formerly occupied by the open neural groove. Shortly after this fusion the neural tube and the superficial ectoderm become somewhat separated from each other leaving no trace of their former continuity.

The Neural Crests

There are cells located near the apices of the neural folds which are not involved in the fusion of either the superficial ectoderm or the neural plate. These cells form a pair of longitudinal aggregations extending one on either side of the mid-line in the angles between the superficial ectoderm and the neural tube (Fig. 35, A, B). With the fusion which closes the neural tube these two cell masses become, for a time, confluent in the mid-line (Fig. 35, C, D). But because this aggregation of cells arises from paired primordia and soon again separates into right and left components, it should be regarded as a paired structure. On account of its position dorsal to the neural tube it is called the neural crest.

When first established the neural crest is continuous antcroposteriorly. As development proceeds, its cells migrate ventrolaterally on either side of the spinal cord and at the same time become segmen tally clustered. The mctamerically arranged cell groups thus derived from the neural crest give rise to the dorsal root ganglia of the spinal nerves, and, in the cephalic region, to the ganglia of the sensory cranial nerves (Figs. 59 and 60).

Early Differentiation of the Brain

In dealing with the establishment of the cephalic region we noted the marked enlargement of the anterior portion of the neural plate and commented on the fact that this enlargement presaged the establishment of the brain. When the neural tube is formed by the folding of the neural plate, the cephalic part of the tube is of larger diameter corresponding to the greater size of the original neural plate in the future brain region (see Fig. 36).

The configuration of the primordial brain shows several things of interest. Not the least significant of these is the series of local enlargements, called neuromeres, which indicate its underlying metameric organization. Concerning the precise homologies of individual enlargements in the brain of a mammalian embryo with specific neuromeres of ancestral forms there is by no means complete agreement. The controversies center about the fusion of neuromeres in the more rostral parts of the brain. There are at least 11 enlargements recognizable in the embryonic brain, but only the more caudal ones show their individuality clearly. Some of the more rostral enlargements undoubtedly represent several neuromeres. In all probability there are as many as 15 neuromeres represented in the vertebrate brain. However this may be, for the beginning student the fact that metamerism is unmistakably present is to be emphasized rather than the controversies concerning the homologies of neuromeres.

Fig. 36. Drawings (X 35) showing early stages in the formation of the brain of the pig. (Based on reconstructions made from series loaned by the Carnegie Institute.) A, Neural plate of 7-somite embryo, dorsal view. B, Neural plate of 13-somite embryo, lateral view. C, Same, anterior aspect. D, Brain of 17-somite embryo, lateral view. E, Brain of 24-somite embryo, lateral view.

Almost from its first appearance the brain shows certain indications of regional differentiation. In early stages we can recognize three fairly definite regions which later become subdivided to form the five divisions characteristic of the adult brain. The three primary regional divisions are known as the fore-brain, mid-brain, and hind-brain, or more technically, as prosencephalon^ mesencephalon^ and rhombencephalon. The prosencephalon is the broadest of the three divisions because of the presence of evaginations from its lateral walls which are the first indications of the formation of the eye. These evaginations are known as the primary optic vesicles (Fig. 36).

In the extreme rostral portion of the prosencephalon complete closure of the neural folds is somewhat delayed. There remains here, for a time, an opening known as the anterior neuropore (Fig. 29).

The mesencephalon is marked off by slight constrictions in the walls of the neural tube from the prosencephalon anteriorly, and somewhat less distinctly, from the rhombencephalon which lies posterior to it. In this early stage the mesencephalon shows no indication of local specialization presaging the formation of specific structures.

The most interesting feature of the hind-brain or rhombencephalon at this stage is the definite indication of neuromeric enlargements already mentioned (Fig. 36). Posteriorly the rhombencephalon grades without abrupt transition into the more slender part of the neural tube which will become the spinal cord.

At the extreme posterior end of the developing spinal cord, closure of the neural folds is delayed, just as it was anteriorly. The opening which thus persists for a time at the posterior end of the neural tube is known as the sinus rhomboidalis (Fig, 29).

The Digestive System

The Primitive Gut

Even before the body of the embryo takes shape, the formation of the digestive system has been initiated by the establishment of the entodermal layer of the spherical blastocyst (Fig. 16). At the time the blastocyst undergoes elongation (Figs. 17 and 18), the primitive gut, as the space within the entoderm is called, becomes correspondingly elongated. In fact it occupies nearly all the space within the blastocyst (Fig. 21).

When the mesoderm has been formed and split into somatic and splanchnic layers, the splanchnic mesoderm becomes closely associated with the entoderm as the splanchnopleure. Thus the primitive gut, very early in development, acquires a double-layered wall (Figs. 22 and 26). The entodermal component of the splanchnopleure gives rise to the epithelial lining of the gut tract and to its glands. The associated layer of splanchnic mesoderm becomes differentiated into the muscular and connective-tissue layers of the gut wall.

The Delimitation of the Embryonic Gut

By the time the wall of the primitive gut has received its mesodermal reinforcement the region which is to become the embryonic body is beginning to be more clearly defined. Formerly merely a disk-shaped area of the blastocyst distinguishable from the extra-embryonic portion of the germ layers by reason of its greater thickness, the body of the embryo now begins to be bounded by definite folds. These folds increase in depth, undercut the embryo, and finally, except for the communicating belly-stalk, separate it from extra-embryonic structures. Ibe folds which thus definitely t\stablish the boundaries between intra- and extra -embryonic regions are known as the limiting body folds or simply the body folds.

The formation of the body folds plays an important part in determining the configuration and relations of the gut tract of a young embryo. The ‘^folding off” of the embryo begins with a ventral bending of the margins of the embryonic area so that the developing body takes on a marked dorsal convexity. Then the undercutting of these depressed margins cephalically and caudally, together with rapid increase in the length of the embryonic body, cause the embryo to overhang the extra-embryonic layers. The part of the embryo that juts out from the blastocyst anteriorly is the head (Fig. 37, B) and the portion which, slightly later, comes to project in a similar manner posteriorly, is the tail (Fig. 37, C). The folds of somatopleure which undercut the head and tail are known respectively as the subcephalic and subcaudal folds.

Coincidently the down-foldings on either side of the embryo become more definite, emphasizing its lateral boundaries. These folds are known as the lateral body folds {lateral limiting sulci) (Fig. 26, D). Toward the head they are continuous with the subcephalic folds and toward the tail with the subcaudal folds. The progressive deepening of all these circumscribing folds and the continued growth of the body itself constrict the connection of the embryo with the extra-embryonic membranes, and initiate the formation of the belly-stalk. The same folding process establishes the lateral and ventral body-walls of the embryo.

The superficial foldings which thus establish the boundaries between the embryonic body and extra-embryonic portions of the germ layers have their counterparts in the deeper lying layers. The changes which take place in the configuration of the splanchnopleure during this process bring about the division of the primitive gut into an intra-embryonic portion, and an extra-embryonic portion known as the yolk-sac.

Fig, 37. Sagittal sections of pig embryos to show establishment and early regional differentiation of the gut. The drawings indicate schematically conditions: A, in the primitive streak stage; B, at the beginning of somite formation; C, in embryos having about 15 somites; D, in embryos having about 25 somites.

The Fore-gut

The first part of the primitive gut to be definitely incorporated into the embryo is that portion lying beneath the head. With the forward growth of the head and its concomitant undercutting by the subcephalic fold, an entodermally lined pocket is established in the cephalic region. This is the fore-gut (Fig. 37, B) Anteriorly the fore-gut ends blindly, posteriorly it remains in open communication with the rest of the primitive gut. The opening from the undifferentiated portion of the primitive gut into the fore-gut is known as the anterior intestinal portal.

The Hind-gut

In a similar manner a pocket of the primitive gut, known as the hind-gut, is formed beneath the caudal portion of the embryo (Fig. 37, B). The hind -gut ends blindly at its caudal extremity. Anteriorly it retains open communication with the rest of the primitive gut cavity by way of an opening termed the posterior intestinal portal.

The Mid-gut

Beneath the body of the embryo, between fore- and hind-gut, is a region of the primitive gut which is destined to be included within the body, but which* as yet has no floor. This region is known as the mid-gut (Fig. 37, B). As the embryo is constricted ofi' from the extra-embryonic layers by the progress of the subcephalic and subcaudal folds, the fore-gut and hind-gut are increased in extent at the expense of the mid-gut (cf. Fig. 37, B, C, D). The mid -gut is finally diminished until it opens ventrally by a very small aperture. This narrowed opening from the mid-gut to the yolk-sac is the yolkstalk. When the extra-embryonic membranes are taken up we shall give further consideration to the fate of the yolk-sac.

The Stomodaeum and Proctodaeum

When first separated from the yolk-sac the embryonic gut ends blindly both cephalically and caudally. There are no indications of either oral or anal openings. Soon, however, there appear two depressions in the surface of the body which sink in to meet the gut. One of these depressions, the stomodaeum, is located on the ventral surface of the head in the future oral region. The other, the proctodaeum, is located caudally in the future anal region.

The stomodaeal depression gradually becomes deeper until its floor makes contact with the entoderm of the fore-gut (Fig. 37, C). The thin layer of tissue formed by the apposition of stomodaeal ectoderm to fore-gut entoderm is known as the oral plate. Not long after the first appearance of the stomodaeum, the oral plate ruptures, establishing the anterior opening of the gut (Fig. 37, D). Growth of surrounding structures further deepens the original stomodaeal depression and it becomes the oral cavity. The region of the oral plate in the embryo becomes, in the adult, the region of transition from oral cavity to pharynx.

Somewhat later in development than the time at which the oral opening is established, the proctodaeum breaks through to the hindgut, forming the cloacal opening. Subsequent differentiation in this region results in the separation of the originally single cloacal aperture, into anal and urogenital openings (see C'hap. 10).

Pre-oral and Post-cloacal Gut

Neither the stomodaeum nor the proctodaeum break through into the gut at its extreme end. There remains cephalic to the stomodaeal opening a small region of the fore-gut which is designated as the pre-oral gut. The comparable region of the hind-gut caudal to the proctodaeal opening is known as the post-cloacal gut (Fig. 37, D). Pre-oral and post-cloacal gut are of interest primarily as embryological landmarks helpful in following changing relations in the cephalic and caudfil regions. They disappear later in development without giving rise to any definite structures.

Fio. 39. Projection drawing (X 17) of a lightly stained and cleared 5 mm. pig embryo.

Fig. 40. Longitudinal section of 5 mm. pig embryo (X 25). The caudal end of an embryo in this stage of development is usually somewhat twisted to one side (see Fig. 31). For this reason sections wliich cut the cephalic region in the sagittal plane pass diagonally through the posterior part of the body. For a schematic plan of a completely sagittal section of an embryo of about this age sec figure 37, D. Abbreviation: Rath., Rathke’s.

fm. 41. Tramvarse sections of 5 mm. pig iwtoy# (X 18).

Yolk -stalk extending into extra- embryonic coctom



Rt, subcard, vein

Right iimbiiicol vein

embryo (Fig. 39), and with longitudinal section of embryo of same pfe

Early Regional Differentiation of the Gut

The special structures and organs which arise from various parts of the gut tract will be considered subsequently in some detail. We cannot, however, ignore altogether the local differentiations which appear in young embryos indicating the regions where some of the more conspicuous organs will develop. The names of these primordial local differentiations clearly indicate their role in organ formation.

The fore-gut posterior to the stomodaeal opening becomes greatly expanded laterally and at the same time somewhat compressed dorsoventrally. This region is the pharynx (Figs. 38, A-C, and 40). A series of lateral bays of the main pharyngeal lumen extend to either side. These are the pharyngeal pouches. Each pouch lies opposite one of the external grooves between adjacent, branchial arches (Fig. 41, B, C). The tissue between the floor of a branchial groove and the tip of its associated pharyngeal pouch is reduced to a thin membrane and sometimes may even break through transitorily to form an open gill cleft reminiscent of those in water-living ancestral forms.

From the extreme caudal part of the pharynx a medial ventral diverticulum arises which is destined to play an important part in the formation of the respiratory organs. Cephalically this diverticulum is little more than a furrow in the floor of the pharynx {laryngo-tracheal groove^ Fig. 41, D). At its caudal extremity the outgrowth projects ventrad, entirely free of the gut tract. This blind end of the diverticulum (Fig. 40) soon bifurcates to form the lung buds.

Immediately caudal to the point at which the respiratory primordia appear, the gut narrows abruptly to form the esophagus and then dilates again in the region which will become the stomach. Just beyond the stomach is a conspicuous group of outgrowths from which are derived the pancreas, the liver and the gall-bladder (Figs. 40, 41, G, and 46). Caudal to this region there is as yet little local diflerentiation.

=The Early Differentiation of the Mesoderm

The Somites

In the preceding chapter we traced the differentiation of the mesoderm into three primary divisions — dorsal, intermediate, and lateral. Comment was made, also, on the formation of the segmentally arranged somites in the dorsal mesoderm. The cells in a somite are not destined to a common fate. In fact these c.ells as a group have a wider diversity of developmental potentialities than any sharply localized aggregation of cells with which we have to deal. It is, therefore, a matter of especial interest to see the various steps by which they become, so to speak, sorted out, grouped according to their potentialities, and finally highly specialized in various ways (see Fig. 27).

When first formed a somite appears as a solid mass of cells without any definite organization — a mere local thickening of the dorsal mesoderm (Fig. 42, A). This initial mass grows rapidly in bulk and its cells take on a definite radial arrangement (Fig. 42, B). At the same time its boundaries become more clear cut and a small but quite definite lumen appears in its center (Fig. 42, B). This lumen, known as the myocoele^ increases in size until the somite appears as a hollow vesicle with thick outer walls (Fig. 42, C).

By this time local differences within the somite are becoming apparent. Three regions are recognized and named on the basis of their later history. The dorso-mesial part of a somite is composed of cells which will form the skeletal muscles developing at that segmental level of the body. For this reason it is called the myotome (Fig. 42, B, C).

The ventro-lateral portion of the somite is made up of cells which have been believed to migrate out, become aggregated close under the ectoderm and give rise to the connective-tissue layer (dermis) which underlies the epidermis. Accordingly it has been called the dermatome (cutis plate). While some cells from this region of the somite undoubtedly ijire contributed to the formation of the deep layers of the skin, the conviction has been gaining ground that many, perhaps most, of them take part in the formation of muscle. Furthermore the connective-tissue layer of the skin is known to receive many cells from the somatic mesoderm generally, and from the diffuse mesenchyme in the cephalic region where there are no somites. The term dermatome is so firmly fixed that it is probably unwise to attempt to discard it, but we should bear in mind that while it does contribute to the dermis it probably does not do so any more extensively than other regions of the mesoderm which lie in close proximity to the ectoderm.

The third region of the somite is the so-called sclerotome consisting of cells which migrate ventro-mesially from the original compact mass

Fig. 42. Drawings ( X 120) of transverse sections of pig embryos of various ages to show formation and early differentiation of somites. (From series in the Carnegie Collection.) A, Beginning of somite formation. B, 7-somite embryo. C, 1 6-somitc embryo. D, 30-somite embryo.

(Fig. 42, G, D). These cells become concentrated about the neural tube and notochord, eventually giving rise to the vertebrae.

The Intermediate Mesoderm

There are, even at this early stage of development, changes becoming apparent in the intermediate mesoderm which foreshadow the formation of the embryonic excretory organs known as the mesonephroi. But the differentiation of other closely related parts of the urogenital system is as yet very slight. It therefore seems advisable to postpone consideration of the system as a whole and dismiss these early changes with a word of comment.

The mesonephroi are paired excretory organs conspicuous in young embryos but becoming rudimentary later in development. The name mesonephros (middle kidney) implies the existence in the vertebrate group of a more cephalic and a more caudal kidney. The cephalic kidney (pronephros) is vestigial in mammalian embryos. The caudal kidney (metanephros) becomes the permanent kidney of the adult.

Transverse sections of pig embryos with 16 or 17 somites show the mesonephric ducts as cords of cells arising on either side of the body where somatic and splanchnic mesoderm merge into the intermediate mesoderm (Fig. 38, F). Just mesial to the mesonephric duct, cells from the intermediate mesoderm become aggregated into a solid mass known as the nephrogenic cord (Fig. 38, F). Where development has progressed somewhat farther the nephrogenous cord can be seen to have given rise to a series of hollow vesicles, the primordial mesonephric tubules (Figs. 38, E, 39, and 40). These tubules soon make connection with the mesonephric duct, becoming at the same time much elongated and tortuous (Fig. 41, G). Meanwhile the mesonephric duct ^becomes patent and establishes an outlet into the hind-gut.

Somatic and^«£ipkHEielmieLay«mof the Lateral Mesoderm. Very early in development the lateral mesoderm becomes split into somatic and splanchnic layers with the coelom between (Figs. 22, 26, and 42). As long as the somites and the intermediate mesoderm remain undifferentiated the place of transition from somatic to splanchnic mesoderm is quite obvious and definite (Fig. 42, B, C). After the intermediate mesoderm has become organized into nephric tubules and no longer connects the somite with the lateral mesoderm, the line of demarcation between splanchnic and somatic layers is less readily determined. If the location of the mesonephric tubules is taken as a landmark, however, the point at which somatic and splanchnic mesoderm become continuous may be established with sufficient definiteness for all practical purposes (Fig. 38, E). Since the mesonephros grows primarily ventro-mesially, it is the splanchnic mesoderm which is pushed out to cover the mesonephros as it increases in size (cf. Fig. 38, E, with 41, G). At this stage the point of transition from somatic to splanchnic mesoderm can be approximated as the place at which the mesodermal lining of the coelom bends sharply from the inner face of the body-wall to be reflected over the lateral surface of the mesonephros.

Similarly in the cardiac region somatic mesoderm ends at the angles of the coelom on either side of the pharynx (Fig. 38, C). The splanchnic mesoderm covers the entodermal wall of the pharynx, forms the dorsal mesocardium, and is reflected to form the outer covering of the heart wall. It is this characteristic relationship of the two layers which accounts for their names. The somatic mesoderm lines the body-wall (somatic) face of the coelom. The splanchnic mesoderm forms the supporting membranes (denoted by the prefix meso-, e.g., meso-cardium, meso-gaster) of organs suspended in the coelom and covers the visceral (splanchnic) surfaces which project into the coelom.

The Circulatory System

The mammalian embryo, having practically no yolk available as food, is dependent for its survival and growth on the prompt establishment of relations with the circulation of the mother. This implies the necessity of a precocious development of the vascular system in the embryo, for the maternal circulation remains confined within the uterine walls, and the embryonic circulation must grow to it. Until this is accomplished the embryo is dependent on what food material it can obtain by direct absorption from the fluid within the uterine cavity — a method entirely inadequate to provide for the growth of the embryo except in its very early stages when its bulk is inconsiderable.

The Heart. In mammalian embryos the heart arises from paired primordia situated ventro-laterally beneath the pharynx. The fact that the heart, a median unpaired structure in the adult, arises from paired primordia which at first lie widely separated on either side of the mid-line is likely to be troublesome unless its significance is understood at the outset. The paired condition of the heart at the time of its origin is correlated with the fact that the embryonic body at first lies spread out flat on the surface of the blastocyst. The primordia of certain anatomically ventral structures arising at an early stage of development, therefore, first appear as separate halves lying on either side of the mid-line. With the folding under of the lateral margins of the embryonic area which brings the lateral walls of the body into their definitive position, the embryo is closed ventrally, and potentially median structures which arose as separate halves are established in the mid-line.

Fig. 43. Sections cut transversely through the cardiac region of pig embryos of various ages to show the origin of the heart from paired primordia. (Projection diagrams X 50, from series in the Carnegie Collection.) A, 5-somite embryo. B, 7-somite embryo. C, 10-somite embryo. D, 13-somite embryo.

The primordial heart is double-layered, as well as paired right and left. The inner layer is called the endocardium because it is destined to form the internal lining of the heart. The outer layer is known as the epi-myocardium because it will give rise both to the heavy muscular layer of the heart wall (myocardium) and to its outer covering (epicardium).

The endocardium appears first in the form of irregular clusters and cords of mesenchymal cells lying between the splanchnic mesoderm and the entoderm. These cells become organized into two main strands lying one on either side of the gut. Soon after their establish contact and fuse with each other. This fusion occurs in such a manner that the limbs of the mesodermic folds next to the endocardium fuse with each other forming an outer layer of the heart no longer interrupted ventrally; and so that the limbs of the folds which line the pericardial coelom ventrally also fuse with each other to form an unbroken layer (Fig. 43, B, C). Thus the originally paired right and left coelomic chambers become confluent to form a median unpaired pericardial cavity in the same process which establishes the heart as a median structure. Dorsally the right and left epi-myocardial layers become contiguous, but here they do not fuse immediately as happens ventral to the heart. They persist for a time as a double-layered supporting membrane called the dorsal mesocardium. In this manner the heart is established as a nearly straight, double-walled tube suspended mesially in the most cephalic part of the coelom.

Blood Vessels

While these changes have been occurring in the cardiac region, the main vascular channels characteristic of young embryos are making their appearance. The cephalic prolongations of the endocardial tubes beyond the region in which the heart itself is formed constitute the start of the main efferent channels or aortae. The aortae are further extended by a process entirely similar to that involved in the formation of the endocardial tubes themselves. Cords and knots of cells of mesodermal origin^ become aggregated along the course of the developing vessel. These strands of cells are then hollowed out to form tubes walled by a single layer of thin, flattened cells (endothelial cells). Where the main blood vessels are about to become established there is found first a meshwork of these small channels with their delicate endothelial lining. Gradually some of these primitive channels are enlarged and straightened to form the main vessels and their walls are reinforced by the addition of circularly disposed connective-tissue fibers and smooth muscle cells. In this manner the

  • By some writers these cells are called mesenchymal because of their similarity to the rest of the mesenchyme in their mesodermal origin, in their sprawling shapes, and in their migratory proclivities. By others they are called angioblasts (collectively ‘Hhe angioblast”) because of the part they play in the formation of blood vessels. Unfortunately, because of a long-standing controversy, the very appropriate term angioblast carries with it the special connotation that all cells of this type originate from a common center and migrate thence to all places where vascular endothelium is formed. (This is the so-called “angioblast theory” of the origin of vascular endothelium.) Recent experimental evidence indicates that cells with vasculogenctic potentialities arise in many places by direct differentiation from local mesoderm.

Fig. 45. Schematic plan of the circulation in a young pig embryo. At this stage all the blood vessels are paired (right and left), the entire circulatory system except for the heart being bilaterally symmetrical. Only the vessels on the side toward the observer are shown in the figure.

primitive efferent channels are prolonged from the heart cephalad beneath the pharynx as the ventral aortae. They then bend laterad and dorsad about the pharyngeal walls to form the aortic arches^ and finally turn caudad to extend nearly the entire length of the embryo as the dorsal aortae (Fig. 45).

At first there is but a single pair of aortic arches which is located in the tissue of the first branchial arch (mandibular arch). Later in development five additional pairs of arches connecting the ventral and dorsal aortae are formed. Each of these aortic arches lies in one of the branchial arches caudal to the mandibular (Figs. 41, B, C, and 47). At present we are not interested in the history of individual aortic arches but merely in the fact that blood passes by way of one or more pairs of aortic arches around the pharynx from the ventrally located heart to the dorsally located aortae which form the main distributing trunks of the embryonic circulation (Fig. 45).

The vessels serving to collect the blood which is distributed to all parts of the embryo by branches from the aortae are called the cardind veins. They arise somewhat later than the aortae but by an entirely similar process. There are two pairs of these vessels, the anterior cardinal veins draining the cephalic, and the posterior cardinal veins draining the caudal region of the body. Under favorable conditions the position of the cardinal veins can be made out in lightly stained and cleared entire embryos (Fig. 39). Their relations are best seen, however, in parasagittal sections (Fig. 47). At the level of the heart the anterior and the posterior cardinal veins on either side of the body unite as the common cardinal veins (ducts of Cuvier) (Figs. 45 and 47). The common cardinals are short trunks which at once turn ventro-mesiad and become confluent with the omphalomesenteric veins in the sinus venosus. The sinus venosus in turn discharges into the atrial part of the heart. The entrance of the sinus venosus into the atrium is guarded by a pair of delicate flaps called the valvulae venosae (Fig. 46).

In addition to the vessels limited in their distribution to the body of the embryo, there are conspicuous channels leading beyond the confines of the body to the yolk-sac and to the allantois. The main arteries from the aorta to the yolk-sac are called the omphalomesenterics and their terminal branches the vitellines. The main vessels leading to the allantois are known as the allantoic arteries^ or, especially in mammals, as the umbilical arteries.

The aflferent channels which lead from the yolk-sac to the heart are the omphalomesenteric veins. Near the heart these vessels arise as extra-cardiac continuations of the endocardial tubes (Fig. 44). The fusion which takes place in the heart does not at this stage as yet involve the omphalomesenteric veins and they enter the sinus venosus as paired channels.

Distally the omphalomesenteric veins are extended along the walls of the gut toward the yolk-sac. In the splanchnopleure of the yolk-sac the main omp'halomesenteric vessels are continuous with a rich plexus of small tributaries, the vitelline vessels (Fig. 45). These smaller blood vessels can be traced into prevascular cords of mesodermal cells as yet not hollowed out. In these cellular cords are frequent knot-like enlargements, known as blood islands, containing not only cells which are destined to form the endothelium of blood vessels but also cells which will give rise to blood corpuscles.

Blood Islands. In the difierentiation of a blood island the peripherally located cells become flattened and somewhat separated from the central cells (Fig. 48, B), Then they arrange themselves as a coherent layer a single cell in thickness, which invests the remaining cells of the island (Fig. 48, C). Meanwhile similar changes have been occurring in the neighboring islands and, with their progressive enlargement, adjacent blood islands coalesce (Fig. 48, C). Continuation of this process results in the transformation pf the original endothelial vesicles into a plexus of anastomosing endothelial tubes, the primordial capillary bed of the yolk-sac.

Meanwhile fluid has accumulated within the endothelial vesicles and the central cells have become rounded, forming primitive blood corpuscles (Fig. 48, C). When these vesicles with their contained

Fig. 46. Drawing of parasagittal section of 5.5 mm. pig embryo. (Projection outlines X 22.) The section was taken from the series to the right of the mid-line, at a plane favorable for showing the entrance of the sinus venosus into the right atrium.

corpuscles have become confluent to form capillaries and the capillaries have acquired open communication with the vitelline arteries on the one hand and the vitelline veins on the other, all the conditions necessary for active circulation of blood have been established. Under the pumping action of the heart which begins at this time, the fluid accumulated in the blood islands acts as a vehicle conveying the corpuscles formed in the blood islands of the yolk-sac to all parts of the body.

Fig. 47. Drawing of a parasagittal section of a 5.5 mm. pig embryo. (Projection outlines X 22.) The section was taken from the series to the left of the midrline, at a plane especially favorable for showing the cardinal veins.

Fig. 48. Three stages in the development of blood islands.

A, The primordial blood island is merely a knot of mesodermal cells. (3somite embryo.)

B, The beginning of specialization of peripheral cells to form endothelium and central cells to form corpuscles is clearly indicated. (13-somite embryo.)

C, The upper island has a complete endothelial wall which has become continuous with that of neighboring vascular channels as shown by the fact that most of the corpuscles formed within it have moved out into the blood stream. The lower island has not yet acquired a complete endothelial wall nor open communication with neighboring vessels, and the corpuscles formed within it still densely pack its lumen. (20-somite embryo.) All figures drawn from yolk-sac blood islands of pig embryos in the Carnegie Collection.

Changes similar to those described for the yolk-sac occur in the allantois so that, by the time the circulation of blood actually begins, there is a rich plexus of small vessels in the allantois. This plexus is fed by the allantoic branches of the aortae and blood is returned from it to the heart by way of a pair of large veins, the allantoic (umbilical) veins. These vessels traverse the allantoic stalk and the lateral bodywall, emptying into the posterior end of the heart along with the omphalomesenteric and the common cardinal veins (Fig, 45).

The circulatory system of a mammalian embryo at this stage in its development can be analyzed into three distinct sets of afferent and efferent channels. Each set of main channels with its interpolated capillary bed can be conveniently designated as a circulatory arc. One of these is known as the intra-embryonic arc because it consists of vessels which lie wholly within the body of the embryo. The blood, pumped by the heart, is distributed to all parts of the embryo over the aortae. Small branches from the aortae break up loeally in various parts of the body into capillaries, thus bringing the blood into intimate relation with the developing tissues. The blood is then collected by the cardinal veins and returned by way of the ducts of Cuvier to the heart.

The other two arcs are, the vitelline which runs to the yolk-sac, and the allantoic or umbilical to the allantois (Fig. 45). Both these arcs start within the embryo, for the heart serves as a common receiving and pumping station, and the aortae as a common distributing main for all three of the circulatory arcs. But because their main vessels extend outside the body with their terminal ramifications in the extra-embryonic membranes, these latter arcs are ordinarily spoken of as extra-embryonic.

Later we shall trace the origin of the main systemic vessels of the adult from the primitive channels of the intra-embryonic circulation. We shall see the vitelline arc transformed into the hepatic-portal circulation, and the allantoic arc highly developed to become the placental circulation. It is, therefore, of prime importance that the primitive ground plan of the circulation as graphically summarized in figure 45 be fixed clearly in mind as a basis on which to build.

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