Book - Embryology of the Pig 6
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Chapter 6. The Extra-Embryonic Membranes and the Relation of the Embryo to the Uterus
I. The Formation of the Extra-embryonic Membranes
In dealing with the establishment of the embryonic body we have already seen that the germ layers extend far beyond the region in which the embryo itself develops. These peripheral portions of the germ layers give rise to membranes which are of service as a means of protection and as a means of securing food from the blood of the mother. Because they are not incorporated in thd body of the embryo but discarded at the time of birth, they are called extra-embryonic membranes. These membranes are the yolk-sac, the amnion, the serosa, and the allantois. Their designation as extra-embryonic should not lead us to lose sight of their vital importance to the embryo during its intra-uterine existence.
In mammals, although there is virtually no yolk accumulated in the ovum, a large yolk-sac is formed just as if yolk were present. Such persistence of a structure, in spite of the loss of its original function, is not an uncommon phenomenon in evolution and has given rise to the biological aphorism that â€˜â€˜morphology is more conservative than physiology.â€ Not only, does the yolk-sac itself persist, even the blood vessels characteristically associated with it in its functional condition appear in the empty yolk-sac of mammalian embryos (Fig. 45). When the mammalian yolk-sac is spoken of as a vestigial structure, therefore, we must bear in mind that such a statement has reference primarily to its function. Morphologically it is of considerable size in young embryos.
The yolk-sac may be defined as that part of the primitive gut which is not included within the body when the embryo is â€œfolded off.â€ In dealing with the formation of the digestive tube we have already sufficiently considered the processes which thus establish the boundaries between the yolk-sac and the intra-cmbryonic portion of the original gut cavity (Fig. 37). The peripheral extent of the yolk sac is unusually great in young pig embryos correlated with enormous elongation of the blastocyst (Fig. 18). From the yolk-st^lk, the yolk-sac extends beneath the embryo, beyond its head in one direction and beyond its tail in the other, nearly to the ends of the blastocyst (Fig. 49, A).
Fig. 49. Schematic diagrams showing the relations of the extra-embryonic membranes. The great length of the chorionic vesicle of the pig precludes showing the entire vesicle in correct proportions. In A, sections of the vesicle have been omitted. In B and C, the vesicle has been shortened from its true proportions.
A represents conditions in embryos of 15-20 somites; B, conditions in embryos of 4-6 mm. (after flexion); and C, conditions in embryos of 30 mm. or more.
During this, the period of its greatest relative development, the yolk-sac serves as an organ purveying nutritive material to the embryo. Separated from the uterine wall only by the thin outer layer of the blastocyst, its abundant vessels are in a position which maker absorption from the uterus readily possible. The food material and oxygen thus absorbed are transported by way of the vitelline circulation (Fig. 45) to the growing embryo. When, somewhat later, the allantois becomes highly developed it takes over this function and the yolk-sac decreases rapidly in size (Fig. 49, B), finally becoming a shriveled sac buried in the belly-stalk (Figs. 49, C, and 65). The yolk-sac, therefore, exhibits another point of special interest in addition to being an organ which persists after the cessation of its original function. The persistent yolk-sac and its associated vessels are utilized by the embryo in a new manner. Containing no stored food materials, the mammalian yolk-sac turns about, so to speak, and absorbs from the uterus through its external surface. The fact that the function is temporary, later being taken over by the allantois, makes this physiological opportunism in seizing on a new source of supplies none the less striking.
The amnion arises as a layer of somatopleure which enfolds the developing embryo. When the amniotic sac is completed it becomes filled with a watery fluid in which the embryo is suspended. This suspension of the embryo in a fluid-filled sac, by equalizing the pressure about it, serves as a protection against mechanical injury. At the same time the soft tissues of the growing embryo, being bathed in fluid, do not tend to form adhesions with consequent malformations. The functional significance of the amnion is emphasized by the fact than an amnion appears only in the embryos of non-water-living forms. This has led to designating the embryos of fishes and Amphibia, which form no amnion, as â€˜â€˜anamniotes,â€ and those of birds, reptiles, and mammals, which do form an amniotic sac during development, as â€˜"amniotes.â€
In all mammalian embryos the amnion is formed at an exceedingly early stage of development. In some forms (e.g., man) the amniotic cavity appears even before the body of the embryo has taken definite shape. When the amnion is thus precociously established the processes in its formation are, as it were, hurried through and consequently they are difficult to analyze. In the pig the amnion is formed in more leisurely fashion, the process being quite similar to that in reptiles and birds.
The first indication of amnion formation in pig embryos becomes evident shortly after the primitive streak stage. The embryonic disk appears to settle into the blastocyst so that it is overhung on all sides by the extra-embryonic somatopleure (Figs. 25 and 26). As the embryo grows in length its head and tail push deeper and deeper into these folds of somatopleure (cf. Fig. 37, B, C). At the same time the folds themselves become more voluminous, growing centripetally over the embryo from the region of its head, its sides, and its tail. These circumferential folds of somatopleure are known as the amniotic folds. For convenience in description we recognize cephalic, caudal, and lateral regions of the amniotic folds, although in reality they are all directly continuous with each other.
Progress of the amniotic folds soon brings them together above the mid-dorsal region of the embryo to complete the amniotic sac (Figs. 37, C, and 49). Where the amniotic folds close, there persists for a time a cord-like mass of tissue between the amnion and the outer layer of the blastocyst (Fig. 49, B). In time even this trace of the fusion is obliterated and the embryo in its amnion lies free in the blastocyst (Fig. 49, C).
The amnion is attached to the body of the embryo where the bodywall opens ventrally in the region of the yolk-stalk (Fig. 50). As development advances this ventral opening becomes progressively smaller, its margins being known finally as the umbilical ring. Meanwhile the yolk-stalk and the allantoic stalk, in the same process of ventral closure, are brought close to each other, to constitute, with their associated vessels, the belly-stalk (Fig. 49, C). The amnion is now reflected from its point of continuity with the skin of the body at the umbilical ring, to constitute the covering of the belly-stalk, before it is recurved as a free membrane enclosing a fluid-filled space about the embryo.
The Serosa. The outer layer of the mammalian blastocyst has various names. In the inner-celFmass stage, the outer layer of the blastocyst is most commonly called the trophoblast (Fig. 15, C). Since the primary germ layers are not at this time differentiated, this term is far more appropriate than ectoderm, which implies a layer having relations which make it stand in contrast with entoderm, or with entoderm and mesoderm. The premature use of the term ectoderm would seem to have nothing to recommend it, and has been responsible for much confusion in thinking and much pointless discussion.
Fig. 50. Drawing (X 12) of pig embryo having about 28 pairs of somites (crown-rump length 4 mm.; age approximately 17 days). The embryo has been removed from the chorionic vesicle with the amnion and allantois intact but the distal portion of the yolk-sac has been cut off. (Cf. Fig. 49, .)
When the germ layers have been differentiated, the outer layer of the blastocyst is directly continuous with the ectoderm of the embryo (Fig. 19). There is then justification for calling it more specifically trophectoderm. The prefix troph- (Greek root, to nourish) suggests the part this layer plays in the acquisition of food materials from the uterus and at the same time sets it off from the ectoderm of the embryonic disk.
Still later in development when the mesoderm splits and its somatic layer becomes associated with the ectoderm, the name trophectoderm is no longer appropriate (Fig. 22). Three different names are in common use to cover this double layer. Extra-embryonic somatopleure is a term following the general embryological usage in designating by the suffix -pleure the double layers formed by the secondaiy association of somatic and splanchnic mesoderm with ectoderm and entoderm respectively. Trophoderm is widely used by those working especially in mammalian embryology. Those especially interested in comparative embryology are quite likely to use the term serosa for this layer, thus emphasizing its very evident homology with the part of the extra-embryonic somatopleure so designated in birds and reptiles. This multiplicity of names is not difficult to master if we understand they are all appropriate terms selected to emphasize somewhat different aspects of the same structure. Trophoblast, then, means the outer layer of the blastocyst before ectoderm, entoderm, and mesoderm are differentiated. Trophectoderm is the extra-embryonic part of the ectoderm when the germ layers have been differentiated, and trophoderm (serosa) (extra-embryonic somatopleure) is trophectoderm reinforced by a layer of somatic mesoderm.
In the formation of the amnion, the somatopleure is thrown into folds surrounding the embryo. The inner limb of the fold, as we have seen, becomes the amnion (Fig, 37, C). The outer limb of the fold is commonly called the serosa. It is, perhaps, in just this designation of the outer limb of the amniotic fold that the term serosa is most often encountered. After the amnion has been completed, interest in this outer layer centers about its role in food absorption and the term trophoderm is most commonly used to designate it.
Almost as soon as the hind-gut of the embryo is established, there arises from it a diverticulum known as the allantois (Fig. 37). The allantoic wall, because of its manner of origin, is necessarily composed of splanchnopleure. As the allantois increases in Gxtent its original communication with the hind-gut is narrowed to a cylindrical stalk, while its distal portion becomes enormously dilated (Figs. 37 and 49).
Externally the developing allantois appears at first as a crescentic enlargement directly continuous with the caudal part of the embryo (Figs. 29 and 30). Its dilated distal portion continues to grow with great rapidity until it is a large semilunar sac free of the caudal end of the embryo except for the stalk by which it maintains its original connection with the hind-gut (Fig. 50).
The allantois early acquires an abundant blood supply by way of large branches from the caudal end of the aorta (Figs. 45 and 51). In the allantoic wall these arteries break up into a maze of thin-walled vessels. As the yolk-sac circulation undergoes retrogressive changes, this plexus of allantoic vessels becomes progressively more highly developed and soon takes over entirely the function of metabolic interchange between fetus and mother.
Fig. 51. Drawing showing the main vascular channels in a pig embryo of about 17 days (27 pairs of somites). Based on cleared preparations of injected embryos loaned by Dr. C. H. Heuscr, and on Sabinâ€™s figures of similar material.
II. The Relation of the Embryo and Its Membranes to the Uterus
Having seen the manner of origin and the relations of the various extra-embryonic membranes to the embryo, we must now turn our attention to their relations to the uterus. It will be recalled that the ova are fertilized in the uterine tube shortly after their discharge from the ovary. It takes from three to four days for the ova to pass through the uterine tube so that by the time the embryos reach the uterine cavity the cleavage divisions are well under way (Figs. 14, C, D, and 15, A).
Fig. 52. Uterus of pregnant sow opened to show distribution of embryos. (After Corner.) Note the approximately uniform spacing of the embryos in the two horns of the uterus in spite of the fact that the corpora lutea indicate the origin of two ova from one ovary and seven from the other.
Fig. 53. Schematic diagrams indicating the intra-uterine relations of the embryos and their membranes: A, in the elongated blastocyst stage; B, when the blastocysts have been dilated to form the chorionic vesicles characteristic of the later stages of pregnancy.
Fig. 54. Drawing of pig embryo in unruptured chorionic vesicle. (After Grosser.) Compare with figures 49, C., and 53, B.
The Spacing of Embryos in the Uterus
Normally, of course, some of the ova fertilized were formed in one ovary and some in the other. The corpora lutea which develop from the ruptured ovarian follicles leave unmistakable evidence of the number of ova contributed by each ovary. Usually the numbers are approximately equal, but it not infrequently happens that, at a given estrus, one ovary is far more prolific than the other. If all the ova originating on one side remained in the corresponding horn of the uterus there would be crowding of the embryos on one side and unutilized space on the other. Such a condition does not ordinarily occur. The mechanism by which the embryos are arranged within the uterus is not known, but the careful observations of Comer show that they do tend to become uniformly distributed in the two horns of the uterus regardless of the side on which the ova were liberated (Fig. 52).
This spacing of the embryos apparently takes place before the elongation of the blastocysts occurs. Even when elongated, the blastocysts do not spread over as much space in the uterus as one might suppose. The lining of the uterus is extensively folded and the threadlike blastocysts follow along these folds, so that a blastocyst a meter long will occupy perhaps no more than 10-15 cm. of a uterine horn (Fig. 53, A).
The attenuated condition of the blastocyst does not persist long. With the increase in the extent of the allantois and growth of the embryo, the blastocyst becomes greatly dilated and somewhat shortened. Thus altered in shape and appearance it is commonly called a chorionic vesicle. The allantois never grows to quite the entire length of the chorionic vesicle and there remains an abrupt narrowing at either extremity of the vesicle where the allantois ends (Figs. 49, C, and 54).
Where these undeveloped terminal portions of neighboring chorionic vesicles lie close to each other the uterus remains undilated, sharply marking off the region where each embryo is located (Fig. 53, B). One of the local enlargements of the uterus containing an embryo and its membranes is known as a loculus.
While these changes have been occurring in the general relations of the embryo and its membranes to the uterus there have been further specializations of the fetal membranes themselves. In its peripheral growth the allantois comes into contact with the trophoderm (extra-embryonic somatopleure) (serosa) and becomes fused with it. The new layer thus formed by the fusion of allantoic splanchnopleure with extra-embryonic somatopleure is the chorion (Fig. 49, C). Lying in intimate contact with the uterine mucosa externally, and having in its double mesodermal layer the rich plexus of allantoic vessels communicating with the fetal circulation, it is well situated to carry on the functions of metabolic interchange between the fetus and the uterine circulation of the mother.
In many mammals the chorion becomes fused with the uterine mucosa. In the pig, although the chorion and the uterine mucosa lie in close contact with each other, they can always be peeled apart (Fig. 57, A). When the uterine mucosa and the chorion do not actually grow together and can readily be separated from one another without the tearing of either, they are said to constitute a contact placenta (semi-placenta). Most mammals show a more highly specialized condition in which the fetal (chorionic) and maternal (mucosal) portions of the placenta actually grow together so they cannot be pulled apart without hemorrhage. So intimately do they become fused in fact, that when, shortly following the birth of the fetus itself, the extra-embryonic membranes are delivered as the so-called ^^after-birth,â€ a large part of the uterine mucous membrane is pulled away with the chorion. In contrast with the primitive contact type such a placenta is called a burrowing or â€œtrueâ€ placenta. An understanding of placental relationships, in such a form as the pig where all the component parts are clearly identifiable furnishes the best possible background for interpreting the more complex types of placentae where there are extensive fusions between fetal and maternal portions.
Fig. 55. Semischernatic diagram showing the structure of the chorion and its relation to the uterine wall. Reference to figure 49, C, will show the general relations of the region here depicted. The area indicated in this figure by the rectangle is shown in detail in figure 56.
Fig. 56. Detailed drawing of small area of placenta of 27 mm. pig embryo showing the structure of its chorionic and uterine portions where they lie in contact. (After Grosser.) For relations of area drawn see figure 55.
Fig. 57. A, Small portion of pig placenta drawn about natural size, to show the manner in which the chorion may readily be separated from the uterine mucosa without injury to either layer. (After Grosser.)
B, Small area of chorion of 19 cm. pig embryo showing its uterine surface magnified. (After Grosser.) For location of area depicted see rectangle in A. Note the minute irregular local elevations (chorionic villi of primitive type) which are superimposed on the major folds shown in A, further increasing the surface of contact between chorion and uterine mucosa.
C, Semischematic diagram of section in region of areola. (After Zeitzschmann.) The areolae fit into depressions in the uterine mucosa near the orifice of uterine glands. The villi of the areolae are somewhat more highly developed than those of the general chorionic surface and are believed to be especially active in absorbing nutritive materials from the uterine secretions.
Even in its fully developed condition the chorion of the pig is a relatively simple structure. The entodermal lining of the allantoic cavity becomes much reduced in thickness, forming a delicate singlelayered epithelial covering on the internal face of the chorion. The mesoderm of the allantoic wall fuses With that of the overlying trophoderm and this combined layer becomes differentiated into a primitive type of gelatinous connective tissue (mucoid connective tissue) (Fig. 55). In this connective-tissue layer the blood vessels of the allantoic plexus are located. With the fusion of the allantoic mesoderm to the trophoderm these vessels invade the trophoderm and their terminal arborizations come to lie close to the trophectoderm (Fig. 55). The trophectoderm becomes a highly developed simple columnar epithelium with its surface exposure enormously increased by abundant foldings.
This plicated epithelial surface of the chorion follows the folds in the surface of the uterine mucosa so that the chorionic and the uterine epithelium lie in contact with each other except for a thin film of uterine secretion (Fig. 56). Under the uterine epithelium is a loosely woven layer of connective tissue richly supplied with maternal vessels. Thus there intervenes between fetal and maternal vessels only a scanty amount of connective tissue and the epithelial layers of the uterine mucosa and of the chorion. ^ Through these layers food materials and oxygen are passed on from maternal to fetal vessels and the waste products of metabolism are absorbed from the fetal blood stream by the maternal blood stream. What part the epithelial layers may play in this transfer is uncertain. It seems not unlikely that the chorionic epithelium is active in absorption as is the case with the yolk-sac epithelium in Sauropsida. Whether or not this is the case it certainly constitutes no serious impediment to the reciprocal transfer of materials between fetus and mother which is the function of the placenta.
^ Because the fetal and maternal vessels both retain the integrity of their endothelium, and because both chorionic and uterine epithelium persist, a placenta of this type may be designated technically epitheliochorial. The term is used by students of comparative placentation in contrast with endotheliochorial which designates a type of placenta such as that occurring in the cat or dog where the uterine epitheliuln is lost but the endothelium of the uterine vessels is retained; and in contrast also with the term hemochorial which designates the type of placenta occurring in man and a number of other mammals in which maternal blood comes in direct contact with chorionic tissue without the intervention of either uterine epithelium or the endothelium of maternal vessels.
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Cite this page: Hill, M.A. (2021, May 14) Embryology Book - Embryology of the Pig 6. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Book_-_Embryology_of_the_Pig_6
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