Book - The Elements of Embryology - Mammalian 2
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Embryonic Membranes and Yolk-Sac
IN the Mammalia the early stages in the development of the embryonic membranes are nearly the same as in Aves ; but during the later stages the allantois enters into peculiar relations with the uterine walls, and the two, together with the interposed portion of the sub zonal membrane or false amnion (the nature of which will be presently described), give rise to a very characteristic Mammalian organ the placenta into the structure of which it will be necessary to enter at some length. The embryonic membranes vary so considerably in the different forms that it will be advantageous to commence with a description of their development in an ideal case.
We may commence with a blastodermic vesicle closely invested by the delicate remnant of the zona radiata at the stage in which the medullary groove is already established. Around the embryonic area a layer of mesoblast would have extended for a certain distance ; so as to give rise to an area vasculosa, in which however the blood-vessels would not have become definitely established. Such a vesicle is represented diagrammatically in Fig. 114, I. Somewhat later the embryo begins to be folded off first in front and then behind (Fig. 114, 2). These folds result in a constriction separating the embryo and the yolk-sac (ds), or as it is called in Mammalian embryology, the umbilical vesicle. The splitting of the mesoblast into a splanchnic and a somatic layer has taken place, and at the front and hind end of the embryo a fold (ks) of the somatic mesoblast and epiblast begins to rise up and grow over the head and tail of the embryo. These two folds form the commencement of the amnion. The head and tail folds of the amnion are continued round the two sides of the embryo till they meet and unite into a continuous fold. This fold grows gradually upwards, but before it has completely enveloped the embryo the blood-vessels of the area vasculosa become fully developed. They are arranged in a manner not very different from that in the chick.
The following is a brief account of their arrangement in the rabbit :
- The outer boundary of the area, which is continually extending further and further round the umbilical vesicle, is marked by a venous sinus terminalis (Fig. 114, st). The area is not, as in the chick, a nearly complete circle, but is in front divided by a deep indentation extending inwards to the level of the heart. In consequence of this indentation the sinus terminalis ends in front in two branches, which bend inwards and fall directly into the main vitelliue veins. The blood is brought from the dorsal aortse by a series of lateral vitelline arteries, and not by a single pair as in the chick. These arteries break up into a more deeply situated arterial network, from which the blood is continued partly into the sinus terminalis, and partly into a superficial venous network.The hinder end of the heart is continued into two vitelline veins, each of which divides into an anterior and a posterior branch. The anterior branch is a limb of the sinus terminalis, and the posterior and smaller branch is continued towards the hind part of the sinus, near which it ends. On its way it receives, on its outer side, numerous branches from the venous network. The venous network connects by its anastomoses, the posterior branch of the vitelline vein and the sinus terminalis.
|Fig. 114. Five diagrammatic figures illustrating the formation of the foetal membranes of a mammal. (From Kolliker.)
Shortly after the establishment of the circulation of the yolk-sac the folds of the amnion meet and coalesce above the embryo (Fig. 114, 3 and 4, am). After this the inner or true amnion becomes severed from the outer or false amnion, though the two sometimes remain connected by a narrow stalk. The space between the true and false amnion is a continuation of the body cavity. The true amnion consists of a layer of epiblastic epithelium and generally also of somatic mesoblast, while the false amnion consists as a rule of epiblast only; though it is possible that in some cases (the rabbit ?) the mesoblast may be continued along its inner face.
Before the two limbs of the amnion are completely severed the epiblast of the umbilical vesicle becomes separated from the subjacent mesoblast and hypoblast of the vesicle (Fig. 114, 3), and, together with the false amnion (sh) with which it is continuous, forms a complete lining for the inner face of the zona radiata. The space between this membrane and the umbilical vesicle with the attached embryo is obviously continuous with the body cavity (vide Figs. 114, 4 and 115). To this membrane Turner has given the appropriate name of subzonal membrane : by Von Baer it was called the serous envelope. It soon fuses with the zona radiata, or at any rate the zona ceases to be distinguishable.
While the above changes have been taking place the whole blastodermic vesicle, still enclosed in the zona, has become attached to the walls of the uterus. In the case of the typical uterus with two tubular horns, the position of each embryo, when there are several, is marked by a swelling in the walls of the uterus, preparatory to the changes in the wall which take place on the formation of the placenta. In the region of each swelling the zona around the blastodermic vesicle is closely embraced in a ring-like fashion by the epithelium of the uterine wall. The whole vesicle assumes an oval form, and it lies in the uterus with its two ends free. The embryonic area is placed close to the mesometric attachment of the uterus. In many cases peculiar processes or villi grow out from the ovum (Fig. 114, 4, sz) which fit into the folds of the uterine epithelium, The nature of these processes requires further elucidation, but in some instances they appear to proceed from the zona (rabbit) and in other instances from the subzonal membrane (dog). In any case the attachment between the blastodermic vesicle and the uterine wall becomes so close at the time when the body of the embryo is first formed out of the embryonic area, that it is hardly possible to separate them without laceration ; and at this period from the 8th to the 9th day in the rabbit it requires the greatest care to remove the ovum from the uterus without injury. It will be understood of course that the attachment above described is at first purely superficial and not vascular.
During the changes above described as taking place in the amnion, the allantois grows out from the hindgut as a vesicle lined by hypoblast, but covered externally by a layer of splanchnic mesoblast (Fig. 114, 3 and 4, at)1 . It soon becomes a flat sac, projecting into the now largely developed space between the subzonal membrane and the amnion, on the dorsal side of the embryo (Fig. 115, ALC}. In some cases it extends so as to cover the whole inner surface of the subzonal membrane ; in other cases again its extension is much more limited. Its lumen may be retained or may become nearly or wholly aborted. A fusion takes place between the subzonal membrane and the adjoining mesoblastic wall of the allantois, and the two together give rise to a secondary membrane round the ovum known as the chorion. Since however the allantois does not always come in contact with the whole inner surface of the subzonal membrane the term chorion is apt to be somewhat vague ; in the rabbit, for instance, a considerable part of the so-called chorion is formed by a fusion of the wall of the yolk-sac with the subzonal membrane (Fig. 116). The region of the chorion which gives rise to the placenta may in such cases be distinguished as the true chorion from the remaining part which will be called the false chorion.
(1The hypoblastic element in the allantois is sometimes very much reduced, so that the allantois maybe mainly formed of a vascular layer of mesoblast.)
The mesoblast of the allantois, especially that part of it which assists in forming the chorion, becomes highly vascular ; the blood being brought to it by two allantoic arteries continued from the terminal bifurcation of the dorsal aorta, and returned to the body by one, or rarely two, allantoic veins, which join the vitelline veins from the yolk-sac. From the outer surface of the true chorion (Fig. 114, 5, ch. z, 116) villi grow out and fit into crypts or depressions which have in the meantime made their appearance in the walls of the uterus1. The villi of the chorion are covered by an epithelium derived from the subzonal membrane, and are provided with a connective-tissue core containing an artery and vein and a capillary plexus connecting them. In most cases they assume a more or less arborescent form, and have a distribution on the surface of the chorion varying characteristically in different species. The walls of the crypts into which the villi are fitted also become highly vascular, and a nutritive fluid passes from the maternal vessels of the placenta to the foetal vessels by a process of diffusion; while there is probably also a secretion by the epithelial lining of the walls of the crypts, which becomes absorbed by the vessels of the foetal villi. The above maternal and foetal structures constitute together the organ known as the placenta. The maternal portion consists essentially of the vascular crypts in the uterine walls, and the foetal portion of more or less arborescent villi of the true chorion fitting into these crypts.
|Fig. 115. Diagram of the foetal membranes of a mammal. (From Turner.)
Structures which either are or have been at an earlier period of development continuous with each other are represented by the same character of shading.
While the placenta is being developed the folding off of the embryo from the yolk-sac becomes more complete; and the yolk-sac remains connected with the ileal region of the intestine by a narrow stalk, the vitelline duct (Fig. 114, 4 and 5 and Fig. 115), consisting of the same tissues as the yolk-sac, viz. hypoblast and splanchnic mesoblast. While the true splanchnic stalk of the yolk-sac is becoming narrow, a somatic stalk connecting the amnion with the walls of the embryo is also formed, and closely envelopes the stalk both of the allantois and the yolk-sac.
(1These crypts have no connection with the openings of glands in the walls of the uterus. They are believed by Ercolani to be formed to a large extent by a regeneration of the lining tissue of the uterine walls.)
The somatic stalk together with its contents is known as the umbilical cord. The mesoblast of the somatopleuric layer of the cord develops into a kind of gelatinous tissue which cements together the whole of the contents. The allantoic arteries in the cord wind in a spiral manner round the allantoic vein. The yolk-sac in many cases atrophies completely before the close of intra-uterine life, but in other cases it, like the other embryonic membranes, is not removed till birth. The intra-embryonic portion of the allantoic stalk gives rise to two structures, viz. to (1) the urinary bladder formed by a dilatation of its proximal extremity, and to (2) a cord known as the urachus connecting the bladder with the wall of the body at the umbilicus. The urachus, in cases where the cavity of the allantois persists till birth, remains as an open passage connecting the intra- and extra-embryonic parts of the allantois. In other cases it gradually closes, and becomes nearly solid before birth, though a delicate but interrupted lumen would appear to persist in it. It eventually gives rise to the ligamentum vesicae medium.
At birth the foetal membranes, including the foetal portion of the placenta, are shed ; but in many forms the interlocking of the foetal villi with the uterine crypts is so close that the uterine mucous membrane is carried away with the foetal part of the placenta. It thus comes about that in some placentae the maternal and foetal parts simply separate from each other at birth, and that in others the two remain intimately locked together, and both are shed together as the after-birth. These two forms of placenta are distinguished as nondeciduate and deciduate, but no sharp line can be drawn between the two types. Moreover, a larger part of the uterine mucous membrane than that actually entering into the maternal part of the placenta is often shed in the deciduate Mammalia, and in the non-deciduate Mammalia it is probable that the mucous membrane (not including vascular parts) of the maternal placenta is either shed or absorbed.
Comparative history of the Mammalian foetal membranes
Two groups of Mammalia the Monotremata and the Marsupialia are believed not to be provided with a true placenta. Nothing is known of the arrangement of the foetal membranes in the former group of animals (Monotremata). In the latter (Marsupialia) the yolksac is large and vascular, and is, according to Owen, attached to the subzonal membrane. The allantois on the other hand is but small, and is not attached to the subzonal membrane; it possesses however a vascular supply.
Observations have hitherto been very limited with regard to the foetal membranes of this group of animals, but it appears highly probable that both the yolk-sac and the allantois receive nutriment from the walls of the uterus.
All Mammalia other than the Monotremata and Marsupialia have a true allantoic placenta. The placenta presents a great variety of forms, and we propose first to treat the most important of these in succession, and then to give a general exposition of their mutual affinities.
The discoidal placenta is found in the Rodentia, Insectivora, and Cheiroptera. The Rabbit may be taken as an example of this type of placenta.
In the pregnant female Rabbit several ova are generally found in each horn of the uterus. The general condition of the foetal-membranes at the time of their full development is shewn in Fig. 116.
The embryo is surrounded by the amnion, which is comparatively small. The yolk-sac (ds) is large and attached to the embryo by a long stalk. It has the form of a flattened sac closely applied to about two-thirds of the surface of the subzonal membrane. The outer wall of this sac, adjoining the subzonal membrane, is formed of hypoblast only ; but the inner wall is covered by the mesoblast of the area vaaculosa, as indicated by the thick black line (fd). The vascular area is bordered by the sinus terminalis (st). In an earlier stage of development the yolk-sac had not the compressed form represented in the figure. It is, however, remarkable that the vascular area never extends over the whole yolk-sac ; but the inner vascular wall of the yolksac fuses with the outer wall, and with the subzonal membrane, and so forms a false chorion, which receives its blood supply from the yolk-sac. This part of the chorion does not develop vascular villi.
The allantois (al) is a simple vascular sac with a large cavity. Part of its wall is applied to the subzonal membrane, and gives rise to the true chorion from which there project numerous vascular villi. These fit into corresponding uterine crypts. It seems probable, from BischofFs and Kolliker's observations, that the subzonal membrane in the area of the placenta becomes attached, by means of villi, to the uterine wall even before its fusion with the allantois. In the later periods of gestation the intermingling of the maternal and fcetal parts of the placenta becomes very close, and the placenta is truly deciduate. The cavity of the allantois persists till birth. Between the yolk-sac, the allantois, and the embryo, there is left a large cavity filled with an albuminous fluid.
|Fig. 116. Diagrammatic longitudinal section of a rabbit's ovum at an advanced stage of pregnancy. (From Kolliker after Bischoff.)
The metadiscoidal type of placenta is found in Man and the Apes. The placenta of Man may be conveniently taken as an example of this type.
The early stages in the development of the foetal membranes in the human embryo have not been satisfactorily observed ; but it is known that the ovum, shortly after its entrance into the uterus, becomes attached to the uterine wall, which in the meantime has undergone considerable preparatory changes. A fold of the uterine wall appears to grow round the blastodermic vesicle, and to form a complete capsule for it, but the exact mode of formation of this capsule is a matter of inference and not of observation. During the first fortnight of pregnancy villi grow out, over the whole surface of the ovum. The further history of the early stages is extremely obscure : what is known with reference to it will be found on p. 335 et seq. ; we will here take up the history at about the fourth week.
At this stage a complete chorion has become formed, and is probably derived from a growth of the niesoblast of the allantois (unaccompanied by the hypoblast) round the whole inner surface of the subzonal membrane. From the whole surface of the chorion there project branched vascular processes, covered by an epithelium. The allantois is without a cavity, but a hypoblastic epithelium is present in the allantoic stalk, though not forming a continuous tube. The blood-vessels of the chorion are derived from the usual allantoic arteries and vein. The general condition of the embryo and of its membranes at this period is shewn diagrammatically in Fig. 114, 5. Around the embryo is seen the amnion, already separated by a considerable interval from the embryo. The yolk-sac is shewn at ds. Eelatively to the other parts it is considerably smaller than it was at an earlier stage. The allantoic stalk is shewn at al. Both it and the stalk of the yolk-sac are enveloped by the amnion, am. The chorion with its vascular processes surrounds the whole embryo.
It may be noted that the condition of the chorion at this stage is very similar to that of the normal diffused type of placenta, described in the sequel.
While the above changes are taking place in the embryonic membranes, the blastodermic vesicle greatly increases in size, and forms a considerable projection from the upper wall of the uterus. Three regions of the uterine wall, in relation to the blastodermic vesicle, are usually distinguished; and since the superficial parts of all of these are thrown off with the after- birth, each of them is called a decidua. They are represented at a somewhat later stage in Fig. 117. There is (1) the part of the wall reflected over the blastodermic vesicle, called the decidua reflexa (dr) ; (2) the part of the wall forming the area round which the reflexa is inserted, called the decidua serotina (ds) ; (3) the general wall of the uterus, not related to the embryo, called the decidua vera (du).
The decidua reflexa and serotina together envelop the chorion (Fig. 114. 5), the processes of which fit. into crypts in them. At this period both of them are highly and nearly uniformly vascular. The general cavity of the uterus is to a large extent obliterated by the ovum, but still persists as a space filled with mucus, between the decidua reflexa and the decidua vera.
The changes which ensue from this period onwards are fully known. The amnion continues to dilate (its cavity being tensely filled with amniotic fluid) till it comes very close to the chorion (Fig. 117, am); from which, however, it remains separated by a layer of gelatinous tissue. The villi of the chorion in the region covered by the decidua reflexa, gradually cease to be vascular, and partially atrophy, but in the region in contact with the decidua serotina increase and become more vascular and more arborescent (Fig. 117, z). The former region becomes known as the chorion Iceve, and the latter as the chorion frondosum. The chorion frondosum, together with the decidua serotina, gives rise to the placenta.
The umbilical vesicle (Fig. 117, rib\ although it becomes greatly reduced in size and flattened, persists in a recognisable form till the time of birth.
The decidua reflexa, by the disappearance of the vessels in the chorion Iseve, becomes non-vascular. Its tissue and that of the decidua vera undergo changes which we do not propose to describe here ; it ultimately fuses on the one hand with the chorion, and on the other with the decidua vera. The membrane resulting from its fusion with the latter structure becomes thinner and thinner as pregnancy advances, and is reduced to a thin layer at the time of birth.
|Fig. 117. Diagrammatic section of pregnant human uterus with contained foetus. (From Huxley after Longet.)
The placenta has a somewhat discoidal form, with a slightly convex uterine surface and a concave embryonic surface. At its edge it is continuous both with the decidua reflexa and decidua vera. Near the centre of the embryonic surface is implanted the umbilical cord. As has already been mentioned, the placenta is formed of the decidua serotina and the foetal villi of the chorion frondosum. The fcetal and maternal tissues are far more closely united than in the placenta of the rabbit. The villi of the chorion, which were originally comparatively simple, become more and more complicated, and assume an extremely arborescent form. At birth the whole placenta, together with the fused decidua vera, and reflexa, with which it is continuous, is shed ; and the blood-vessels thus ruptured are closed by the contraction of the uterine walls.
The metadiscoidal placenta of Man and Apes and the discoidal placenta of the Eabbit are usually classified by anatomists as discoidal placentae, but it must be borne in mind that they differ very widely.
In the Eabbit there is a dorsal placenta, which is co- extensive with the area of contact between the allantois and the subzonal membrane, while the yolk-sac adheres to a large part of the subzonal membrane. In Apes and Man the allantois spreads over the whole inner surface of the subzonal membrane ; the placenta is on the ventral side of the embryo, and occupies only a small part of the surface of the allantois.
Another form of deciduate placenta is known as the zonary. This form of placenta occupies a broad zone of the chorion, leaving the two poles free. It is found in the Carnivora, Hyrax, Elephas, and Orycteropus.
In the Dog, which may be taken as a type, there is a large vascular yolk-sac formed in the usual way, which does not however fuse with the chorion. It has at first an oval shape, and persists till birth. The allantois first grows out on the dorsal side of the embryo, where it coalesces with the subzonal membrane, over a small discoidal area, and there is thus formed a rudimentary discoidal placenta closely resembling that of the Rabbit.
The area of adhesion between the outer part of the allantois and subzonal membrane gradually spreads over the whole interior of the subzonal membrane, and vascular villi are formed over the whole area of adhesion except at the two extreme poles of the ovum.
With the full growth of the allantois there is formed a broad placental zone, with numerous branched villi fitting into corresponding pita which are not true glands but special developments of the uterine surface. The maternal and foetal structures become closely interlocked and highly vascular ; and at birth a large part of the maternal part is carried away with the placenta ; some of it however still remains attached to the muscular wall of the uterus. The zone of the placenta diminishes greatly in proportion to the chorion as the latter elongates, and at the full time the breadth of the zone is not more than about one-fifth of the whole length of the chorion.
At the edge of the placental zone there is a very small portion of the uterine mucous membrane reflected over the non-placental part of the chorion, so as to form a small reflexa analogous with the reflexa in Man.
The most important of the remaining types of placenta are the diffuse and the polycotyledonary, and these placente are for the most part non-deciduate. In the diffuse placenta, found in the Horse, Pig, Lemurs, etc., the allantois completely envelopes the embryo, and villi are formed on all parts of the chorion, excepting over a small area at the two poles.
In the polycotyledonary placenta, which is characteristic of the Ruminantia, the allantois grows round the whole inner surface of the subzonal membrane ; the placental villi are however not uniformly distributed, but collected into patches or cotyledons, which form as it were so many small placentae. The foetal villi of these patches fit into corresponding pits in thickened patches of the wall of the uterus.
Comparative histology of the Placenta
It does not fall within the province of this work to treat from a histological standpoint the changes which take place in the uterine walls during pregnancy. It will, however, be convenient to place before the reader a short statement of the relations between the maternal and foetal tissues in the different varieties of placenta.
The simplest known condition of the placenta is that found in the pig (Fig. 118 II.). The papilla-like foetal villi fit into the maternal crypts. The villi (v) are formed of a connective tissue core with capillaries, and are covered by a layer of very flat epithelium (e) derived from the subzonal membrane. The maternal crypts are lined by the uterine epithelium (e), immediately below which is a capillary plexus. The maternal and fcetal vessels are here separated by a double epithelial layer. The same general arrangement holds good in the diffused placentae of other forms, and in the polycotyledonary placenta of the Ruminantia, but the foetal villi in the latter (III.) acquire an arborescent form. The maternal vessels retain the form of capillaries.
In the deciduate placenta a much more complicated arrangement is usually found. In the typical zonary placenta of the fox and cat (IV. and V.), the maternal tissue is broken up into a complete trabecuiar meshwork, and in the interior of the trabeculse there run dilated maternal capillaries (d'). The trabeculse are covered by a more or less columnar uterine epithelium (e), and are in contact on every side with foetal villi. The capillaries of the foetal villi preserve their normal size, and the villi are covered by a flat epithelial layer (e).
In the Sloth (VI.) which has a discoidal placenta the maternal capillaries become still more dilated, and the epithelium covering them is formed of very flat polygonal cells.
Fig. 118. Histology of the placenta. diagrammatic representations of the minute structure of the placenta. (From Turner.)
- F. the foetal ; M. the maternal placenta ; e. epithelium of chorion ; e'. epithelium of maternal placenta ; d. foetal bloodvessels ; d'. maternal blood-vessels ; v. villus.
I. Placenta in its most generalized form. II. Structure of placenta of a Pig. III. Of a Cow. IV. Of a Fox. V. Of a Cat.
VI. Structure of placenta of a Sloth. On the right side of the figure the flat maternal epithelial cells are shewn in situ. On the left side they are removed, and the dilated maternal vessel with its blood-corpuscles is exposed.
VII. Structure of Human placenta. In addition to the letters already referred to, ds, ds. represents the decidua serotina of the placenta ; t, t. trabeculae of serotina passing to the foetal villi ; ca. curling artery ; up. utero-placental vein ; x. a prolongation of maternal tissue on the exterior of the villus outside the cellular layer e', which may represent either the endothelium of the maternal blood-vessel or delicate connective tissue belonging to the serotina, or both. The layer e' represents maternal cells derived from the serotina. The layer of foetal epithelium cannot be seen on the villi of the fully-formed human placenta.
In the human placenta (VII.), as in that of Apes, the greatest modification is found. Here the maternal vessels have completely lost their capillary form, and have become expanded into large freely communicating sinuses (d f ). In these sinuses the foetal villi hang for the most part freely, though occasionally attached to their walls by strands of tissue (t). In the late stages of fcetal life there is only one epithelial layer (e} between the maternal and fcetal vessels, which closely invests the fcetal villi, but is part of the uterine tissue. In the foetal villi the vessels retain their capillary form.
Evolution of the placenta
Excluding the marsupials whose placentation is not really known, the arrangement of the foetal membranes of the Rabbit is the most primitive observed. In this type the allantois and yolk-sac both function in obtaining nutriment from the mother ; and the former occupies only a small discoidal area of the subzonal membrane. In all higher types the allantois gradually spreads out over the whole inner surface of the subzonal membrane and its importance increases ; while that of the yolk-sac as a nutritive organ decreases. In the diffuse type of placenta simple villi are present over nearly the whole surface of the chorion. In the remaining types the villi become more complicated and restricted to a smaller area (meta-discoidal, zonary, &c.) of the chorion ; though in the early stages they are more scattered and simpler, in some cases occupying nearly the whole surface of the chorion. It therefore seems probable that the placenta of Man has been derived not directly from the discoidal placenta of the Rabbit, but from the diffuse placenta such as is seen in the Lemurs, etc., and that generally the zonary, cotyledonary, &c. types of placenta have been derived from the diffuse by a concentration and increase in the complexity of the foetal villi.
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