McMurrich1914 Chapter 5
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McMurrich JP. The Development Of The Human Body. (1914) P. Blakiston's Son & Co., Philadelphia, Pennsylvania.
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Chapter V. The Yolk -Stalk, Belly-Stalk, and Fetal Membranes
The conditions to which the embryos and larvse of the majority of animals must adapt themselves are so different from those under which the adult organisms exist that in the early stages of development special organs are very frequently developed which are of use only during the embryonic or larval period and are discarded when more advanced stages of development have been reached. This remark applies with especial force to the human embryo which leads for a period of nine months what may be termed a parasitic existence, drawing its nutrition from and yielding up its waste products to the blood of the parent. In- order that this may be accomplished certain special organs are developed by the embryo, by means of which it forms an intimate connection with the walls of the uterus, which, on its part, becomes greatly modified, the combination of embryonic and maternal structures producing what are termed the deciduce, owing to their being discarded at birth when the parasitic mode of life is given up.
Furthermore, it has already been seen that many peculiar modifications of development in the human embryo result from the inheritance of structures from more or less remote ancestors, and among the embryonic adnexes are found structures which represent in a more or less modified condition organs of considerable functional importance in lower forms. Such structures are the yolk-stalk and vesicle, the amnion, and the allantois, and for their proper understanding it will be well to consider briefly their development in some lower form, such as the chick.
At the time when the embryo of the chick begins to be constricted off from the surface of the large yolk-mass, a fold, consisting of ectoderm and somatic mesoderm, arises just outside the embryonic area, which it completely surrounds. As development proceeds the fold becomes higher and its edges gradually draw nearer together over the dorsal surface of the embryo (Fig. 64, A, Af), and finally meet and fuse (Fig. 64, B and C), so that the embryo becomes enclosed within a sac, which is termed the amnion and is formed by the fusion of the layers which constituted the inner wall of the fold. The layers of the outer wall of the fold after fusion form part of the general ectoderm and somatic mesoderm which make up the outer wall of the ovum and together are known as the serosa, corresponding to the chorion of the mammalian embryo. The space which occurs between the amnion and the serosa is a portion of the extraembryonic ccelom and is continuous with the embryonic pleuroperitoneal cavity.
Fig. 64. - Diagrams Illustrating the Formation of the Amnion and Allantois in the Chick. Af, Amnion folds; Al, allantois; Am, amniotic cavity; Ds, yolk-sac. - (Cegenbaur.)
In the ovum of the chick, as in that of the reptile, the protoplasmic material is limited to one pole and rests upon the large yolk mass. As development proceeds the germ layers gradually extend around the yolk-mass and eventually completely enclose it, the yolkmass coming to lie within the endodermal layer, which, together with the splanchnic mesoderm which lines it, forms what is termed the yolk-sac. As the embryo separates from the yolk-mass the yolksac is constricted in its proximal portion and so differentiated into a yolk-stalk and a yolk-sac, the contents of the latter being gradually absorbed by the embryo during its growth, its walls and those of the stalk being converted into a portion of the embryonic digestive tract.
In the meantime, however, from the posterior portion of the digestive tract, behind the point of attachment of the yolk-sac, a diverticulum has begun to form (Fig. 64, A, Al). This increases in size, projecting into the extra-embryonic portion of the pleuroperitoneal cavity and pushing before it the splanchnic mesoderm which lines the endoderm (Fig. 64, B and C) . This is the allantois, which, reaching a very considerable size in the chick and applying itself closely to the inside of the serosa, serves as a respiratory and excretory organ for the embryo, for which purpose its walls are richly supplied with blood-vessels, the allantoic arteries and veins.
Toward the end of the incubation period both the amnion and allantois begin to undergo retrogressive changes, and just before the hatching of the young chick they become completely dried up and closely adherent to the egg-shell, at the same time separating from their point of attachment to the body of the young chick, so that when the chick leaves the egg-shell it bursts through the driedup membranes and leaves them behind as useless structures.
Turning now to the human embryo, it will be found that the same organs are present, though somewhat modified either in the mode or the extent of their development. A welldeveloped amnion occurs, arising, however, in a very different manner from what it does in the chick; a large yolk-sac occurs even though it contains no yolk; and an allantois which has no respiratory or excretory functions is present, though in a somewhat degenerated condition. It has been seen from the description of the earliest stages of development that the processes which occur in the lower forms are greatly abbreviated in the human embryo. The enveloping layer, instead of gradually extending from one pole to enclose the entire ovum, develops in situ during the stages immediately succeeding segmentation, and the extra-embryonic mesoderm, instead of growing out from the embryo to enclose the yolk-sac, splits off directly from the enveloping layer. The earliest stages in the development of the amnion are not yet known for the human embryo, but from the condition in which it is found in the Peters embryo (Fig. 37) and in the embryo v.H. of von Spee (Fig. 39) it is probable that it arises, not by the fusion of the edges of a fold, as in the chick, but by a vacuolization of a portion of the inner cellmass, as has been described as occurring in the bat (p. 54). It is, then, a closed cavity from the very beginning, the floor of the cavity being formed by the embryonic disk, its posterior wall by the anterior surface of the belly-stalk, while its roof and sides are thin and composed of a single layer of flattened ectodermal cells lined on the outside by a layer of mesoderm continuous with the somatic mesoderm of the embryo and the mesoderm of the belly-stalk (Fig. 65, A).
When the bending downward of the peripheral portions of the embryonic disk to close in the ventral surface of the embryo occurs, the line of attachment of the amnion to the disk is also carried ventrally (Fig. 65, B), so that when the constriction off of the embryo is practically completed, the amnion is attached anteriorly to the margin of the umbilicus and posteriorly to the extremity of the band of ectoderm lining what may now be considered the posterior surface of the belly-stalk, while at the sides it is attached along an oblique line joining these two points (Fig. 65, B and C, in which the attachment of the amnion is indicated by the broken line).
Leaving aside for the present the changes which occur in the attachment of the amnion to the embryo (see p. 116), it may be said that during the later growth of the embryo the amniotic cavity increases in size until finally its wall comes into contact with the chorion, the extra-embryonic body-cavity being thus practically obliterated (Fig. 65, D), though no actual fusion of amnion and chorion occurs. Suspended by the umbilical cord, which has by this time developed, the embryo floats freely in the amniotic cavity, which is filled by a fluid, the liquor amnii, whose origin is involved in doubt, some authors maintaining that it infiltrates into the cavity from the maternal tissues, while others hold that a certain amount of it at least is derived from the embryo. It is a fluid with a specific gravity of about 1.003 an( ^ contains about 1 per cent, of solids, principally albumin, grape-sugar, and urea, the last constituent probably coming from the embryo. When present in greatest quantity - that is to say, at about the beginning of the last month of pregnancy - it varies in amount between one-half and threefourths of a liter, but during the last month it diminishes to about half that quantity. To protect the epidermis of the fetus from maceration during its prolonged immersion in the liquor amnii, the sebaceous glands of the skin at about the sixth month of development pour out upon the surface of the body a white fatty secretion known as the vernix caseosa.
Fig. 65. - Diagrams Illustrating the Formation of the Umbilical Cord.
The heavy black line represents the embryonic ectoderm; the dotted line represents the line of reflexion of the body ectoderm into that of the amnion. Ac, Amniotic cavity ; Al, allantois; Be, extra-embryonic ccelom; Bs, belly-stalk; Ch, chorion; P, placenta; Uc, umbilical cord; V, chorionic villi; Ys, yolk-sac.
During parturition the amnion, as a rule, ruptures as the result of the contraction of the uterine walls and the liquor amnii escapes as the "waters," a phenomenon which normally precedes the delivery of the child. As a rule, the rupture is sufficiently extensive to allow the passage of the child, the amnion remaining behind in the uterus, to be subsequently expelled along with the deciduae.
Occasionally it happens, however, that the amnion is sufficiently strong to withstand the pressure exerted upon it by the uterine contractions and the child is born still enveloped in the amnion, which, in such cases, is popularly known as the "caul," the possession of which, according to an old superstition, marks the child as a favorite of fortune.
As stated above, the liquor amnii varies considerably in amount in different cases, and occasionally it may be present in excessive quantities, producing a condition known as hydramnios. On the other hand, the amount may fall considerably below the normal, in which case the amnion may form abnormal unions with the embryo, sometimes producing malformations. Occasionally also bands of a fibrous character traverse the amniotic cavity and, tightening upon the embryo during its growth, may produce various malformations, such as scars, splitting of the eyelids or lips, or even amputation of a limb.
The probable mode of development of the yolk-sac in the human embryo, and its differentiation into yolk-stalk and yolk- vesicle have already been described (p. 86). When these changes have been completed, the vesicle is a small pyriform structure lying between the amnion and the chorionic mesoderm, some distance away from the extremity of the umbilical cord (Fig. 65, D), and the stalk is a long slender column of cells extending from the vesicle through the umbilical cord to unite with the intestinal tract of the embryo. The vesicle persists until birth and may be found among the decidual tissues as a small sac measuring from 3 to 10 mm. in its longest diameter. The stalk, however, early undergoes degeneration, the lumen which it at first contains becoming obliterated and its endoderm also disappearing as early as the end of the second month of development. The portion of the stalk which extends from the umbilicus to the intestine usually shares in the degeneration and disappears, but in about 3 per cent, of cases it persists, forming a more or less extensive diverticulum of the lower part of the small intestine, sometimes only half an inch or so in length and sometimes much larger. It may or may not retain connection with the abdominal wall at the umbilicus, and is known as Meckel's diverticulum.
This embryonic rudiment is of no little importance, since, when present, it is apt to undergo invagination into the lumen of the small intestine and so occlude it. How frequently this happens relatively to the occurrence of the diverticulum may be judged from the fact that out of one hundred cases of occlusion of the small intestine six were due to an invagination of the diverticulum.
In the reptiles and birds the yolk-sac is abundantly supplied with blood-vessels by means of which the absorption of the yolk is carried on, and even although the functional importance of the yolk-sac as an organ of nutrition is almost nil in the human embryo, yet it still retains a well-developed blood-supply, the walls of the vesicle, especially possessing a rich network of vessels. The future history of these vessels, which are known as the vitelline vessels, will be described later on.
The Allantois and Belly-stalk
It has been seen that in reptilian and avian embryos the allantois reaches a high degree of development and functions as a respiratory and excretory organ by coming into contact with what is comparable to the chorion of the mammalian embryo. In man it is very much modified both in its mode of development and in its relations to other parts, so that its resemblance to the avian organ is somewhat obscured. The differences depend partly upon the remarkable abbreviation manifested in the early development of the human embryo and partly upon the fact that the allantois serves to place the embryo in relation with the maternal blood, instead of with the external atmosphere, as is the case in the egg-laying forms. Thus, the endodermal portion of the allantois, instead of arising from the intestine and pushing before it a layer of splanchnic mesoderm to form a large sac lying freely in the extra-embryonic portion of the body-cavity, appears in the human embryo before the intestine has differentiated from the yolk-sac and pushes its way into the solid mass of mesoderm which forms the belly-stalk (Fig. 65, A). To understand the significance of this process it is necessary to recall the abbreviation in the human embryo of the development of the extra-embryonic mesoderm and body-cavity. Instead of growing out from the embryonic area, as it does in the lower forms, this mesoderm develops in situ by splitting off from the layer of enveloping cells and, furthermore, the extra-embryonic body-cavity arises by a splitting of the mesoderm so formed before there is any trace of a splitting of the embryonic mesoderm (Fig. 38). The belly-stalk, whose development from a portion of the inner cell-mass has already been traced (p. 68), is to be regarded as a portion of the body of the embryo, since the ectoderm which covers one surface of it resembles exactly that of the embryonic disk and shows an extension backward of the medullary groove upon its surface (Fig. 66). The mesoderm, therefore, of the belly-stalk is to be regarded as a portion of the embryonic mesoderm which has not yet undergone a splitting into somatic and splanchnic layers, and, indeed, it never does undergo such a splitting, so that there is no body-cavity into which the endodermal allantoic diverticulum can grow.
Fig. 66. - Transverse Section through the belly-stalk of an Embryo of 2.15 mm.
Aa, Umbilical (allantoic) artery; All, allantois; am, amnion; Va, umbilical (allantoic) vein. - (His.)
But this does not account for all the peculiarities of the human allantois. In the birds, and indeed in the lower oviparous mammals, the endodermal portion of the allantois is equally developed with the mesodermal portion, the allantois being an extensive sac whose cavity is rilled with fluid, and this is also true of such mammals as the marsupials, the rabbit, and the ruminants. In man, however, the endodermal diverticulum never becomes a sac-like structure, but is a slender tube extending from the intestine to the chorion and lying in the substance of the mesoderm of the belly-stalk (Fig. 65, D), the greater portion of which is to be regarded as homologous with the relatively thin layer of splanchnic mesoderm covering the endodermal diverticulum of the chick. An explanation of this disparity in the development of the mesodermal and endodermal portions of the human allantois is perhaps to be found in the altered conditions under which the respiration and secretion take place. In all forms, the lower as well as the higher, it is the mesoderm which is the more important constituent of the allantois, since in it the blood-vessels, upon whose presence the physiological functions depend, arise and are embedded. In the birds and oviparous mammals there are no means by which excreted material can be passed to the exterior of the ovum, and it is, therefore, stored up within the cavity of the allantois, the allantoic fluid containing considerable quantities of nitrogen, indicating the presence of urea. In the higher mammals the intimate relations which develop between the chorion and the uterine walls allow of the passage of excreted fluids into the maternal blood; and the more intimate these relations, the less necessity there is for an allantoic cavity in which excreted fluid may be stored up. The difference in the development of the cavity in the ruminants, for example, and man depends probably upon the greater intimacy of the union between ovum and uterus in the latter, the arrangement for the passage of the excreted material into the maternal blood being so perfect that there is practically no need for the development of an allantoic cavity.
The portion of the endodermal diverticulum which is enclosed within the umbilical cord persists until birth in a more or less rudimentary condition, but the intra-embryonic portion extending from the apex of the bladder to the umbilicus becomes converted into a solid cord of fibrous tissue termed the urachus.
Occasionally a lumen persists in the urachal portion of the allantois and may open to the exterior at the umbilicus, in which case urine from the bladder may escape at the umbilicus.
Since the allantois in the human embryo, as well as in the lower forms, is responsible for respiration and excretion, its blood-vessels are well developed. They are represented in the belly-stalk by two veins and two arteries (Fig. 66), known in human embryology as the umbilical veins and arteries. These extend from the body of the embryo out to the chorion, there branching repeatedly to enter the numerous chorionic villi by which the embryonic tissues are placed in relation with the maternal.
The Umbilical Cord
During the process of closing in of the ventral surface of the embryo a stage is reached in which the embryonic and extra-embryonic portions of the body-cavity are completely separated except for a small area, the umbilicus, through which the yolk-stalk passes out (Fig. 65, B). At the edges of this area in front and at the sides the embryonic ectoderm and somatic mesoderm become continuous with the corresponding layers of the amnion, but posteriorly the line of attachment of the amnion passes up upon the sides of the belly-stalk (Fig. 65, B), so that the whole of the ventral surface of the stalk is entirely uncovered by ectoderm, this layer being limited to its dorsal surface (Fig. 66). In subsequent stages the embryonic ectoderm and somatic mesoderm at the edges of the umbilicus grow out ventrally, carrying with them the line of attachment of the amnion and forming a tube which encloses the proximal part of the yolk-stalk. The ectoderm of the belly-stalk at the same time extending more laterally, the condition represented in Fig. 65, C, is produced, and, these processes continuing, the entire belly-stalk, together with the yolk-stalk, becomes enclosed within a cylindrical cord extending from the ventral surface of the body to the chorion and forming the umbilical cord (Fig. 65, D).
From this mode of development it is evident that the cord is, strictly speaking, a portion of the embryo, its surfaces being completely covered by embryonic ectoderm, the amnion being carried during its formation further and further from the umbilicus until finally it is attached around the distal extremity of the cord.
Fig. 67. -Transverse Sections of the Umbilical Cord of Embryos of (A) 1.8 cm.
and (B) 25 cm. al, Allantois; c, coelom; ua, umbilical artery; uv, umbilical vein; ys, yolk-stalk.
In enclosing the yolk-stalk the umbilical cord encloses also a small portion of what was originally the extra-embryonic bodycavity surrounding the yolk-stalk. A section of the cord in an early stage of its development (Fig. 67, A) will show a thick mass of mesoderm occupying its dorsal region; this represents the mesoderm of the belly-stalk and contains the allantois and the umbilical arteries and vein (the two veins originally present in the belly-stalk having fused), while toward the ventral surface there will be seen a distinct cavity in which lies the yolk-stalk with its accompanying blood-vessels. The portion of this ccelom nearest the body of the embryo becomes much enlarged, and during the second month of development contains some coils of the small intestine, but later the entire cavity becomes more and more encroached upon by the growth of the mesoderm, and at about the fourth month is entirely obliterated. A section of the cord subsequent to that period of development will show a solid mass of mesoderm in which are embedded the umbilical arteries and vein, the allantois, and the rudiments of the yolk-stalk (Fig. 67, B).
When fully formed, the umbilical cord measures on the average 55 cm. in length, though it varies considerably in different cases, and has a diameter of about 1.5 cm. It presents the appearance of being spirally twisted, an appearance largely due, however, to the spiral course pursued by the umbilical arteries, though the entire cord may undergo a certain amount of torsion from the movements of the embryo in the later stages of development and may even be knotted. The greater part of its substance is formed by the mesoderm, the cells of which become stellate and form a recticulum, the meshes of which are occupied by connective-tissue fibrils and a mucous fluid which gives to the tissue a jelly-like consistence, whence it has received the name of Wharton's jelly.
To understand the developmental changes which the chorion undergoes it will be of advantage to obtain some insight into the manner in which the ovum becomes implanted in the wall of the uterus. Nothing is known as to how this implantation is effected in the case of the human ovum; it has already been accomplished in the youngest ovum at present known. But the process has been observed in other mammals, and what takes place in Spermophilus, for example, may be supposed to give a clue to what occurs in the human ovum. In the spermophile the ovum lies free in the uterine cavity up to a stage at which the vacuolization of the central cells is almost completed (Fig. 68, A). At one region of the covering layer the cells become thicker and later form a syncytial projection or knob which comes into contact with the uterine mucosa (Fig. 68, B), and at the point of contact the mucosa cells undergo degeneration, allowing the knob to come into relation with the deeper tissues of the uterus (Fig. 68, C), the process apparently being one in which the mucosa cells are eroded by the syncytial knob. It seems probable that in the human ovum the process is at first of a similar nature and that as the covering layer cells come into contact with the deeper layers of the uterus, these too are eroded, and, the uterine blood-vessels being included in the erosion process, an extravasation of blood plasma and corpuscles occurs in the vicinity of the burrowing ovum. In the meantime the ovum has increased considerably in size, its growth in these early stages being especially rapid, and the area of contact consequently increases in size, entailing continued erosion of the uterine mucosa. At the same time, too, the uterine tissues surrounding the ovum grow up around it, forming at first as it were a circular wall (Fig. 69, A), and eventually completely enclose it, forming an envelope known as the decidua capsularis or rejiexa. The blood extravasation is now contained within a closed space bounded on the one hand by the uterine tissues and on the other by the wall of the ovum (Fig. 69, B).
Fig. 68. - Successive Stages in the Implantation of the Ovum of the Spermophile . a, syncytial knob; k, inner cell-mass. - (Rejsek.)
Fig. 69. - Diagrams Illustrating the Implantation of the Ovum. ac, amniotic cavity; bs, belly-stalk; cf, chorion frondosum; cl, chorion laeve;Jc, decidua capsularis; ic, inner cell-mass; s, space surrounding ovum which becomes the intervillous space; um, uterine mucosa; v, chorionic villus; ys, yolk-sac.
Fig. 70. - Section of an Ovum of i mm. A Section of the Embryo Lies in the Lower Part of the Cavity of the Ovum. D, Decidua; E.U., uterine epithelium; Sch, blood-clot closing the aperture left by the sinking of the ovum into the uterine mucosa. - (From Strahl, after Peters.)
The youngest known human ova have already reached approximately this stage. Thus, the Peters ovum (Fig. 70) had already sunk deeply into the uterine mucosa, the point of entrance being indicated by a gap in the decidua capsularis, closed in this case by a patch of coagulated blood (Sch). The uterine tissues in the immediate vicinity of the ovum were much swollen and apparently somewhat necrotic and their blood-vessels could be seen to communicate with the space between the wall of the ovum and the maternal tissues. This space, however, was converted into an irregular network of blood lacunae by anastomosing cords of cells, which arose from the wall of the ovum and extended through the space to the maternal tissues ; these cords of cells are represented in Fig. 70 by the darker masses projecting from the wall of the ovum and scattered among the paler blood lacunae. This stage of implantation of the ovum is shown diagrammatically in Fig. 69, B, where, for simplicity's sake, the cell cords are represented merely as processes radiating from the ovum without reaching the maternal tissues.
The cell cords are derivatives of the trophoblast and are, therefore, of embryonic origin. If examined under a higher magnification than that shown in Fig. 70 they will be seen to be composed of an axial core of cells with distinct outlines, enclosed within a layer of protoplasm which lacks all traces of cell boundaries, although it contains numerous nuclei, being what is termed a syncytium or Plasmodium. The original trophoblast has thus become differentiated into two distinct tissues, a cellular one, which has been termed the cyto-trophoblast, and a plasmodial one, which, similarly, is known as the plasmodi-trophoblast and is the tissue that comes into contact with the maternal blood contained in the lacunar spaces and with the maternal tissues, in connection with these latter sometimes developing into masses of considerable extent. To this plasmoditrophoblast may be ascribed the active part in the destruction of the maternal tissues and probably also the absorption of the products of the destruction for the nutrition of the growing ovum. For up to this stage the ovum has been playing the role of a parasite thriving upon the tissues of^ its host.
The food material that the ovum thus obtains may conveniently be termed the embryotroph and the type of placentation which obtains up to this stage and for some time longer may be termed the embryotrophic type. But even in the Peters ovum the preparation for another type has begun. In earlier stages the cell cords were entirely trophoblastic, but in this ovum (Fig. 70) processes from the chorionic mesoderm may be seen projecting into the bases of the cell cords, and in later stages these processes extend farther and farther into the axis of each cord, the anastomoses of the cords disappear and the cords themselves become converted into branching processes, the chorionic villi, which project from the entire surface of the ovum (Fig. 71) into the surrounding space, which may now be termed the intervillous space, and are bathed by the maternal blood which it contains. Toward the maternal surface of the space some masses of the trophoblast still persist, uniting the extremities of certain of the villi to the enclosing uterine wall, such villi being termed fixation villi to distinguish them from the majority, which project freely into the intervillous space. Later, when the embryonic blood-vessels develop, those associated with the allantois extend outward into the chorionic mesoderm and thence send branches into each villus. The second type of placentation, the hcemotrophic type, is thus established, the fetal blood contained in the vessels of the villi receiving nutrition through the walls of the villi from the maternal blood contained in the intervillous space, and, similarly, transferring waste products to it.
Fig. 71. - Entire Ovum Aborted at about the Beginning of the Second Month. Xi 1/2. - (Grosser.)
At first, as stated above, the villi usually cover the entire surface of the ovum, but later, as the ovum increases in size, those villi which are remote from the attachment of the belly-stalk to the chorion are placed at a disadvantage so far as their blood supply is concerned
Fig. 72. - Two Villi prom the Chorion of an Embryo of 7 mm.
and gradually disappear, and this process continues until, finally, only those villi are retained which are in the immediate region of the belly-stalk (Fig. 69, C), these persisting to form the fetal portion of the placenta. By these changes the chorion becomes differentiated into two regions (Fig. 69, C), one of which is destitute of villi and is termed the chorion lave, while the other provided with them, is known as the chorion frondosum.
Fig. 73. - Transverse Sections through Chorionic Villi in (4) the Fifth and (B) the Seventh Month of Development.
cf, Canalized fibrin; Ic, Langhans cells; s, syncytium. - (A which is more highly magnified than B, from Szymonowicz; B from Minot.)
Occasionally one or more patches of villi may persist in the area that normally becomes the chorion lseve and thus accessory placenta (-placenta succenturiatce) , varying in number and size, may be formed.
The villi when fully formed are processes of the chorion, branching profusely and irregularly (Fig. 72), and each consists of a core of mesoderm, containing blood-vessels, enclosed within a double layer of trophoblastic tissue (Fig. 73, A). The inner layer consists of a sheet of well defined cells arranged in a single series; it is derived from the cyto-trophoblast and forms what is known as the layer of Langhans cells. The outer layer is syncytial in structure and is formed from the plasmodi-trophoblast.
Fig. 74. - Mature Placenta after Separation from the Uterus. c, Cotyledons; eh, chorion, amnion, and decidua vera; urn, umbilical cord. - (Kollmann.) As development proceeds the villi, which are at first distributed evenly over the chorion frondosum, become separated into groups termed cotyledons (Fig. 74) by the growth into the intervillous space of trabecular from the walls of the uterus, the fixation villi becoming connected with these septa as well as with the general uterine wall. The ectoderm of the villi also undergoes certain changes with advancing growth, the layer of Langhans cells disappearing except in small areas scattered irregularly in the villi, and the syncytium, though persisting, undergoes local thickenings which become replaced, more or less extensively, by depositions of fibrin (Fig. 73, B, cf).
The changes which occur during the later stages of development in the chorion are very similar to those described for the villi.
Fig. 75. - Section through the Placental Chorion of an Embryo of Seven Months. c, Cell layer; ep, remnants of epithelium; fb, fibrin layer; mes, mesoderm. - (Minot.)
Thus, the mesoderm thickens, its outermost layers becoming exceedingly fibrillar in structure, while the ectoderm differentiates into two layers, the outer of which is syncytial while the inner is cellular, and later still, as in the villi, the syncytial layer is replaced in irregular patches by a peculiar form of fibrin which is traversed by flattened anastomosing spaces and to which the name canalized fibrin or fibrinoid has been applied (Fig. 75).
It has been pointed out (p. 26) that in connection with the phenomenon of menstruation periodic alterations occur in the mucous membrane of the uterus. If during one of these periods a fertilized ovum reaches the uterus, the desquamation of portions of the epithelium does not occur nor is there any appreciable hemorrhage into the cavity of the uterus; the uterine mucosa remains in what is practically the ante-menstrual condition until the conclusion of pregnancy, when, after the birth of the fetus, a considerable portion of its thickness is expelled from the uterus, forming what is termed the decidua. In other words, the sloughing of the uterine tissue which concludes the process of menstruation is postponed until the close of pregnancy, and then takes place simultaneously over the whole extent of the uterus. Of course, the changes in the uterine tissues are somewhat more extensive during pregnancy than during menstruation, but there is an undoubted fundamental similarity in the changes during the two processes.
Fig. 76. - Diagram showing the Relations of the Fetal Membranes.
Am, Amnion; Ch, chorion; M, muscular wall of uterus; C, decidua capsularis; B, decidua basalis; V, decidua vera; F, yolk-stalk.
Fig. 77. - Surface View op Half of the Decldua Vera at the End of the Third Week of Gestation.
d, Mucous membrane of the Fallopian tubes; ds, prolongation of the vera toward the cervix uteri; pp., papillae; rf, marginal furrow. (Kollmann.)
The human ovum comes into direct apposition with only a small portion of the uterine wall, and the changes which this portion of the wall undergoes differ somewhat from those occurring elsewhere. Consequently it becomes possible to divide the deciduae into (1) a portion which is not in direct contact with the ovum, the decidua vera (Fig. 76, V) and (2) a portion which is. The latter portion is again capable of division. The ovum becomes completely embedded in the mucosa, but, as has been pointed out, the chorionic villi reach their full development only over that portion of the chorion to which the belly-stalk is attached. The decidua which is in relation to this chorion frondosum undergoes much more extensive modifications than that in relation to the chorion laeve, and to it the name of decidua basalts (decidua serotina) (Fig. 76, B) is applied, while the rest of the decidua which encloses the ovum is termed the decidua capsularis (decidua rejlexa) (C).
The changes which give rise to the decidua vera may first be described and those occurring in the others considered in succession.
(a) Decidua vera
On opening a uterus during the fourth or fifth month of pregnancy, when the decidua vera is at the height of its development, the surface of the mucosa presents a corrugated appearance and is traversed by irregular and rather deep grooves (Fig. 77). This appearance ceases at the internal orifice, the mucous membrane of the cervix uteri not forming a decidua, and the deciduae of the two surfaces of the uterus are separated by a distinct furrow known as the marginal groove.
Fig. 78. - Diagrammatic Sections of the Uterine Mucosa, A, in the Nonpregnant Uterus, and B, at the Beginning of Pregnancy. c, Stratum compactum; gl, the deepest portions of the glands; m, muscular layer; sp, stratum spongiosum. - (Kundrat and Engelmann.)
In sections the mucosa is found to have become greatly thickened, frequently measuring i cm. in thickness, and its glands have undergone very considerable modification. Normally almost straight (Fig. 78, A), they increase in length, not only keeping pace with the thickening of the mucosa, but surpassing its growth, so that they become very much contorted and are, in addition, considerably dilated (Fig. 78, B). Near their mouths they are dilated, but not very much contorted, while lower down the reverse is the case, and it is possible to recognize three layers in the decidua, (1) a stratum compactum nearest the lumen of the uterus, containing the straight but dilated portions of the glands; (2) a stratum spongiosum, so called from the appearance which it presents in sections owing to the dilated and contorted portions of the glands being cut in various planes; and (3) next the muscular coat of the uterus a layer containing the contorted but not dilated extremities of the glands is found. Only in the last layer does the epithelium of the glands retain its normal columnar form; elsewhere the cells, separated from the walls of the glands, become enlarged and irregular in shape and eventually degenerate.
In addition to these changes, the epithelium of the mucosa disappears completely during the first month of pregnancy, and the tissue between the glands in the stratum compactum becomes packed with large, often multinucleated cells, which are termed the decidual cells and are probably derived from the connective tissue cells of the mucosa.
After the end of the fifth month the increasing size of the embryo and its membranes exerts a certain amount of pressure on the decidua, and it begins to diminish'in thickness. The portions of the glands which lie in the stratum compactum become more and more compressed and finally disappear, while in the spongiosum the spaces become much flattened and the vascularity of the whole decidua, at first so pronounced, diminishes greatly.
(b) Decidua capsularis
The decidua capsularis has also been termed the decidua reflexa, on the supposition that it was formed as a fold of the uterine mucosa reflected over the ovum after this had attached itself to the uterine wall. Since, however, the attachment of the ovum is to be regarded as a process of burrowing into the uterine tissues (see p. 119), the necessity for an upgrowth of a fold is limited to an elevation of the uterine tissues in the neighborhood of the ovum to keep pace with its increasing size. Since it is part of the area of contact with the ovum it possesses no epithelium upon the surface turned toward the ovum, although in the earlier stages its surface is covered by an epithelium continuous with that of the decidua vera, and between it and the chorion there is a portion of the blood extravasation in which the villi formed from the chorion laeve float. Glands and blood-vessels also occur in its walls in the earlier stages of development.
As the ovum continues to increase in size the capsularis begins to show signs of degeneration, these appearing first over the pole of the ovum opposite the point of fixation. Here, even in the case of the ovum described by Rossi Doria, the cavity of which measured 6X5 mm. in diameter, it has become reduced to a thin membrane destitute of either blood-vessels or glands, and the degeneration gradually extends throughout the entire capsule, the portion of the blood space which it encloses also disappearing. At about the fifth month the growth of the ovum has brought the capsularis in contact throughout its whole extent with the vera, and it then appears as a whitish transparent membrane with ho trace of either glands or blood-vessels, and it eventually disappears by fusing with the vera.
(c) Decidua basalis
The structure of the decidua basalis, also known as the decidua serotina, is practically the same as that of the vera up to about the fifth month. It differs only in that, being part of the area of contact of the ovum, it loses its epithelium much earlier and is also the seat of extensive blood extravasations, due to the erosion of its vessels by the chorionic trophoblast. Its glands, however, undergo the same changes as those of the vera, so that in it also a compactum and a spongiosum may be recognized. Beyond the fifth month, however, there is a great difference between it and the vera, in that, being concerned with the nutrition of the embryo, it does not partake of the degeneration noticeable in the other deciduae, but persists until birth, forming a part of the structure termed the placenta.
This organ, which forms the connection between the embryo and the maternal tissues, is composed of two parts, separated by the intervillous space. One of these parts is of embryonic origin, being the chorion frondosum, while the other belongs to the maternal tissues and is the decidua- basalis. Hence the terms placenta fetalis and placenta uterina frequently applied to the two parts. The fully formed placenta is a more or less discoidal structure, convex on the surface next the uterine muscularis and concave on that turned toward the embryo, the umbilical cord being continuous with it near the center of the latter surface. It averages about 3.5 cm. in thickness, thinning out somewhat toward the edges, and has a diameter of 15 to 20 cm., and a weight varying between 500 and 1250 grams. It is situated on one of the surfaces of the uterus, the posterior more frequently than the anterior, and usually much nearer the fundus than the internal orifice. It develops, in fact, wherever the ovum happens to become attached to the uterine walls, and occasionally this attachment is not accomplished until the ovum has descended nearly to the internal orifice, in which case the placenta may completely close this opening and form what is termed a placenta prcevia.
If a section of a placenta in a somewhat advanced stage of development be made, the following structures may be distinguished: On the inner surface there will be a delicate layer representing the amnion (Fig. 79, Am), and next to this a somewhat thicker one which is the chorion (Cho), in which the degenerative changes already mentioned may be observed. Succeeding this comes a much broader area composed of the large intervillous blood space in which lie sections of the villi (vi) cut in various directions. Then follows the stratum compactum of the basalis, next the stratum spongiosum (Z)')> next the outermost layer of the mucosa (D"), in which the uterine glands retain their epithelium, and, finally, the muscularis uteri (Mc)
These various structures have, for the most part, been already described and it remains here only to say a few words concerning the special structure of the basal compactum and concerning certain changes that take place in the intervillous space.
Fig. 79. - Section through a Placenta of Seven Months' Development.
Am, Amnion; cho, chorion; D, layer of decidua containing the uterine glands ;(Mc, muscular coat of the uterus; Ve, maternal blood-vessel; Vi, stalk of a villus; vi, villi in section. - (Minoi.)
The stratum compactum of the basal decidua forms what is termed the basal plate of the placenta, closing the intervillous space on the uterine side and being traversed by the maternal blood-vessels that open into the space. The formation of canalized fibrin, already mentioned in connection with the decidua vera and the syncytium of the villi, also occurs in the basal portion of the decidua, a definite layer of it, known as NitabucJi's fibrin stria, being a characteristic constituent of the basal plate and patches of greater or less extent also occur upon the surface of the plate. Leucocytes also occur in considerable abundance in the plate and their presence has been taken to indicate an attempt on the part of the maternal tissues to resist the erosive action of the parasitic ovum. From the surface of the basal plate processes, termed placental septa, project into the intervillous space, grouping the villi into cotyledons and giving attachment to some of the fixation villi (Fig. 80). Throughout the greater extent of the placenta the septa do not reach the surface of the chorion, but at the periphery, throughout a narrow zone, they do come into contact with the chorion and unite beneath it to form a membrane which has been termed the closing plate. Beneath this lies the peripheral portion of the intervillous space, which, owing to the arrangement of the septa in this region, appears to be imperfectly separated from the rest of the space and forms what is termed the marginal sinus (Fig. 80).
Attention has already been called to the formation of canalized fibrin or fibrinoid in connection with the syncytium of the villi. In the later stages of pregnancy there may be produced by this process masses of fibrinoid of considerable size, lying in the intervillous space; these, on account of their color, are termed white infarcts and may frequently be observed as whitish or grayish patches through the walls of the placenta after its expulsion. Red infarcts produced by the clotting of the blood, also occurs, but with much less regularity and frequency.
The Separation of the Deciduae at Birth
At parturition, after the rupture of the amnion and the expulsion of the fetus, there still remain in the uterine cavity the deciduae and the amnion, which is in contact but not fused with the deciduae. A continuance of the uterine contractions, producing what are termed the "after-pains," results in the separation of the placenta from the uterine walls, the separation taking place in the deep layers of the spongiosum, so that the portion of the mucosum which contains the undegenerated glands remains behind. As soon as the placenta has separated, the separation of the decidua vera takes place gradually though rapidly, the line of separation again being in the deeper layers of the stratum spongiosum, and the whole of the deciduae, together with the amnion, is expelled from the uterus, forming what is known as the "after-birth." Hemorrhage from the uterine vessels during and after the separation of the deciduae is prevented by the contractions of the uterine walls, assisted, according to some authors, by a preliminary blocking of the mouths of the uterine vessels by certain large polynuclear decidual cells found during the later months of pregnancy in the outer layers of the decidua basalis. The regeneration of the uterine mucosa after parturition has its starting-point from the epithelium of the undegenerated glands which persist, this epithelium rapidly evolving a complete mucosa over the entire surface of the uterus.
The complicated arrangement of the human placenta is, of course, the culmination of a long series of specializations, the path along which these have proceeded being probably indicated by the conditions obtaining in some of the lower mammals. The Monotremes resemble the reptiles in being oviparous and in this group of forms there is no relation of the ovum to the maternal tissues such as occurs in the formation of a placenta. In the other mammals viviparity is the rule and this condition does demand some sort of connection between the fetal and maternal tissues. One of the simplest of such connections is that seen in the pig, where the chorionic villi of the ovum fit into corresponding depressions in the uterine mucosa, this tissue, however, undergoing no destruction, and at birth the villi simply withdraw from the depressions of the mucosa, leaving it intact. This type of placentation is an embryo trophic one, and since there is no separation of deciduae from the uterine wall after pregnancy it is also of the indeciduate type. In the sheep the placentation is also embryotrophic and indeciduate, but destruction of the maternal mucosa does take place, the villi penetrating deeply into it and coming into relation with the connective tissue surrounding the maternal blood-vessels. Another step in advance is shown by the dog, in which even the connective tissue around the maternal vessels in the placental area undergoes almost complete destruction so that the chorionic villi are separated from the maternal blood practically only by the endothelial lining of the maternal vessels. In this case the mucosa undergoes so much alteration that the undestroyed portions if it are sloughed off after birth as a decidua, so that the placentation, like that in man, is of the deciduate type. It still represents, however, an embryotrophic type, although closely approximating to the haemotrophic one found in man, in which, as described above, the destruction of the maternal tissues proceeds so far as to open into the maternal blood-vessels, so that the fetal villi are in direct contact with the maternal blood.
If these various stages may be taken to represent steps by which the conditions obtaining in the human placenta have been evolved, the entire process may be regarded as the result of a progressive activity of a parasitic ovum. In the simplest stage the pabulum supplied by the uterus was sufficient for the nutrition of the parasite, but gradually the ovum, by means of its plasmodi-trophoblast, began to attack the tissues of its host, thus obtaining increased nutrition, until finally, breaking through into the maternal blood-vessels, it achieved for itself still more favorable nutrition, by coming into direct contact with the maternal blood.
In addition to the papers by Beneke and Strahl, Bryce and Teacher, Frassi, Jung, and Herzog, cited in Chapter III, the following may be mentioned:
E. Cova: " Ueber ein menschliches Ei der zweiten Woche," Arch, fur Gynaek., lxxxiii, 1907.
L. Frassi: "Ueber ein junges menschliches Ei in situ," Arch, fiir mikr. Anal., lxx, 1907.
O. Grosser: "Vergleichende Anatomic und Entwicklungsgeschichte der Eihaute und der Placenta mit besonderer Berticksichtigung des Menschen," Wien, 1909.
H. Happe: "Beobachtungen an Eihauten junger menschlicher Eier," Anat. Hefte, xxxii, 1906.
W. His: "Die Umschliessung der menschlichen Frucht wahrend der friihesten Zeit. des Schwangerschafts," Archiv fiir Anat. und Physiol., Anat. Abth., 1897.
M. Hofmeier: "Die menschliche Placenta," Wiesbaden, 1890.
F. Keibel: "Zur Entwickelungsgeschichte der Placenta," Anat. Anzeiger, iv, 1889.
J. Kollmann: "Die menschlichen Eier von 6 mm. Grosse," Archiv fiir Anat. und Physiol., Anat. Abth., 1879.
G. Leopold: "Ueber ein sehr junges menschliches Ei in situ," Arb. aus der Frauenklinik in Dresden, rv, 1906.
F. Marchand: "Beobachtungen an jungen menschlichen Eiern," Anat.Hefte, xxi, 1903.
J. Merttens: "Beitrage zur normalen und pathologischen Anatomie der menschlichen Placenta," Zeitschrift fiir Geburtshiilfe und Gynaekol., xxx and xxxi, 1894.
C. S. Minot: "Uterus and Embryo," Journal of Morphol., n, 1889.
G. Paladino: "Sur la genese des espaces intervilleux du placenta humain et de leur premier contenu, comparativement a la meme partie chez quelques mammiferes,"Archives Ital. de Biolog., xxxi and xxxn, 1899.
H. Peters: "Ueber die Einbettung des menschlichen Eies und das friiheste bisher bekannte menschliche Placentationsstadium," Leipzig und Wien, 1899.
J. Rejsek: "Anheftung (Implantation) des Sangetiereies an die Uteruswand, insbesondere des Eies von Spermophilus citellus," Arch, fiir mikrosk. Anat., lxiii, 1964.
T. Rossi Doria: "Ueber die Einbettung des menschlichen Eies, studirt an einemkleinen Eie der zweiten Woche," Arch, fiir Gynaek., lxxvi. 1905.
C. Ruge: "Ueber die menschliche Placentation," Zeitschrift fur Geburtshiilfe und Gynaekol., xxxix, 1898.
Siegenbeek van Hetjkelom: "Ueber die menschliche Placentation," Arch. f. Anat. undPhys., Anat. Abth., 1898.
F. Graf Spee: "Ueber die menschliche Eikammer und Decidua reflexa," Verhandle. des Anat. Gesellsch., xii, 1898.
H. Strahl: "Die menschliche Placenta," Ergebn der Anat. und Enlwickl., II, 1893. "Neues uber den Bau der Placenta," ibid, vi, 1897.
"Placentaranatomie," ibid., viii, 1899.
R. Todyo: "Ein junges menschliches Ei," Arch, fiir Gynaek., xcv, 1912.
Van Cauwenberghe : "Recherches sur la role du Syncytium dans la nutrition embryonnaire de la femme," Arch, de Biol., xxiii, 1907.
J. C. Webster: "Human Placentation," Chicago,
1901. E. Wormser: "Die Regeneration der Uterusschleimhaut nach der Geburt," Arch. fiir Gynaek., lxix, 1903.
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McMurrich JP. The Development Of The Human Body. (1914) P. Blakiston's Son & Co., Philadelphia, Pennsylvania.
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