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Uterogestation - Placentation

Extra-Embryonic Phenomena of Development

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Sharpey W. Thomson A. and Schafer E.A. Quain's Elements of Anatomy. (1878) William Wood and Co., New York.

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1878 Elements of Anatomy: The Ovum | The Blastoderm | Fetal Membranes | Placenta | Musculoskeletal | Neural | Gastrointesinal | Respiratory | Cardiovascular | Urogenital
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Incapsulation of the Ovum in the Decidua

The further history of the chorion may be best given along with that of the structures by which the ovum is fixed in the uterus, and organic union established between it and the maternal system. This union is efFected by the close interpenetration of the vascular villi covering the surface of the chorion with a soft and spongy layer of substance, Avhicli is the product of a rapid enlargement or a sort of hypertrophy of the lining membrane of the uterus. To the latter substance the names of decidua and cadwa are given, from the circumstance that it is separated from the uterus at birth along with the foetus and its membranes. Not only is the ovum from an early period completely imbedded in a covering of decidua, but there takes place at a somewhat later period of uterogestation, in a limited area of one side, a greater enlargement of the vascular chorionic villi, and in close combination with, and surrounding these villi, a corresponding increased development of the decidual substance, by which there is produced the large discoid mass of complex structure, named the placenta (or uterine cake), through which the nourishment of the foetus and the ai'ration of its blood arc mainly carried on during the latter three-fourths of the period of uterogestation.

This placenta continues to increase in size with the foetus and its membranes, and as pregnancy advances, considerable changes take place in the relations of some of these parts, for the fuller comprehension of which it will be necessary to state the successive steps by which the ovum becomes fixed in the uterine decidua, or mcajisulaied, and the manner in which the development of the chorion, decidua, and placenta proceeds.

Earliest Observed Human Ova

In two distinct cases of young unimpregnated women who died at the time cf the invasion of the menstrual flow, Mr. H. Letheby detected an ovum covered by the granullu- cells in the first part of the Fallopian tube, on the same side on which a ruptured follicle was found in the ovary (Trans. Roy. Soc. Lond. 1852, p. 7). But the fecundated human ovum has not yet been traced in the course of its descent in the Fallopian tube, nor has it, indeed, been seen in a satisfactory manner previous to the time when it is already imbedded in the uterine decidua. The history of the most authentic cases of the earliest ova observed in this state leads to the conclusion that the fecundated human ovum does not reach the cavity of the uterus before the seventh or eighth day after its escape from the ovary ; and that already by the twelfth or thirteenth day, if not even sooner, it has acquired a chorion covered with villi, and has become imbedded in the decidua.

The human ovum, like that of mammals generally, probably undergoes very little enlargement during its descent through the tube and the occurrence of segmentation, and its diameter on arriving in the cavity of the uterus does not probably surpass one hundredth, or at most one eightieth of an inch'. A very rapid expansion, however, of the whole ovum no doubt occurs immediately after its entrance into the uterus, in the same manner as observed in a number of mammals, so that within two or three days, its size may have increased to ten or twelve times its original diameter. Having at first only the zona pellucida for its external envelope (or primitive chorion), and being nearly smooth on the surface, it soon acquires, by a process previously described, its new chorion, on which are formed the permanent villi ; and by the time when it has become incapsulated, according to Reichert's observations in the recently published very careful description of an ovum which was probably of the thirteenth or fourteenth day after impregnation, it has a diameter of nearly one-fourth of an inch, and is beset with villi over a considerable part, but not the whole of its surface (see fig. 517).

Several other examples of incapsulated ova of nearly the same period have been observed, as by Von Baer, Velpeau, Wharton Jones, Coste, and Allen Thomson ; but in all these, with the exception of one observed by Velpeau, and supposed to be of the tenth day, or earlier, the whole surface of the ovum was already uniformly covered with short thick set chorionic villi.

The ova observed by Velpeau and Wharton Jones were like that described by Reichert of a period previous to the formation of an embiyo, and may be stated as probably of from ten to foui-teen days old. (Velpeau, " Ovologie Hiimaine," Paris, 1833 ; WTiarton Jones in Trans. Roy. Soc. of Lond.," 1837, p. 339 ; Reichert, " Beschr. einer Friihzeit. Menschl. Frucht, &c.," Berlin, 1873.) Those of Von Baer, Pockels, Coste, and Allen Thomson are of the period immediately following, or from fourteen to eighteen days after impregnation. (Von Baer, " Entwickelungsgeschichte," p. 270 ; Pockels in "Oken"s Isis," 182.5 ; Coste, " Hist. Gen. et Partic. du Developpement," 1817 ; Allen Tliomson, Contrib. to the Hist., c., of the Human Ovum before the Third Week after Conception," in " Edin. Med. and Sui'g. Joum." No. 140.)

Formation of Decidua

Before the arrival of the ovum in the uterus, the lining membrane of that cavity undoubtedly undergoes a preparatory change, by which the formation of the decidua is commenced, and this change, in its first stages, or up to the time of the arrival of the ovum in the uterus, corresponds closely with that which takes place at every successive menstrual period in the uterine membrane. This process consists essentially in a thickening or hypertrophy of the lining membrane, and is mainly due to an extremely rapid proliferation of the subepithelial cells and fibro-cellular tissue, and an increased development of the blood-vessels and glands.

Fig. 518. — The Decidua opened and viewed from before (after W. Hunter).

This is a representation of the thickened membrane of the uterus thrown off as the product of abortion at a very early period of gestation. A and B mark superiorly tue passage of two bristles through the openings from the Fallopian tubes into the cavity of the uterus (cavity of the decidua), and inferiorly the exit of the bristles at the os uteri. In these three situations the torn edges are seen where the decidua has been separated from the continuous part of the mucous membrane. At 0, where an ovum has probably been lodged, the inner part of the decidua is made to bulge towards the cavity of the uterus, and begins to form decidua reflexa.

The formation of a complete decidua within the uterus has been observed in several cases in which, although the ovum was not discovered, or had not yet arrived in the uterus, there was reason to believe impregnation had occurred six or eight days previously. (Von Baer, E. H. Weber.) And a similar condition has been observed in several examples of extra-uterine pregnancy (Hunter and others) ; from which ii, appears that the earlier changes connected with the formation of the decidua are independent of the presence of the ovum in the uterus.

When the ovum has been recently imbedded in the decidua, it forms a swelling or projection of the surface within the uterine cavity, on opening into which the villous chorion is found surrounded by the substance of the decidua or thickened mucous membrane ; but the covering of this substance which passes over the free surface of the. ovum, or that which is towards the uterine cavity, is thinner and simpler in its structure than at the place of attachment of the ovum and in other parts of the uterine surface.

The most projecting part or summit of the swelling formed by the imbedded ovum more especially is somewhat different from the rest, and indicates, by a sort of cicatricial mark, a place where the substance of the decidua, as it gradually covered in the ovum, may be supposed to have finally closed.

The decidual thickening of the mucous memln-ane affects nearly equally the whole of the lining of the uterine cavity, but towards the OS internum, and the openings of the Fallopian tubes, the thickening gradually decreases, and the membrane assumes the unaltered condition which is maintained in these passages. By the fifth or sixth week, when the ovum has reached a diameter of from an inch to an inch and a half, and the nterus has nearly donbled the size which ii presents in the nnimpregnated state, the swelling formed by the ovum and decidua projects strongly like a tumour within the uterine cavity. The membrane of the other parts of the uterus has also undergone progressive increase in its thickness by decidual hypertrophy, so that, having become, as it were, too wide for the capacity of the cavitA', it is thrown into a number of grooves enclosing irregular folds and mammillary projections, but still exhibiting throughout the peculiar features of the mucous lining membrane.

Fig. 519.— View of the Interior of the Human Gravid Uterus at the Twenty-fifth Day (from Farre after Coste).

?t,_ uterine wall ; o, villi of the chorion of the ovnm : dv, decidua vera and enlarged iitenne glands ; dr, decidua reflexa, di\-ided round the margin of the ovum, and turned down so as to expose its pitted surface, which has been removed from the ovum. The right ovary is divided, and shows in section the plicated condition of the earlv conius luteum.

Fig. 520.— DiAGRAJiMATic View of a Transverse Section op the Uterus at the Sea'enth or Eighth Week of Pregnancy.

c, c, c', the cavity of the uterus wLich Ijocoraes the cavity of the deciJua, oiiening at r, c, the cornua, into the Fallopian tubes, and at c', into the cavity of the cervix, which is closed by a plug of luucus ; dr, decidua vera ; the tlat shade indicates the thickened subepithelial structure, the radiated lines the glandular tubes between this and the muscular wall ; (//•, decidua rcflexa with the sparser villi imbedded in its substance ; (Is, decidua serotina, involving the more developed chorionic villi of the commencing placenta, and forming also between these and the muscular wall a layer outside which the glandular tubes are represented ; ch, chorion, with its villi; u, umbilical vessels of the foetus passing into these, and in the umbilical cord ; aJ, remains of the allantoid pedicle ; am, amnion ; i/, und.ilical vesicle ; v/', its duct, connected with /, the intestine of the embrv-o. The placenta is shown as if it were situated at the fundus, but may be supposed to be on the posterior wall of the uterus.

On the side of the ovum which is towards the uterine wall, there is also a layer of decidua, in which the villi of the chorion are imbedded. At this place, from the sixth to the eighth week, these villi begin to be more thickly set and of larger size, aud to undergo a more complex ramification than on the other sides; and as at the same time there is a corresponding increase in the decidual substance, these villi become more and more closely involved in it, and there is thus established the commencement of that more intimate combination of foetal villi and decidual substance, which by its progressive development in the two or three following weeks, gives rise to the formation of the placenta.

By the clianges now described there has become apparent the distinction of the three portions of decidua usually recognised by authors, viz., decidua vera, decidua reflexa, and -decidua serofi/ia. The first of these is that portion of the altered membrane which lines the general cavity of the uterus in every part except that occupied by the attachment of the ovum ; the decidua reflexa is that w'hich covers the ovum as it projects into the uterine cavity, and which is continuous with the decidua vera at the base of the swelling.

The name of decidua serotina has been somewhat variously employed by authors ; but may, in the meantime, be most suital)ly applied to the whole of the decidual substance inteiwening between the ovum and the uterus, and which may include, therefore, both that which is concerned in the formation of the placenta, and the distinct layer of decidual substance which at a later period is found covering the uterine surface of the placenta.

The cavity of thee decidua, which intervenes between the decidua vera and decidua reflexa, and which subsists during the first half of the period of pregnancy, is obviously the same as the original uterine cavity, and, so long as it remains open, naturally communicates with the Fallopian tubes at the upper angles, and the canal of the cervix at the lower. In the last three months of pregnancy, however, this cavity is completely obliterated by the union of the decidua vera and reflexa into one layer over the whole of their extent, so that when afterwards much extended and I'educed to a comparatively thin and irregular stratum of substance, they are at birth thrown off as one membrane along with the other envelopes of the foetus.

x\s the human ovum has never been observed in the progress of its mcapsulation, the exact manner in which this occurs is still involved in doubt. From the various observations, however, already referred to on early ova which have undergone recent incapsulation, and the knowledge of what occurs in animals, it may be conjectured, as first suggested by ^Sharpey (Baly's transl. of "Mailer's Physiology," 1842, p" 1580) that the minute ovum when it arrives in the uterus may be sunk or imbedded in the soft or spongy substance of the mucous membrane, and that when it subsequently enlarges it carries with it, or there is formed round it a covering of the membrane, the substance of which is at the same time undergoing a rapid decidual development, and that this substance continuing to grow with the ovum and expanding with it, constitutes the decidua reflexa. The entire similarity of the structure of the decidua reflexa at its base with that of the decidua vera is in favour of the view that it owes its origin to a similar mode of production.

The formation of the decidua is, as has already been stated, to be attributed mainly to a great increase in the development of the subepithelial tissue. Its substance, accordingly, consists in great measure of the cells, round and spindle-shaped, and cell-fibres which belong to that tissue ; but these are mingled with much larger irregularlyformed multi-nuclear cells, which increase in number as pregnancy advances, and which are peculiarly characteristic of the structm'e of the outer layers of the decidua.

The blood-vessels and the glands of the mucous membrane also undergo great enlargement and modification. The whole of the decidua vera and the basilar part of the reflexa are at first penetrated by blood-vessels derived from those of the uterus, more especially in the latter part of the second and first, half of the third months, Avhen the decidual structure may be considered as having reached its highest degree of development. After this time the blood-vessels of the decidua reflexa, and later those of the whole lining decidua of the uterus, except in the immediate vicinity of the placenta, shrink and ultimately disappear, so that the united decidua becomes in the end wholly non-vascular. The same retrograde process, or atrophy and disappearance, occurs in the blood-vessels of the chorionic villi by which the decidua reflexa is penetrated, and, although the villi themselves never entirely disappear, but may be traced even in the advanced stages of pregnancy as sparse and shrivelled irregular arborescent processes, the blood-vessels very soon begin to shrink and disappear from all the villi ^hich do not form part of the placental structure.

The uterine glands also become enlarged during the development of the decidua, being both elongated in their deeper convoluted portions, which are directed towards the muscular wall of the uterus, and undergoing a peculiar change not yet fully understood, in the parts next their openings on the inner surface. Over the surface of the whole decidua vera, as it lines the uterine cavity, and also on the decidua reflexa, except at its most projecting part, a number of irregular pits are visible to the naked eye, which are frequently so numerous as to give the membrane a reticulated or sieve-like appearance (cups of Montgomery). These pits are really the uterine glands enlarged and altered soon after the commencement of pregnancy, as first clearly shown by Sharpey (Midler's Physiology by Baly, 1842, p. 1579), and the fact has since been observed by others (Kolliker, Coste). The villi embedded in the decidua do not, however, occupy the cavities of these pits, but so far as yet ascertained, are rather sunk in the interglandular hypertro])hie*d substance of the decidua between them (Schroder van der Kolk, Kolliker, and Priestley. See the lectures by the latter " On the Development of the Gravid Uterus," 1860, p. 24). Upon the more exact relation of the villi to the uterine glands in the placenta, further remarks will be made hereafter.

From what has been previously stated, it will be seen that there is at first a general union or interpenet ration of the villi of the chorion with the vascular decidna, the more extended part of this union taking place in the decidua reflexa, and the remainder in that portion of the decidua which is interposed between the ovum and the uterus, and which has been already referred to as decidua serotina, but which might from its relation to the formation of the placenta with propriety be named decidua ^jJaccnicdls. Already in the latter half of the second month of pregnancy, the villi on the uterine side of the chorion become larger and more ramified than those which run into the decidua reflexa ; and as these latter, while they still continue to grow to some extent, do not increase in number, or in the complication of their ramifications, they gradually become more sparse, thin, and elongated, and lose their vascularity.

Fig. 521. — View of the Dissection of the Pregxant Uterus op Forty Days after Conception (from Leisliman after Coste).

a, the embryo sho-OTi within the amnion ; c, the chorion opened, the umbilical vesicle seen lying between it and the amnion ; d, deep surface of decidua reflexa where it has been turned back from the ovum ; r, r, remainder of the decidua i-eflexa projecting from the uterine surface ; v, decidua vera, of which the glandular structure is shown in the cut edges ; x x , openings of the Fallopian tubes seen within the uterus. In the uterine wall the distinction is sho-rtai between the outer muscular part with the wide uterine vessels, and the glandular and decidual internal parts.

An active process of increase meanwhile is going on in the villi placed on the uterine side of the ovum, by which, while they become larger and longer, and penetrate more deeply into the uterine decidua, they also become more minutely and extensively ramified. Changes at the same time occur in the disposition of the decidua placentalis, by which it receives into its substance, and is more and more intimately interlocked with the developing chorionic villi. In the earlier stages, as up to the eighth or ninth week, the foetal and maternal structures may be separated by the withdrawal of the villi from the recesses of the decidua in which they are sunk ; but by the middle of the third month, this becomes no longer possible in consequence of the closer combination or interlocking of the two structures ; in the remaining half of the third month the union becomes more intimate, and by the middle of the fourth month the completion of the placenta is etfected by the continued increase in size and modification of the structure of the maternal and foetal elements.

Structure of the Placenta

At the time when the placenta has attained its characteristic form and peculiar structure, or after the fourth month of pregnancy, it forms a large discoid or lenticular mass interposed in a limited space between the foetal membranes and the uterus. It presents a fcetal and a uterine surface, the former having implanted into it, usually near the middle, the umbilical cord, which carries to the placenta the umbilical arteries and veins of the foetus, and is covered by a tubular prolongation of the amnion, passing over it from that membrane Avhere it lines the placenta to the abdominal integument of the foetus. The placenta continues to increase in size with the foetus, and when it has attained its full dimensions, it has a width of from seven to eight inches, and a thickness of about one inch and a quarter. But towards the circumference it rapidly thins, where it becomes continuous with the chorion and decidua. The foetal surface is covered by the chorion and amnion, and presents the larger divisions of the umbilical vessels before they dip into the substance. The uterine surface shows a subdivision into a number of large lobes, sometimes called cotyledons, which are covered with a layer of decidua (d. serotina) passing over the whole of this surface, and sending septal prolongations into the placenta between the lobes, which in some places run almost as far as the fcetal surface.

The more uniform substance of the placenta (parenchyma) within these lobes, consists, on the one hand, of highly-developed and complicated tufts of foetal villi, which adhere to the chorion by vascular stems of considerable size and strength, and subdivide again and again into very complex ramifications ; and on the other, of certain dilated vascular spaces continuous with the uterine vessels, the outlines of which follow closely the ramifications of the villi throughout every inflection of their surface. These spaces are doubtless to be regarded as belonging to the maternal system, but their exact nature it is very difficult to determine in the fully formed condition. They probably originally. possess walls of their own, and arc contained in abounding substance of uterine or decidual tissue : but this has become so reduced in thickness, or so closely united with, and so nearly assimilated in its structure to the villi, that it has been found difficult to follow it with certainty, and its existence has even by many been entirely denied. The relations of these two parts of the placenta, as ascertained by the observation of their gradual development in the growth of the placenta, and their comparative anatomy in animals, will be referred to hereafter.

The whole of the placental mass, together with the layer of decidua on its external or uterine surface, and the united decidua vera and reflexa are separated at birth along with the foetus and its membranes.

Fig. 522. — Vertical Section thkough the jiiddle part of the Placenta and the XJterixe Wall (from Farre after Wagner).

the preparation was from a woman who died in the thirtieth week of gestation : the fines V, u, run through the wall of the uterus to the outer surface of the placenta ; d, the decidua serotina ; p, the tufts of foetal vascular villi, of which two larger divisions are separated by decidual septa, as at dp ; /, the placental end of the umhilical cord ; am, the amnion ; cli, the chorion ; vf, divided fcetal blood-vessels ; r, stems of vascular villi ; us, uterine sinuses or veins ; a, a, coiled arteries passing into the placenta.

Circulation of Blood in the Placenta

The existence of a distinct circulation of blood in the fcetal and in the maternal vessels of the placenta, discovered by the Hunters, has long been placed beyond doubt by the experimental investigations of all those who have injected the two sets of vessels with sufficient care and success. The nature of the distribution of the vessels is very different in the two parts of the placenta. In the tufts of foetal villi, the umbilical arteries and veins, possessed of distinct coats, undergo gradual subdivision by ramification into smaller and smaller tubes, until they at last reach capillary minuteness, and the terminal capillaries run in long and tortuous loops which pass from one extreme branch of the villi to another, within the fibrous core of which they are situated, and SchriJder van der Kolk has described also a tiner superficial network of capillaries distributed below the epithelial covering of the stems and larger branches of the villi. By artificial injections fluids can be made to pass with perfect precision from the umbilical arteries through the capiharies of the villi into the veins, or in the reverse direction from the veins into the arteries. Nor does there ever occur, except from visible accidental rupture of the vessels, either extravasation of the injected material into the intervening tissue, nor any escape into the maternal sinuses.

Fig. 523.— Small Portion of Placenta SHOWING THE FcETAL ViLLISlightly Magnified (from Leishman after Weber).

The uterine blood-passages, on the other hand, are of the nature of irregular spaces, into which the maternal blood is poured directly by numerous small coiled arteries which, as shown by the Hunters, pierce the external decidua at the uterine surface of the placenta, and open into these blood-spaces without the intervention of any capillary subdivision. The result of artificial injection of the blood-vessels in the pregnant uterus equally demonstrates the nature of the circulation in the maternal part of the placenta, for it is easy to show by this method, that a fluid thrown into the uterine arteries fills at once all the maternal bloodspaces of the placenta, surrounding everywhere the chorionic or foetal villi, and returns thence into the uterine veins b_y a number of slanting venous channels, the ntero-jilaccntal sinuses, provided with delicate coats, which issue from the placenta at its uterine surface by piercing the decidua serotiua, and which are most numerous towards the circumference of the organ, where they are in communication with the so-called circular vein or circular sinus previously referred to. Some of these veins may even be traced for some distance into the placenta, in the septa of decidual substance, which are prolonged from the external decidiia serotina bet-^veen the lobes. (For an excellent account of the evidence in favour of the foregoing views, supported by original observations, see Professor Turner's observations on the " Structure of the Human Placenta," in Proceed. Eoy. Sec. of Edin., May, 1872, and in Journ. of Anat., vol. vii., p. 120.)

Fig. 524. — Chorionic Villits from: the Placenta AT THE Twelfth Week. Enlarged ISO Diameters (from Leisliman after Ecker).

From a to h, the epithelial coveriDg is left entire ; from a to a it ha.s been removed and the fibrons core with the capillary blood-vessels is shown.

Farther Consideration of the Structure of the Placenta

Two doubtful points respecting the structure of the placenta still requne consideration, viz.. 1st, the extent to which uterine tissue is included in or penetrates into, or remains as a constituent of the maternal part ; and 2nd, the relation of the interpenetration of the fojtal villi and the uterine decidua to the glandular or other structures of the uterus.

In regard to the first of these points, the views of anatomists still differ gi'eatly ; for, on the one hand, some hold that there is no vestige of iiterine tissue left in connection with the maternal blood-spaces, at all events in the deeper two-thirds of the thickness of the placenta, and that consequently the maternal blood cii-culating in the placenta is in direct contact with the epithelial covering of the foetal villi ; while others are inclined to regard that epithelial covering, or some part of the structure which appears to belong to the foetal villi, as really containing some of the elements of the decidua.

Goodstr, indeed, described a double cellular covering of the placental villi, regarding the external layer as of uterine, and the internal as of foetal origin. But later anatomists have not succeeded in confirming these observations, and it does not appear certain that there is more than one obvious layer of cells over the surface of the foetal villi. If, therefore, we assume the existence of a layer of uterine cells in the fully formed placenta, we are reduced to the necessity of supposing that it has either replaced that of the chorionic villi, or has become closely incorporated with it (Kolliker).

The observation of the gradual penetration of the decidua by the villi in the earlier stages of placental formation, the possibility of separating the foetal and maternal structui'es from each other during a certain time, and the undoubted presence both of decidual tissue and of uterine blood-vessels possessed of walls of their own in the commencement, — all supply convincing proof that uterine elements of structure have originally existed in the placenta, and have contributed to its formation along with those derived from the fostus ; but the condition of the uterine elements in the more advanced stages of placental gi'owth, if they really then exist, still requires further investigation.

The actual enlargement of the uterine capillaries of the decidua into the form of vascular spaces has been traced by Virchow (Archiv, vol. iii. p. 450), and Priestley has observed the capillary form of the maternal vessels surrounding the villi in a product of abortion of the 8th week (Lectures on the Development of the Gravid Uterus, 1860, p. 02). Some anatomists, indeed, as Schroder Van der Kolk, afiirm that they have been able to detect the remains of a vascular wall in connection with the blood-spaces of some parts of the placenta (Waamemingen over het Maaksel van de Menschelijke Placenta : Amsterdam. 18G1), so that it may with reason be surmised that the change which takes place in the stnictirre of the parts forming the human placenta, in the course of the fourth month, is mainly of the nature of a rapid thinning and absorption of the elements of the decidual tissue and vascular walls.

The view that the placenta originally consists of uterine as well as foetal elements combined receives the fullest confirmation from the study of the comparsr tive anatomy of the various forms of simple placental structures in animals, as, in the diffyge placenta of the pachydermata, solipeds, cetacea and some other animals, and in the cotylcdonous placenta of most ruminants ; in both of which only the foetal part of the placental structure undergoes separation from the uterus at birth, — constituting the iwn-dccidnate iorm. of placentation ; and in the various forms of more complete union of the foetal and maternal elements, which occur in the zonal placenta of the camivora, and the discoid of the rodentia, in which more or less of the maternal stmcture derived from decidual formation comes away with the foetal product at birth. But in all forms of placentation of animals, -n-itli the exception of the Simiac, in -v-\-hich the structure probably agrees closely Avith the human, the elements of uterine structure are very clearly present, and the uterine blood-vessels may be recog-nised as such, not being dilated into vide sinuses or lacv\na3, but retaining more or less the capillary form of distribution : although in some instances the capillaries have undergone considerable dilatation, and geem to be passing into the condition of venous sinuses. (For a very instructive view of the structure of the placenta in these various animals and their bearing upon the nature of the i^lacental structure in general, the reader is referred to Professor Turners Lectures on the Structure of the Diffused, the Polycotyledonous and the Zonary forms of Placenta, as published in the Journal of Anatom}- and Physiology, vol. x.. p. 127. Oct. 1875, and separately, 1870. There may also be consulted Yon Baer's Entwickelungsgeschichte. 1839. and Eschricht, De oi-ganis qua3 Respirationi et Xutritioni Foetus Mammalium inserviunt, Hafnia3, 1837.)

It has long been known that in the placentation of various mammals the uterine glands undergo an increased development, and it has been supposed that they enter into structural and functional relation with the fcetal villi ; but more precise knowledge is still wanting as to whether, and to what extent this relation is of the nature of an actual penetration of the cavities of developed and dilated glands hj the chorionic villi. It was shoAAii by Dr. Sharpey (Miiller's Physiology by Baly, vol. ii. p. 1574) that thedecidua which enters into the fonnation of the zonular placenta of the dog. and doubtless of other camivora, consists of a hypertrophical part of the uterine mucous memlu-ane of corresponding figure, in which the main ducts of the largely developed glands become dilated at their orifices into utricular saccules or pouches, that hollow membranous processes containing foetal vessels rise from the chorion, from among the smaller vascular villi, and apply their flattened summits to the widened mouths of the gland ducts, and enter a short way within the saccules : while these receptacles are filled with a whitish semifluid secretion and are lined with an epithelium, which also covers the intriided part of the fcetal process. As pregnancy advances, the chorional and decidual structures are further interlocked, and the arrangement becomes more intricate. Decidual, that is, maternal blood-vessels are abundantly distributed round the villi of the chorion, which they closely cover. These vessels are larger than ordinary capillaries ; they are unsupported by decidual stroma, and are separated along with the rest of the placenta in parturition.

The observations of Shaipey appeared to receive confirmation from the investigations of E. H, Weber (Zusatze zur Lehre vom Baue, kc, der Geschlechtsorgane, 184G,) and Bischoff (Entmck, des Hundeeies, 184.5,), and by many these observations have been held to prove satisfactorily a connection between the glands and placental formation. But Dr. Sharpey has expressed himself cautiously on this general question, and more recent observations tend rather to throw doubts on the penetration of the uterine glands by the villi of the chorion.

More especially the observations of Ercolani (Mem. of the Acad, of Bologna, 1868 and 1870) and of Turner (loc. cit.) seem to show that it is possible that the saccules described by Sharpey may be formed in the uterine decidua of the carnivora independently of the glands ; and they are disposed to think that these saccules are produced rather in the interglandular tissue. From an examination of the whole evidence on this point and an investigation of the structure and formation of the placenta in different animals, contained in the lectiu-es prcAdously quoted. Turner has ascertained that in the diffused fonn of placenta the uterine glands open in the sow into feebly vascular intervals between the vascular crypts in which the foetal chorionic fringes are sunk, and not into these crypts themselves ; that in the mare the glands open on elevated ridges between the vascular crypts which receive the foetal villi, and only in the Cetacea (Orca gladiator) did he find gland apertures in some of the placental crj-pts. Again, the maternal cotyledons of the i-uminants are destitute of utricular glands, and these are confined to the sunk intercotyledonous part of the uterine membrane. In the zonal placenta of the camivora, as previously stated. Turner failed to trace the uterine glands into the recesses of the decidua which receive the prolongations of the chorionic plates and villi, constituting the foetal portion of the placenta ; and he cliilers therefore from Sharpey in regarding the crypts as of new formation and independent of the glands. He is thns led to the general conclusion that m no kinds of placenta do the uterine glands form an essential part of the placental structm'e, and that the uterine crypts which receive the fcetal processes are essentially interglandular in their origin. Nevertheless Turner recognises the existence in all placentas of uterine structiu'al elements of a cellular nature, which he regards as descendants of the epithelial or subepithelial tissue of the uterine mucous membrane, and to which he attributes, as others have done, glandular functions in the preparation of the matter which is absorbed as nourishment by the blood-vessels of the foetal \'illi. Further observations will be requhed to detennine in how far these views admit of application to the stracture of the fully-formed human placenta. (See also Tm'ner's Memoir on the Placentation of Seals. Trans. Roy. Soc. of Edinbm-gh. Vol. xxvii.. 1875.)

With respect also to the relation of the uterine glands to the penetration of the decidua by the viUi in the fonnation of the human placenta, further observations are stiU requii-ed. As iireviously stated, anatomists have failed to trace the villi into the dilated parts of the enlarged glands, and Kolllker after a careful examination of the whole subject, comes to the conclusion (Lectures, p. 1G2) that the villi in becoming involved in the decidua have no pennanent connection with the glands. He affirms indeed that the glands soon shrmk over the whole extent of the decidua. beg'uining to do so as early as in the second month of j^regnancy, and have in great measure disappeared before the chorionic villi are fully connected with the uterine membrane. Reichert. however, in his recent description of an early human ovum affirms that commencing villi actually enter the mouths of uterine glands, and he has given a diagrammatic representation of the prolongation of the tubes of the uterine glands through the decidua to the surface of the membrane, and the small marginal villi of the o\Tim as actually within tenninal portions of the glands, the cavities of which have undergone some degree of ramification. Fui-ther observations will, however, be necessary for the confirmation of a statement so much at variance with the results of most other observers.

General Conclusion

In recapitulation of the preceding description it may be stated that the human placenta is an organ which is formed by the conabination of two different structural elements ; of which one is derived from the foetus or its membranes, and the other from the uterus. The foetal part consists of the developed vascular villi of the chorion, continues to grow and extend itself with the foetus during the whole of uterogestation, and is the seat of a complete circulation of the foetal blood through the capillary ramifications of the umbilical arteries, veins, and capillaries. The uterine element of the placenta originates in a part of the decidua, which is produced by increased growth and transformation of the lining membrane of the uterus and its blood-vessels. "With the hypertrophied structure so produced the villi of the foetal chorion at first interpenetrate, so that in the earlier stages of placental formation the uterine and foetal elements are for a time separable, and may still be distinguished fi-om each other, at the period even when they have become more intimately united. But in the progress of development the uterine elements are so much modified, and finally so completely attenuated or removed, that they almost entirely disappear ; and as along with them the walls of the blood-vessels are either thinned out to the last degree, or are entirely absorbed, there remain only the vascular spaces through which the maternal blood flows. A doubt, however, may still exist as to whether these spaces are, or are not, entirely deprived of any uterine enclosure.

The maternal blood is introduced into these spaces directly by the small coiled uterine arteries, without capillary intervention, and after moving through the whole of the placental spaces in contact with or in closest proximity to the foetal villi, it is returned by numerous veins through the outer decidua serotina into the vascular channels of the uterus. This blood is essentially arterial in its qualities, and may be supposed to perform a double function, viz., 1. to exert an aerating effect on the blood of the foetus through the tissue of the villi and the walls of their minute vessels, and 2. to supply for absorption by the foetal vessels the new materials required in the continued nourishment of the foetus.

The continuity of the decidua vera and decidua reflexa with the decidua serotina, and of all three with the whole thickness of the placenta at its margin, sufficiently demonstrates the actual coonection of the maternal elements in these several structures, and the existence of that connection is fully confirmed by tracing the steps of the primary formation and subsequent development of the two sets of placental elements, by which are ascertained the actual presence of both in the commencement, and the gradual modification and disappearance of the maternal part. The view thus arrived at receives further support from the result of the observation of the varieties of form and structure presented by the placental organisation in different animals.

Separation at Birth and Restoration of the Mucous Membrane of the Uterus

In the act of birth the whole decidual structures which have been formed in human uterogestation are separated from the uterus along with all those belonging to the ovum, and the placentation is thus said to be completely deciduate. Thus in parturition, fi'om the effect of the contraction of the uterine walls and the abdominal muscles, after the usual rupture of the foetal membranes and the discharge of the amniotic fluid, the foetus is first expelled : the placenta is next detached and pressed downwards, carrying with it the layer of serotina by which it is covered on its uterine surface, and along with it necessarily are broken through the coiled arteries and the slanting veins ; the membranes of the ovum follow, consisting of the amnion and chorion, together with the shrunken covering of decidua which in the last stage of pregnancy remains from the union of decidua reflexa with decidua vera matted together into one, which is finally peeled off the whole of the interior of the uterus.

The uterus having now contracted and its cavity being greatly reduced in size, there remains, probably, on the uterine surface a part of the subepithelial or decidual structure of the mucous membrane, in irregular shreds rather than in one continuous layer, and in the deeper part are imbedded the convoluted uterine glands extending outwards into the layer of fibres formed by the muscularis mucosse. These remains of decidua, with the clots of blood resulting from the rupture of the vessels, are gradually cast off with the lochia, a discharge which is at first of a mixed character, but gradually becomes more and more composed of corpuscles similar to the white blood globules, and this is succeeded by the closure and contraction of the vessels, the prolongation of the gland tubes to the surface, the formation of a complete ciliated epithelial lining to the cavity, and the complete restoration of the natural structure of the whole membrane.

1878 Elements of Anatomy: The Ovum | The Blastoderm | Fetal Membranes | Placenta | Musculoskeletal | Neural | Gastrointesinal | Respiratory | Cardiovascular | Urogenital

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