A mammalian placenta, typically, is a structure produced by the apposition or fusion of the extra-embryonic membranes (i.e., chorion) with the endometrium of uterus for the purpose of physiological exchange. It, therefore, follows that the placenta from the point of view of its origin, consists of two parts- a foetal placenta, furnished by the extra-embryonic membranes and a maternal placenta, furnished by the uterine endometrium.
Now, it would be obvious that while on the maternal side a single component, the endometrium is involved; on the foetal side, one has to consider the prospective roles of four elements – amnion, chorion, yolk sac and allantois. The first of these, the amnion, may be ruled out immediately, as it is making no direct contribution to the placenta.
This leaves the other three, of which the chorion, because of its most external position, is the membrane making immediate contact with the endometrium. In chick embryo, the chorion plays its role by way of a vascular supply, which it acquires from the allantois.
In mammals, there are two possible sources of chorionic vascularisation – the vitelline circulation provided by the yolk sac and allanotic circulation, provided by the allantois. Thus, it can be said that in mammals, there exists two essentially different main types of placentae – the chorio-vitelline placenta and the chorio-allantoic placenta.
A. Chorio-vitelline Placenta or Chorionic Placenta or Yolk-Sac Placenta:
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In some marsupials (Didelphys, Macropus), the allantois remains relatively small and never makes contact with the chorion. Whereas the yolk sac becomes very large and fuse broadly with the chorion. In these forms, the chorion gains its blood supply from the network of vitelline blood vessels of yolk sac.
Such a placenta is called yolk sac placenta or chorio-vitelline placenta. In such a foetal placenta, the chorion never advances beyond a smooth membrane is close apposition with the vascular uterine lining, the endometrium. The chorio-vitelline placenta also occurs as a temporary structure in insectivores, rodents and horse (eutherians).
B. Chorio-Allantoic Placenta or Allantoic Placenta:
In some marsupials (e.g., Parameles, Dasyurus), and all eutherians the yolk sac remains rudimentary and the allantois becomes well developed and vascularised to fuse with chorion and to furnish the latter the blood supply. Such a foetal placenta is called chorio-allantoic placenta. In this kind of placenta, the chorion is not smooth, but bears root-like, vascular processes, the villi, which grow out from the chorion into the adjacent maternal tissue.
Thus, in chorio-allantoic placenta, the chorion lined with allantois becomes associated with the uterine wall during the gestation period. In it the allantoic (umbilical) vessel transports the materials. As the embryonic body develops, it separates from various membranes until only the umbilicus cord connects the foetus with the placenta.
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The final part of the placenta consists of chorionic villi formed of outer chorionic ectoderm, inner allantoic endoderm and in between these is a highly vascularised mesodermal connective tissue of double origin-somatic or chorionic and allantoic or splanchnic. The ectodermal cells have mircovilli to increase the absorptive surface.
The trophoblast that gives rise to villi often becomes syncytial on its outer border towards endometrium (syncytiotrophoblast). The maternal part of placenta consists of the epithelium of endometrium and the stroma of endometrium with its glands and the uterine blood vessels.