Origin and significance of mammalian Placenta (Paper one hundred five; Unit four)

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Origin and significance of mammalian Placenta (Paper one hundred five; Unit four)

The term placenta in its broadest sense refers to any region in a viviparous organism where maternal and embryonic tissues of any kind are closely apposed, and which serves as a site for physiological exchange between parent and embryo. The placenta can be defined as a temporary organ which is formed jointly by the extra-embryonic membranes of the foetus and maternal tissue. The developing embryo or foetus of viviparous mammals obtains its nourishment from the maternal uterine tissue. The mode of formation and fusion of the placenta to the uterine wall is called placentation.

The phenomenon of placentation is invariably related with the viviparity of the organism. In all viviparous animals, the development of the young takes place inside the uterus of the mother because the amount of stored yolk of egg does not remain sufficient for the development of an embryo which can lead an independent mode of existence, therefore, the developing embryo has to depend on the mother in lesser or greater degree for nourishment, oxygen supply and other physiological assignments. Such embryos which develop inside the uterus of the mother often get attached with uterine wall to draw necessary substances from the maternal uterine circulation by means of an organ called placenta. The placentae are not found exclusively in mammals but appear also in animals belonging to various groups of the animal kingdom such as in Peripatus, Salpa, Mustelus laevis, and certain lizards. The nature of the tissues entering into the formation of the placenta is not the same in all cases. For example, in placental fishes and reptiles, the yolk sac comes in close relation with the maternal blood stream and vitelline circulation carries the materials to and from the embryo. However, the placentae of mammals have following mode of origin, structure, type and physiology.

Mode of origin and Types of mammalian placentae

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 mammals, there are two possible sources of chorionic vascularisation - the vitelline circulation provided by the yolk sac and allantoic 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 In some marsupials, 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.

B. Chorio-allantoic Placenta or Allantoic Placenta In some marsupials, 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 allantois smooth, but bears root-like, vascular yolk sac processes, the villi, which grow out from endometrium of uterus the chorion into the adjacent maternal tissue. Thus, in chorio-allantoic placenta, the chorion lined with allantois allantois becomes associated with the uterine vitelline blood vessels allantoic blood vessels wall during the gestation period. In it A B 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. 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 give 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.

Classification of chorio-allantoic placenta

A. Morphological Classification of Placenta

D. Classification of Placentae According to the Mode of Implantation

Physiology of placenta

Overview

The document explores the biological significance and various classifications of mammalian placentae, detailing how they facilitate the exchange of nutrients and gases between mother and embryo. It highlights the complexity of placentation across different species.

Key Points

1The placenta is a temporary organ crucial for nutrient exchange in viviparous mammals
2There are two main types of mammalian placenta: chorio-vitelline and chorio-allantoic
3Placentae vary widely in structure and function among different mammalian species
4The classification of placentae can be based on morphology, histology, and implantation type

Details

Category: Biology and Natural Sciences

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