The Classification of Mammalian Placenta!
According to the modes of distribution, pattern of the villi, and the degree of intimacy established between the foetal and maternal tissues, following types of placentae have been recognized:
1. On the degree of contact between chorionic villi and the endometrium.
(a) Non-deciduate placenta:
The chorionic villi are simple and minute. They lie on the crypts of uterine wall apposing endometrium and do not fuse with it. At the time of birth, when parturition (separation of the foetus and its membranes from the uterine wall) occurs, the chorionic villi are simply drawn out from the depression without any damage to the uterine wall, hence no bleeding occurs. This type of placenta found in pig, cattle, horse and other ruminants is known as non-deciduae or semi-placenta.
(b) Deciduate placenta:
In higher eutherian mammals including dog, rabbit and man, the degree of intimacy between maternal and foetal tissues increased. The allanto-choric villi become complex and penetrate deeper into the uterine tissue. Chorionic villi fuse with eroded uterine mucosa to various degrees, so that the chorionic epithelium comes to lie in the connective tissue or into the maternal blood, facilitating the passage of substances from the mother to the foetus and vice versa.
Such a placenta is cast off at the time of birth; there is loss, not only of embryonic membranes but also of encapsulating maternal tissue with extensive haemorrhage. Such a placenta is called deciduate placenta, and the uterine wall participating in the formation of such a placenta is the decidua.
(c) Contra deciduate placenta:
In Таlpa (mole) and Parameles (bandicoot) no part of the placenta is thrown out. The foetal portion of the placenta remains in the uterus, where it is broken up or absorbed in situ by maternal leukocytes; such a condition is called contra deciduate.
2. Type of placenta according to the distribution pattern of chorionic villi:
(a) Diffuse placenta:
Among primates (lemurs) and some ungulates (pig, horse), the chorionic villi remain scattered all over the surface of the chorion and their placentae are correspondingly extensive.
(b) Cotyledonary placenta:
In ruminants (cud-chewing) ungulates such as cattle, sheep and deer, the true villi are grouped in well-spaced prominent rosettes or patches known as cotyledons. The rest of the chorionic surface is smooth.
(c) Zonary placenta:
In carnivores (cat, dog) the villi have developed in the form of a belt or a girdle around the middle of the chorionic sac. Raccoon has incomplete zonary placenta.
(d) Discoidal placenta:
The insectivores, rodents, anthropoid apes, and bats, the chorion is at first all covered with villi, but the villi continue to develop only on the side turned away from the lumen of the uterus, while on the other parts of the chorion, the villi are reduced.
The functional placenta, therefore, has a single disc-shaped villous area. Such a placenta is called mono-Discoidal placenta. In monkeys, the placenta consists of two disc shaped villous areas and is known as bidiscoidal placenta.
3. Type of placenta based on the degree of intimacy between the foetal and maternal tissues:
(a) Epithelio-chorial placenta:
This type is the least modified type of placental condition illustrated by all marsupials, some ungulates and lemurs. The allantois and chorion unite and become jointly vascularized by allantoic blood vessels. This composite membrane forms simple chorionic villi fitting into corresponding pockets or pits in the mucosal lining of the uterus. In this type of placenta all the six tissues (three foetal and three maternal) lie as barriers between the foetal and maternal blood streams.
Therefore, the molecules of nutrients and oxygen during their passage from the mother to foetus pass through (i) the endothelium of maternal blood vessels, (ii) endometrial connective tissue (maternal mesenchyme) ground material of blood vessels, (iii) uterine epithelium, (iv) the epithelium of chorion, (v) allanto-chorionic connective tissue (foetal mesenchyme), and (vi) the endothelium of foetal blood vessels. Over the same path gaseous wastes from the embryo travel in reverse direction. The allantois therefore, not only, serves as an organ of respiration and excretion, as in reptiles and birds, but also as a participant in nutrition.
In this case the relationship is purely one of the apposition, because the chorionic epithelium is merely applying itself against the uterine epithelium. At the time of parturition, the villi can be pulled out of the pocket in which they are embedded, leaving the surface of uterine wall intact. There is no bleeding and damage to any tissue.
The thickness of the partition between the maternal and foetal blood may be decreased by the removal of some of the intervening layers of tissue. Depending upon the layers that disappear, placenta may be further distinguished into different types.
(b) Syndesmochorial placenta:
The epitheliochorial placenta is modified slightly by an advance in the sub group of ruminant ungulates. In these mammals, the prominent villi penetrate deeper in the uterine pits so as to erode the uterine epithelium, which allows the chorionic ectoderm to come into direct contact with the vascular material connective tissue.
Only five barriers in between the foetal and uterine blood streams are present. At the end of gestation, however, the chorionic villi are merely withdrawn, and the maternal mucosa is not torn away. Therefore, no bleeding occurs at the time of parturition.
(c) Endotheliochorial placenta:
In carnivores the foetal-maternal union shows a marked advance in intimacy. Erosion of the maternal uterine epithelium and the underlying connective tissue practically bores the endothelium of maternal blood vessels and the syncytial chorionic epithelium then comes in contact with the endothelial walls of the uterine blood vessels.
Therefore, only four barriers are left between the foetal and maternal blood streams. At birth there is destructive separation of placenta through which the foetal layers with enclosed maternal vessels split off from a deeper regenerative zone of maternal tissue.
(d) Haemochorial placenta:
In lower rodents, insectivores, bats and man, the destruction of the endothelial wall of maternal blood vessels causes the chorionic epithelium to bath directly in maternal blood vessels, which circulates in the lacunae.
This facilitates rapid gaseous exchange and diffusion of nutrients from the maternal blood into the blood vessels of chorionic villi. The villi may be ramified by dendritic structures and form a complicated network. The fusions between chorionic and uterine tissues are so intimate that at birth the placenta tears away as an unit.
(e) Haemendothelial placenta:
In higher rodents and rabbit, the number of barriers between the maternal and foetal blood streams is still reduced. The chorionic villi lose their epithelium and connective tissue layers to such a degree that in most places endothelial lining of their vessels alone separate the foetal blood from the maternal sinuses.
There is evidence that the rate of transfer of substances from the blood of the mother to that of the foetus increases as the number of layer to be crossed decreases. The increase in efficiency of permeability is the same as the order in which the structural type of placenta have been discussed.
It is also natural to assume that this structural series with a progressive thinning of the foetal-maternal barrier indicates the evolutionary sequence. Yet this may not be the case, since the epitheliochorial type is widely scattered and tends to occur in mammals, highly specialized in other respects.