Short notes on the structure of human placenta; what are its functions
In viviparous animals the embryo develops inside the uterus of the mother and only a fully formed offspring comes out of the mother. This is because the amount of nutrition (yolk) stored in the egg is insufficient for the development of the embryo. Therefore the developing embryo has to depend on the maternal tissues for its nourishment. The embryos which develop inside the uterus of the mother will be attached to the uterine wall to obtain the necessary nourishement. This attachment of the embryo to the uterus will be through an organ called placenta.
Basically the placenta has two parts from the point of view of its origin. These are foetal placenta by the extra embryonic membranes of the embryo and maternal placenta provided by the endometrium of the uterus. Placenta can be classified into various categories based on their origin, on their morphological structure, on the distribution of villi and also on their histological structure.
The placenta of human beings is called allantoic or chorio allantoic placenta. Here the yolk sac is rudimentay. The allantois is very well developed, becomes vascularised and fuses with the chorion. Here the chorion bears a number of outgrowths called villi which project into the maternal tissue to help the absorption. Each villus has an outer ectoderm formed from chorion and the endoderm formed by the allantois. Between the two is a highly vascularised mesoderm consisting of the connecting tissue. The mesoderm is dual in origin. It is made up of both chorionic as well as allantoic parts.
The maternal part of placenta consists of the outer layer of the endometrium and also the sroma with its associated glands and the uterine blood vessels.
The chorio-allantoic placenta has been classified into various types on criteria such as morphological structure, anatomical structure etc. Based on the morphological structure the chorio-allantoic placenta of human beings may be classified as deciduous placenta or plamentavera. Here the association between the foetal and maternal tissues is very intimate. The uterine wall gets eroded due to the action of the penetration of embryonic tissues to facilitate two way transportation of materials (from mother to foetus and vice versa). Here the chronic villi fuse with the uterine mucosa to bring out better communication.
At the end of pregnancy the entire placenta comes out causing extensive haemorrhage from the uterine wall during child birth. This haemorrhage is due to the intimate association of foetal and maternal tissues. Based on the anatomical structure, the human placenta is designated as haemochorial placenta. Here there are only three layers from the side of the foetus. The three layers from the side of the mother disappear.
In such cases the endothelial walls of the uterine blood vessel disappear and the chorionic villi directly come into contact with the maternal blood. The chorionic villi are surrounded by sinuses (spaces) without any endothelial covering and into this the maternal blood enters through the areteries of the uterus and the blood from this flows back into the uterine vein.
Functions of placenta
Placenta acts as a link between the foetus and the mother helping in the transportation of nutrients from the latter to the former.
Placenta helps in the exchange of gases such as carbon dioxide and oxygen between the mother and the foetus thus playing an important role in foetal respiration. Oxygen from the maternal blood diffuses into the foetal blood while the carbon dioxide from the foetal blood is taken into the maternal blood.
The waste products produced due to the metabolic activities of the foetus are transferred to the maternal blood with the help of the placenta.
The maternal blood provides antibodies against several diseases to the foetus to keep it healthy.
5. Transport of drugs:
Some of the drugs taken by the mother during pregnancy can pass on into the blood of the foetus through the placenta. Many a time certain drugs taken for ailments following the mother may harm the foetus. Pregnant women should be very careful in the use of drugs even for small ailments. For instance thalidomide which is used as a sedative to prevent morning sickness is known to cause many deformities in the foetus.
6. Storage of nutrition:
Placenta can store materials like fat, glycogen, iron etc.
7. Secretion of hormones:
The placenta can secrete a number of hormones such as progesterone, estrogen, gonadotropin etc., which are helpful in maintaining pregnancy.
8. Transport of pathogens:
Microbial pathogens such as viruses can enter the foetus through the placenta. Particularly those causing syphilis, measles, small pox are known to enter the blood stream of the foetus if the mother gets infected during pregnancy.