Placenta - Human Development
We had said previous in the unit that in the second week of development a primitive uteroplacental circulation is established. Early in this stage slender projections grow out from the trophoblast into the surrounding endometrium. These are the chorionic villi and they become branched. By the end of fourth week they are healthy formed. Like the chorionic villi develop embryonic blood vessels start to form in them and these vessels are continuous with the vessels in the connecting stalk. Matching the chorionic villi, uterine crypts develop. Till about the end of the eighth week the chorionic villi cover the entire surface of the former blastocyst. Like the embryo and chorion enlarge only those villi that are in contact with the endometrium remain. The others degenerate.
The region still in contact along with the endometrium forms the disk shaped placenta. The manner in which the trophoblast interacts with the maternal tissue mainly determines the morphology of the placenta. Very intimate contact between the mother and embryo is attained in the placenta found in the humans and rodents. Here the chorionic villi produce deep into uterine tissue and break -down the maternal blood vessels till they are bathed in maternal blood. This kind is called the hemochorial placenta, where a thin membrane separates the embryonic blood in the capillary of the chorionic villus from the maternal blood in the crypts of the endometrium. This membrane is the placental membrane and is composed of epithelial membrane of the capillary and the membrane of the villus. It is by this membrane that exchange of gases occurs. Oxygen and nutrients pass from the maternal blood to the embryo and carbon dioxide the other wastes from the foetal blood move across the membrane to the mother's blood.
By the 10th week of development, the placenta is fully formed. It starts to secrete estrogen and progesterone. These hormones through their negative feedback on the pituitary and hypothalamus prevent any new follicle from maturing and as well maintain the lining of the uterus. Several substances can move across the membrane by active transport and pinocytosis. The blood of the embryo generally never mixes with that of the mother, though, small molecules and various toxins and viruses do pass through across the placenta and this is the reason that many drugs taken by the mother or certain infections contacted by the mother are passed on to the embryo.