The Probert Encyclopaedia of Medicine

PLACENTA

The placenta is formed from the chorionic villi, small finger-like projections that cover the outer cells of the blastocyst. After implantation of the fertilized ovum, the chorionic villi burrow into the lining of the uterus seeking nourishment; those which have penetrated deepest erode some of the small uterine blood vessels and become bathed in the mother's blood. At this point the burrowing stops and the villi start to multiply and form branches. It is these villi which form the basis of the placenta. The placenta is responsible for the transfer of nourishment from the mother to the fetus, and of the waste products the fetus produces to the mother so that they can be excreted. Two layers of cells keep the fetal circulation in the placenta separate from the maternal blood. Through these cells the vital exchange function of the placenta takes place. Carbon dioxide, waste products and hormones pass from the fetus to the mother. Oxygen, nutrients and hormones are transferred in the opposite direction. The placenta also acts as a barrier to protect the fetus against potentially harmful substances.
Research Placenta