Formation and Role of Placenta

As the developing blastocyst hatches from the zona pellucida (day 5-6 post fertilization) it has increasing nutritional needs. These are met by the development of an association with the uterine wall into which it implants. A series of synchronized morphological and biochemical changes occur in the embryo and the endometrium. The final product of this is the placenta, a temporary organ that affords physiological exchange, but no direct connection between the maternal circulation and that of the embryo. Initially cells in the outer layer of the blastocyst, the trophoblast, differentiate producing an overlying syncytial layer that adheres to the endometrium. The embryo then commences its interstitial implantation as cells of the syncytiotrophoblast pass between the endometrial epithelial cells and penetrate the decidualized endometrium. The invading embryo is first nourished by secretions of the endometrial glands. Subsequently the enlarging syncytiotrophoblast develops spaces that anastomose with maternal vascular sinusoids, forming the first (lacunar) uteroplacental circulation. The villous placental circulation then develops as fingers of cytotrophoblast with its overlying syncytiotrophoblast (primary villi) extend from the chorion into the maternal blood space. The primary villi become secondary villi as they are invaded by extraembryonic mesoderm and finally tertiary villi as embryonic blood vessels develop within them. During the first trimester of pregnancy cytotrophoblasts partially occlude the uterine vessels such that only plasma circulates in the intervillous space. This provides a low oxygen environment for early organ formation. Cytotrophoblasts also replace the endothelium and smooth muscle of endometrial spiral arteries, releasing them from maternal influences. The highly branched villi allow for the passage of respiratory, metabolic and other products between maternal and fetal blood systems across a barrier comprised only of embryonic tissue (a hemochorial placenta). The cytotrophoblasts and overlying syncytiotrophoblasts lining the villi are also the sources of numerous substances including peptide and steroid hormones, and growth factors.