19. The placenta is derived from both maternal and embryonic tissues. Embryonic mesodermal cells come to line the inner surface of the trophoblast. The chorion is formed by the trophoblast plus cells derived from the inner cell mass. It develops fingerlike projections ( chorionic villi). These villi are invaded by newly forming blood vessels, reach the embryo, and form the umbilical arteries and vein.. As the concepts invades the endometrium, lacunae with blood form and the chorionic villi immerse in these lacunae with maternal blood. Two compartments of the endometrium are the decidua basalis which is the part of the endometrium beneath the embryo, and the decidua capsularis --the part of the endometrium that faces the uterine cavity, away from the implantation site.
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As the embryo develops, the villi of the decidua capsularis degenerate because of compression by the embryo, but the decidua basalis grows and expands, By the end of the third month of gestation, the placenta is formed and fully functional. Embryonic/fetal component : Chorionic villi from trophoblast Maternal component: Decidua basalis derived from uterine endometrium One of the important functions of the chorionic villi is to secrete hCG to maintain the corpus luteum (CL).The CL produces progesterone to prevent expulsion of the conceptus. The decidua basalis functions to exchange nutrients, wastes,and gases with the fetus-- by diffusion. The blood of the mother and fetus do not mix, under normal circumstances.