Bruce M. Carlson

Placenta and Extraembryonic Membranes

131 One of the most characteristic features of human embryonic development is the intimate relationship between the embryo and the mother. The fertilized egg brings little with it except genetic material. To survive and grow during intrauterine life, the embryo must maintain an essentially parasitic relationship with the body of the mother for acquiring oxygen and nutrients and eliminating wastes. It must also avoid being rejected as a foreign body by the immune system of its maternal host. These exacting requirements are met by the placenta and extraembryonic membranes that surround the embryo and serve as the interface between the embryo and the mother. The tissues that compose the fetal-maternal interface (placenta and chorion) are derivatives of the trophoblast, which separates from the inner cell mass and surrounds the cellular precursors of the embryo proper even as the cleaving zygote travels down the uterine tube on its way to implanting into the uterine wall (see Fig. 3-17). Other extraembryonic tissues are derived from the inner cell mass. These include the amnion (an ectodermal derivative), which forms a protective fl uidfi lled capsule around the embryo; the yolk sac (an endodermal derivative), which in mammalian embryos no longer serves a primary nutritive function; the allantois (an endodermal derivative), which is associated with the removal of embryonic wastes; and much of the extraembryonic mesoderm, which forms the bulk of the umbilical cord, the connective tissue backing of the extraembryonic membranes, and the blood vessels that supply them. EXTRAEMBRYONIC TISSUES