gynecology and obstetrics
Conception and Prenatal Development
Amniotic sac and placenta
Within 1 or 2 days of implantation, a layer of cells (trophoblast cells) develops around the blastocyst. The progenitor villous trophoblast cell, the stem cell of the placenta, develops along 2 cell lines: Nonproliferative extravillous trophoblast: These cells penetrate the endometrium, facilitating implantation and anchoring of the placenta. Syncytiotrophoblast: These cells produce chorionic gonadotropin by day 10 and other trophic hormones shortly thereafter. An inner layer (amnion) and outer layer (chorion) of membranes develop from the trophoblast; these membranes form the amniotic sac, which contains the conceptus (term used for derivatives of the zygote at any stage—see figure Placenta and embryo at about 11 4/7 weeks gestation ). When the sac is formed and the blastocyst cavity closes (by about 10 days), the conceptus is considered an embryo. The amniotic sac fills with fluid and expands with the growing embryo, filling the endometrial cavity by about 12 weeks after conception; then, the amniotic sac is the only cavity remaining in the uterus. The embryo measures 4.2 cm. Trophoblast cells develop into cells that form the placenta. The extravillous trophoblast forms villi, which penetrate the uterus. The syncytiotrophoblast covers the villi. The syncytiotrophoblast synthesizes trophic hormones and provides arterial and venous exchange between the circulation of the conceptus and that of the mother. The placenta is fully formed by week 18 to 20 but continues to grow, weighing about 500 g by term. 