THE TROPHOBLAST

• Implantation begins with the attachment of the trophoblast to the endometrial epithelium. The trophoblast tissue differentiates into inner cytotrophoblast and outer syncytiotrophoblast. The syncytial layer erodes the maternal endometrial epithelium.( Diagram 2B).

• Continued erosion of maternal tissues results in a network of lacunae which will fill with maternal blood and allow for transfer of oxygen and nutrients to the embryo while disposing of carbon dioxide and waste products.( Diagram 3B). Thus, a primitive uteroplacental circulation is established (second week).

• Extraembryonic mesoderm arises from further delamination of the cytotropho- blast and completely fills the space between the trophoblastic tissues externally and the amnion and primary yolk sac internally.( Diagram 3B).

• Isolated spaces within the extraembryonic mesoderm begin to coalesce at about day 10. This results in the formation of a single large extraembryonic coelom.( Diagram 4B, 5B, 6B).

• Formation of the chorion results from the fusion of one layer of extraembryonic mesoderm with two layers of trophoblast. The extraembryonic coelom is now chorion-lined (chorionic cavity). (Diagram 6B).

• Primary chorionic villi are formed at the end of the second week when lumps of cytotrophoblastic tissue migrate into the syncytiotrophoblast where its processes extend into the endometrium. (Diagram 6B).

• Secondary chorionic villi are formed when the villi acquire extraembryonic mesoderm cores. Tertiary villi are formed from the differentiation of this mesenchymal tissue into the blood capillaries connecting fetal and maternal circulations.
• As the chorionic sac grows blood vessels in the portion of the endometrium overlying the embryo, the decidua capsularis, become compressed and degenerate, leaving smooth chorion (chorion laeve). (Diagram 8).

• The chorionic villi related to the decidua basalis, the portion of the endometrium underlying the embryo, continue to grow in size and complexity, forming the chorion frondosum (fetal placenta). (Diagram 9).

• As the villi invade the decidua basalis (maternal placenta), they leave placental septae, or tissue wedges which divide the placenta into cotyledons.
• Eventually fetal growth will result in the extension of the decidua capsularis across the uterine cavity and fusion with the decidua parietalis, the remaining endometrium.( Diagram 9).