51. Extraembryonic fetal membranes – formation and function – amnion, chorion. Amniotic cavities. Amnio-chorionic membrane.

Amnion:

• Structure: The amnion is a thin, transparent membrane that surrounds the fetus.
• Location: It forms from the epiblast layer of the bilaminar embryonic disc and grows to encase the fetus.
• Function:
• Contains amniotic fluid, which cushions and protects the fetus.
• Maintains a stable temperature and pressure around the fetus.
• Allows for fetal movement, which is important for musculoskeletal development.
• Prevents adhesions between the fetus and the membranes.

Chorion:

• Structure: The chorion is the outermost membrane surrounding the fetus and the amnion. It is formed by the trophoblast layer (specifically the cytotrophoblast and syncytiotrophoblast) and the extra-embryonic mesoderm.
• Location: The chorion is located outside the amnion and forms the outermost layer of the fetal membranes.
• Function:
• Contributes to the formation of the placenta.
• In the early stages of pregnancy, the chorionic villi (finger-like projections) invade the maternal endometrium to establish maternal-fetal blood exchange.
• Produces human chorionic gonadotropin (hCG), which helps maintain the pregnancy by supporting the corpus luteum and stimulating the production of progesterone.