Placenta and Fetal Membranes

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Development of the trophoblast during the first 4 weeks of gestation
- The trophoblast divides into 2 layers – the cytotrophoblast and syncytiotrophoblast – These will form the embryonic component of the placenta (The maternal component is derived from the endometrium)
- Cytotrophoblast: The inner layer of the chorionic villi. Is mitotically active. Cells of the cytotrophoblast migrate outwards and fuse to become the syncytiotrophoblast
- Syncytiotrophoblast: Outer layer of the chorionic villi. It is mitotically inactive.
- Syncitiotrophoblast cells invade the endometrium, creating lacunae (spaces in between them) that fill with maternal blood
- Syncitiotrophoblasts secrete proteolytic enzymes that degrade the ECM and spiral arteries of the decidua. This is what causes the lacunae to fill with maternal blood.
- Lacunae merge to form intervillous spaces.
- Primary villi develop as cytotrophoblast cells migrate into the syncytiotrophoblasts. They comprise of an inner layer of cytotrophoblast and an outer layer of syncytiotrophoblasts with microvilli.
- Secondary villi form as the extraembryonic mesoderm cells migrate into the core of the primary villi. These cells form the mesenchymal core in addition to the inner and outer layers mentioned previously.
- Tertiary (Terminal) villi develop through vascularization. The cytotrophoblast cells disappear with only isolated cytotrophoblast (Langhans) cells remaining. The structure is composed of a mesenchymal core with fetal capillaries, the inner layer of Langhans cells, and an outer layer of syncytiotrophoblast
Functions of the placenta
- Metabolism: Synthesizes glycogen
- Transports gases and nutrients
- Hormone production: hCG, Progesterone, Estriol
- Excretion of fetal waste products
- Protection: Maternal antibodies pass through the placenta to confer passive immunity to the fetus

Placenta and Fetal Membranes

Jeffrey Kalei

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