The chorion is composed of which two layers?

• A. Outer syncytiotrophoblast and inner cytotrophoblast
• B. Outer cytotrophoblast and inner syncytiotrophoblast
• C. Ectoderm and mesoderm
• D. Endoderm and mesoderm

Answer: (A)

The chorion consists of two trophoblastic layers: an outer syncytiotrophoblast and an inner cytotrophoblast. The syncytiotrophoblast forms from fusion of trophoblast cells, creating a continuous multinucleated layer that covers the chorionic villi and interacts with maternal blood—crucial for nutrient exchange and hormone (like hCG) production. The inner cytotrophoblast remains a cellular layer inside, lining the villi in early development and supplying cells that contribute to the growing placental structure. The other options refer to embryonic germ layers (ectoderm/endoderm/mesoderm), which are not layers of the chorion.