Which membrane forms the outer component of the blastocyst?

• A. Amnion
• B. Yolk sac
• C. Trophoblast
• D. Chorion

Answer: (C)

In early development, the blastocyst has an outer cell layer called the trophoblast, which forms the outer component of the blastocyst. This layer later contributes to the chorion, the fetal membrane that surrounds the developing embryo and placenta. The amnion forms a membrane around the inner cell mass and lines the amniotic cavity, while the yolk sac is an early extraembryonic structure involved in early nutrition and blood formation. Although the chorion is the outer fetal membrane later on, it is derived from the trophoblast and develops as development progresses, not as the initial outer component of the blastocyst. Therefore, the outer component of the blastocyst is the trophoblast.