A reproductive endocrinologist is discussing blastocyst development with a couple undergoing in vitro fertilization. She explains that successful implantation requires differentiation of the blastocyst into two major cell populations. One population will eventually form the fetus as well as the amnion and yolk sac, whereas the other will contribute primarily to the fetal portion of the placenta.
Which of the following structures gives rise to the embryo proper?
The correct answer is:
C) Embryoblast (inner cell mass)
By approximately day 5 after fertilization, the morula develops into a blastocyst composed of an outer trophoblast, an inner embryoblast (inner cell mass), and a fluid-filled blastocoel. The embryoblast differentiates into the epiblast and hypoblast, which subsequently give rise to the embryo proper, amnion, yolk sac, and ultimately all three germ layers during gastrulation. In contrast, the trophoblast contributes to the fetal portion of the placenta and mediates implantation.
Answer choice A: Blastocoel, is incorrect. The blastocoel is the fluid-filled cavity within the blastocyst. It provides space for cellular organization but does not give rise to embryonic or placental tissues.
Answer choice B: Cytotrophoblast, is incorrect. The cytotrophoblast is the mitotically active inner layer of the trophoblast. It contributes to placental development and replenishes the syncytiotrophoblast but does not form the embryo.
Answer choice D: Syncytiotrophoblast, is incorrect. The syncytiotrophoblast is the multinucleated outer trophoblastic layer responsible for implantation, maternal tissue invasion, and hCG secretion. It contributes to the placenta rather than the embryo.
Answer choice E: Trophoblast, is incorrect. The trophoblast gives rise to the fetal component of the placenta, including the cytotrophoblast and syncytiotrophoblast. The trophoblast may be confused with the embryoblast because both arise from the blastocyst, but only the embryoblast forms the embryo proper.
Key Learning Point
The embryoblast (inner cell mass) forms the embryo proper and selected extraembryonic tissues (amnion and yolk sac), whereas the trophoblast forms the fetal portion of the placenta.
