A 27-year-old woman presents to the physician with 6 weeks of amenorrhea and a positive home pregnancy test. Serum β-hCG concentration is markedly elevated, confirming an early pregnancy. The hormone responsible for maintaining progesterone production during the first trimester is secreted by a population of multinucleated cells that actively invade the maternal endometrium during implantation.
Which of the following embryonic structures is the source of this hormone?
The correct answer is:
E) Syncytiotrophoblast
Following implantation, the trophoblast differentiates into two layers: the cytotrophoblast, which is mitotically active and gives rise to new trophoblastic cells, and the syncytiotrophoblast, a multinucleated outer layer that invades the maternal endometrium. The syncytiotrophoblast secretes human chorionic gonadotropin (hCG), which rescues and maintains the corpus luteum during early pregnancy. Continued progesterone secretion by the corpus luteum preserves the endometrium until the placenta becomes capable of producing sufficient progesterone near the end of the first trimester.
Answer choice A: Cytotrophoblast, is incorrect. The cytotrophoblast consists of mononuclear cells that proliferate and replenish the syncytiotrophoblast. Although it contributes structurally to placental development, it is not the primary source of hCG.
Answer choice B: Embryoblast, is incorrect. The embryoblast develops into the embryo proper. It differentiates into the epiblast and hypoblast after implantation and does not produce hCG.
Answer choice C: Hypoblast, is incorrect. The hypoblast forms the yolk sac endoderm and contributes to extraembryonic structures. It has no role in trophoblastic invasion or hCG secretion.
Answer choice D: Inner cell mass, is incorrect. The inner cell mass is another term for the embryoblast before differentiation into the epiblast and hypoblast. It forms the embryo but is not responsible for implantation or endocrine support of pregnancy.
Key Learning Point
The syncytiotrophoblast is the invasive outer trophoblastic layer that secretes hCG, maintaining the corpus luteum and progesterone production during early pregnancy until the placenta assumes this endocrine function.
