A 28-year-old woman presents to her physician after obtaining a positive home pregnancy test. She reports regular 28-day menstrual cycles and believes conception occurred approximately 1 week ago. Transvaginal ultrasonography does not yet demonstrate an intrauterine gestational sac. The physician explains that the conceptus is likely just beginning to attach to the endometrial lining.
Which of the following embryonic structures normally initiates implantation into the endometrium?
The correct answer is:
B) Blastocyst
Following fertilization in the ampulla of the uterine tube, the zygote undergoes a series of mitotic divisions known as cleavage while traveling toward the uterus. By approximately day 3 after fertilization, it forms a morula, a solid ball of cells. Fluid then enters the morula to create the blastocyst, which consists of an inner cell mass (embryoblast), an outer trophoblast, and a fluid-filled blastocoel. Around 6–7 days after fertilization, the trophoblast of the blastocyst begins implantation by attaching to and invading the receptive endometrium.
Answer choice A: Bilaminar embryonic disc, is incorrect. The bilaminar embryonic disc forms after implantation begins as the embryoblast differentiates into the epiblast and hypoblast. It is not the structure that initiates implantation.
Answer choice C: Morula, is incorrect. The morula is the developmental stage immediately preceding the blastocyst. It remains enclosed within the zona pellucida and does not implant into the endometrium.
Answer choice D: Primary oocyte, is incorrect. The primary oocyte is arrested in prophase I until ovulation and exists before fertilization. It is not an embryonic structure.
Answer choice E: Zygote, is incorrect. The zygote is the single-cell product of fertilization. It undergoes cleavage to become a morula and subsequently a blastocyst before implantation occurs.
Key Learning Point
Implantation begins approximately 6–7 days after fertilization when the trophoblast of the blastocyst attaches to and invades the endometrium. The developmental sequence is zygote → morula → blastocyst → implantation.
