Explain the formation of the blastocyst from a fertilized ovum, describing the roles of zona pellucida, cortical granule exocytosis, inner cell mass, and trophoblast.
Answer
The blastocyst forms after fertilization when cortical granule exocytosis prevents polyspermy, the zygote divides into a morula, a blastocoel cavity appears, the outer trophoblast differentiates to form the placenta while the inner cell mass forms the embryo, and the blastocyst then implants into the endometrium.
Step-by-step solution
- Fertilization occurs when sperm penetrates the zona pellucida and releases its contents into the oocyte.
- Cortical granule exocytosis modifies zona pellucida, preventing polyspermy (cortical reaction).
- The zygote undergoes successive mitotic divisions forming a morula (8–16 cell ball).
- The morula compacts, and a fluid‑filled cavity, the blastocoel, forms, giving a blastula (blastocyst).
- During this stage, cells differentiate: outer trophoblast forms placenta; inner cell mass becomes future embryo.
- The blastocyst then implants into the endometrium, trophoblast secreting enzymes to penetrate uterine lining.
Common mistake: Forgetting cortical reaction or confusing inner cell mass with trophoblast
Concept tested: Blastocyst formation and implantation
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