Fertilization and Early Embryonic Development
Core concepts
Fertilization requires capacitation, sperm binding to the zona pellucida (ZP2/ZP3 interactions), the acrosome reaction, membrane fusion, and cortical granule release to block polyspermy. After syngamy, cleavage divisions occur without growth, so cell number rises while embryo volume stays roughly constant. Know the timeline: two-cell by day 1–2, morula around day 4, blastocyst by day 5–6, hatching around day 6–7, implantation around day 7–10.
Lineage and cavitation
Compaction establishes polarity. The outer cells become trophectoderm; inner cells become the inner cell mass. Cavitation forms the blastocoel. Understand the difference between the morula (no cavity) and the blastocyst (cavitated, differentiated).
Traps
- Confusing the acrosome reaction (sperm) with the cortical reaction (oocyte).
- Assuming embryo size increases during cleavage - it does not.
- Mixing up hatching (zona breach) with implantation (endometrial attachment).
- Mis-stating which layer forms the placenta versus the fetus.
Worked example
If a question says "an embryo with a visible cavity and distinct inner cell mass is recovered on day 5," that is a blastocyst, not a morula. If the zona is breached, it is hatching. If the embryo is a solid ball of cells with no cavity, it is a morula regardless of day.