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Blastocyst: The Pre-Implantation Embryo Stage Explained
The blastocyst is the pre-implantation mammalian embryo at ~5–6 days post-fertilization (human).
Structure
- Trophectoderm (trophoblast): Outer cell layer → forms placenta, fetal membranes.
- Inner Cell Mass (ICM / embryoblast): Cluster of pluripotent cells → forms the embryo proper (epiblast) and yolk sac (hypoblast).
- Blastocoel: Fluid-filled cavity.
- Zona pellucida: Acellular extracellular matrix (hatches ~day 5–6 for implantation).
Timeline
- Day 0: Fertilization (zygote)
- Day 1–3: Cleavage divisions (2-cell → 8-cell → morula)
- Day 4: Compaction → morula
- Day 5–6: Blastulation → blastocyst (100–200 cells)
- Day 6–7: Hatching → apposition/adhesion to endometrium
- Day 7–10: Implantation
Clinical Relevance
- IVF: Blastocyst-stage transfer (Day 5/6) improves selection, live birth rates.
- PGT-A/PGT-M: Biopsy of trophectoderm cells.
- Ectopic pregnancy: Implantation outside uterus (often fallopian tube).
The blastocyst represents the first lineage specification in human development.
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