Question

Trace the events from fertilization till implantation of blastocyst in human female. Also mention the site where fertilization takes place.

Answer 1

1. Site of fertilization: Fertilization normally occurs in the ampullary (ampulla) region of the fallopian (uterine) tube.
2. Arrival of gametes: Around ovulation, the secondary oocyte is released from the ovary, picked up by the fimbriae, and transported into the uterine tube. Motile sperm swim from the vagina through the cervix and uterus into the uterine tube, so that sperm and oocyte meet in the ampulla.
3. Sperm preparation and entry: Sperm undergo capacitation in the female tract (functional changes that enable fertilisation), and then the acrosome reaction allows enzymes to digest the zona pellucida so a sperm can penetrate to the oocyte membrane.
4. Fusion and prevention of polyspermy: A single sperm fuses with the oocyte membrane, triggering cortical granule release and changes in the zona pellucida that block entry of additional sperm (preventing polyspermy). The sperm nucleus and oocyte nucleus undergo events that produce haploid male and female pronuclei.
5. Formation of the zygote (syngamy): Male and female pronuclei fuse to form a diploid zygote (the first embryonic cell). This is the moment the chromosome complement of the embryo is established.
6. Cleavage (early mitotic divisions): The zygote undergoes repeated mitotic (cleavage) divisions as it is transported through the isthmus of the uterine tube toward the uterus, producing 2-, 4-, 8-, 16-cell stages (blastomeres).
7. Morula: Continued cleavage yields a compacted morula (about 8–16 blastomeres). The embryo is still surrounded by the zona pellucida as it moves toward the uterine cavity.
8. Blastocyst formation: Fluid accumulates within the morula to form a blastocyst: an outer trophoblast layer and an inner cell mass (embryoblast). The blastocyst stage is the stage competent to attach to the endometrium.
9. Attachment and implantation: The trophoblast of the blastocyst adheres to the endometrial epithelium of the uterus (attachment/apposition), then trophoblast cells proliferate and invade into the endometrium so the blastocyst becomes embedded (implantation). As implantation proceeds, the trophoblast contributes to the formation of chorionic villi and the developing placenta.