Q.How is polyspermy avoided in humans?

Fertilisation is the fusion of a haploid sperm with a haploid secondary oocyte at the ampullary-isthmic junction of the fallopian tube, forming a diploid zygote. Sperm first undergo capacitation in the female tract, then penetrate the egg's corona radiata and, via the acrosomal reaction (releasing hyaluronidase), the zona pellucida; the first sperm to fuse with the egg's plasma membrane triggers cortical granules to build a fertilisation membrane that blocks any further sperm — preventing polyspermy. The zygote cleaves mitotically (blastomeres: 2, 4, 8 cells and so on) into a solid morula by about 72 hours, then into a fluid-filled blastocyst (an outer trophoblast layer and an inner cell mass) as progesterone-relaxed fallopian-tube muscle carries it toward the uterus over 4-5 days. The inner cell mass embeds in the endometrium — implantation …