3.12—Mammalian fertilisation

Fertilisation is the fusion of sperm and egg nuclei to form a diploid zygote. It begins when a sperm reaches the egg in the oviduct and its acrosome releases enzymes that digest a path through the zona pellucida.

After the sperm crosses the egg membrane, cortical granules release their contents. The zona pellucida hardens, producing the cortical reaction that blocks other sperm. The sperm and egg nuclei then fuse, restoring the full chromosome number; the zygote divides by mitosis.

The acrosome reaction permits entry, whereas the cortical reaction prevents polyspermy. They occur at different stages and solve opposite problems: access first, exclusion second.

Fertilisation is nuclear fusion, not merely sperm contact with the egg. The first divisions of the zygote are mitotic, so they increase cell number without halving chromosome number.