Fertilisation in Humans

In humans, sperm are deposited in the vagina and swim through the cervix and uterus into the oviduct, where fertilisation normally happens. The female gamete released at ovulation is a secondary oocyte, which has stopped at metaphase of meiosis II. It is surrounded by a jelly-like glycoprotein layer, the zona pellucida, and an outer layer of follicle cells.1. Acrosome reaction. The sperm pushes between the follicle cells. When its head binds to the zona pellucida, the acrosome (a vesicle in the sperm head) releases digestive enzymes (proteases). These digest a path through the zona pellucida.2. Fusion. The sperm cell membrane fuses with the cell membrane of the oocyte and the sperm nucleus enters the cytoplasm.3. Cortical reaction. Fusion triggers cortical granules just under the oocyte membrane to release their contents into the zona pellucida by exocytosis. Their enzymes make the zona pellucida thicken and harden, so no other sperm can enter. This prevents polyspermy, which would give a zygote with too many chromosomes.4. Fusion of nuclei. Sperm entry triggers the secondary oocyte to complete meiosis II, forming an ovum and a second polar body. The haploid sperm nucleus and haploid ovum nucleus then fuse to form the diploid nucleus of the zygote.