Fertilization is the fusion of sperm and egg nuclei to form a diploid zygote.
• On entering the vagina, sperm spend about 7 hours being capacitated, after which they move towards the oviducts, aided by
muscular contractions of the uterus and oviducts, and lashing of tail.
• A spermatozoon comes into contact with the oocyte by random movement.
• Acrosome enzymes hydrolyse a path in the granulosa layer of egg until the sperm head makes contact with zona pellucida.
• Sperm acrosome membrane ruptures to release hydrolytic enzymes (acrosome reaction) and the acrosomal filament pierces
through the oocyte membranes up to the plasma membrane of the oocyte.
• An electrical potential change in the oocyte membrane occurs (fast block), followed by fusion of cortical granules with plasma
membrane to discharge their contents (cortical reaction), which creates an osmotic gradient that draws water into the space
between the plasma membrane and vitelline membrane.
• The two membranes are lifted away and the vitelline membrane hardens (fertilization membrane) to block polyspermy.
• While the sperm tail is lost and disintegrates, the nucleus expands and is now known as pronucleus.
• Entry of a sperm stimulates completion of second meiotic division of the secondary oocyte to form the second polar body, which
disintegrates, and an egg. The haploid male and female pronuclei fuse