Mechanism of Fertilization

Mechanism of fertilization : Semen released at ejaculation contains sperm along with various secretions; the coagulated semen soon liquefies, and the sperm become active. The mechanism of fertilization is as follows:

a. Movement of sperm towards egg : It involves capacitation of the sperm as they reach the vagina, where as many as half of them are demotilised, broken or destroyed; the surviving sperm undergo capacitation over roughly 5–6 hours, during which the acrosome membrane thins, calcium ions enter the sperm, and the tails begin showing rapid, whiplash-like movements. Prostaglandins help activate the sperm, and the vestibular secretions of the female further enhance sperm motility; the sperm themselves swim at an average speed of 1.5–3.0 mm/minute, reaching the ampulla partly by their own swimming and partly aided by contractions of the uterus and fallopian tubes, stimulated by oxytocin from the female — after capacitation, sperm may reach the ampulla within about 5 minutes. Sperm can remain viable for 24–48 hours, while the ovum remains viable for about 24 hours.

b. Entry of sperm into the egg : Of the 200-400 million sperm ejaculated, only a few hundred manage to reach the ampulla, and although many of these reach the ampulla, only a single sperm actually fertilizes the ovum. Once a sperm reaches the egg and lies against it, its acrosome releases lytic enzymes — hyaluronidase and corona-penetrating enzymes — that separate and dissolve the cells of the corona radiata, allowing the sperm head to pass through the zona pellucida. The zona pellucida carries fertilizin receptor proteins (ZP2, ZP3); fertilizin binds to a specific protein on the sperm, antifertilizin, drawing the sperm towards the egg and enhancing fertilization — because this fertilizin–antifertilizin interaction is species-specific, it is also called the compatibility reaction.

Acrosome reaction : As the sperm head touches the zona pellucida in the animal-pole region, the acrosome covering ruptures, releasing lytic enzymes (acrosin/zona lysin) that act on the zona pellucida at the point of contact; this in turn causes an egg reaction, in which a small fertilization cone (cone of reception) forms on the egg membrane. The sperm head touches this cone, producing a weak wave of depolarisation, and the plasma membranes of both cells dissolve at the point of contact, allowing the sperm nucleus and centrioles — but not its other parts — to enter the egg. As soon as the sperm head touches the vitelline membrane, a cortical reaction is triggered, converting the vitelline membrane into a fertilization membrane and deactivating the sperm receptors on the zona pellucida; this creates a distinct perivitelline space around the fertilization membrane and prevents any further sperm from entering — thereby avoiding polyspermy (fertilization by more than one sperm). …