Fertilisation

Fertilisation is the fusion of a haploid sperm with a haploid secondary oocyte to form a diploid zygote, and in humans it normally takes place in the ampullary-isthmic junction of the fallopian tube, generally within about 24 hours of ovulation, since both the released oocyte and any sperm present in the tract remain capable of fertilisation for only a limited window of time.

Sperm deposited in the female reproductive tract at coitus are not immediately capable of fertilising an egg; they must first undergo a series of physiological changes within the female tract, collectively called capacitation, which alter the sperm's plasma membrane and metabolic activity and are a necessary precondition for the later acrosomal reaction. Of the very large number of sperm deposited, only a small fraction survive the journey through the cervix, uterus and fallopian tube to reach the vicinity of the oocyte in the ampulla.

On reaching the secondary oocyte, a capacitated sperm must penetrate two protective layers surrounding it: the outer corona radiata, a loose layer of follicular cells still attached from ovulation, and the inner zona pellucida, a thick glycoprotein coat. Contact with these layers triggers the acrosomal reaction: the acrosome at the tip of the sperm head ruptures, releasing a battery of hydrolytic enzymes that digest a passage through the corona radiata and zona pellucida, allowing the sperm head to reach and make contact with the oocyte's own plasma membrane. Binding and fusion of the sperm and oocyte plasma membranes allows the sperm's haploid nucleus (and, in principle, its other contents) to enter the oocyte's cytoplasm. …