Capacitation

Capacitation is the penultimate step in the maturation of mammalian spermatozoa and is required to render them competent to fertilize an oocyte. This step is a biochemical event; the sperm move normally and look mature prior to capacitation. In vivo this step typically occurs after ejaculation, in the female reproductive tract. In vitro, capacitation can occur in sperm that have either undergone ejaculation or have been extracted from the epididymis.

By secreting sterol binding albumin, lipoproteins, proteolytic and glycosidasic enzymes such as heparin, the uterus aids in the steps of capacitation.

Non-mammalian spermatozoa do not require this capacitation step and are ready to fertilize an oocyte immediately after release from the male. After this capacitation the sperm must undergo activation involving the acrosome reaction.

Result of capacitation
Capacitation involves the destabilisation of the sperm head membrane rendering it more fusigenic. This change is facilitated by the removal of sterols (e.g. cholesterol) and non-covalently bound epididymal/seminal glycoproteins. The result is a more fluid membrane with an increased permiability to Ca2+.

An influx of Ca2+ produces increased intracellular cAMP levels and thus, an increase in motility. Hyperactivation is also part of capacitation and is the result of the increased Ca2+ levels. The tripeptide FPP (fertilization promoting factor) produced by the male is essential for capacitation. It has a synergistic stimulatory effect with adenosine that increases adenyl cyclase activity in the sperm. FPP is found in the seminal fluid, and comes into contact with the spermatozoa upon ejaculation.

Discovery
The discovery of this process was independently reported in 1951 by both Min Chueh Chang and Colin Russell Austin.

Historically, the term "capacitation" has evolved in meaning and this should be taken into account when consulting sources.