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 by incubating sperm that have either undergone ejaculation or have been extracted from the epididymis in a defined medium for several hours.
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The uterus aids in the steps of capacitation by secreting sterol-binding albumin, lipoproteins, proteolytic and glycosidasic enzymes such as heparin.
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 the final maturation step, activation, involving the acrosome reaction.
Historically, the term "capacitation" has evolved in meaning and this should be taken into account when consulting sources.
Capacitation
Capacitation has 2 effects: destabilisation of the acrosomal sperm head membrane which allows it to penetrate the outer layer of the egg, and chemical changes in the tail that allow a greater mobility in the sperm. The changes are 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 permeability to Ca2+.
An influx of Ca2+ produces increased intracellular cAMP levels and thus, an increase in motility. Hyperactivation coincides with the onset of capacitation and is the result of the increased Ca2+ levels. The tripeptide FPP (fertilization promoting peptide) produced by the male is essential for capacitation (high levels of FPP prevent capacitation, the proper concentration occurs after ejaculation in the female reproductive tract where the concentration drops after mixing with vaginal secretions and/or becomes less active due to the pH of the vagina). It has a synergistic stimulatory effect with adenosine that increases adenylyl cyclase activity in the sperm. FPP is found in the seminal fluid (FPP produced in prostate gland), 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.