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, capacitation typically occurs after ejaculation into 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.
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[how?] and this should be taken into account when consulting sources.
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. It has a synergistic stimulatory effect with adenosine that increases adenylyl cyclase activity in the sperm.
The tripeptide, fertilization promoting peptide (FPP), is essential for controlling capacitation. FPP is produced in prostate gland of the man, as a component of the seminal fluid. FPP comes into contact with the spermatozoa during ejaculation, as the sperm and seminal fluid mix. High levels of active FPP prevent capacitation. After ejaculation, the concentration of FPP drops in the female reproductive tract. The vaginal secretions dilute it, and make it less active due to the pH of the vagina, which differs from that of semen.
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