Sperm Capacitation

A spermatozoon released at ejaculation already has the complete structural apparatus assembled during spermiogenesis. It is not yet capable of fertilising an oocyte. It must first undergo further functional maturation within the female reproductive tract, called capacitation, before it can complete the remaining steps of fertilisation.

Capacitation occurs as the spermatozoon transits the female reproductive tract, principally the uterus and uterine tube, over a period generally estimated at several hours. It depends on exposure to the secretions and physiological environment of the female tract.

A spermatozoon retained in the male reproductive tract does not undergo this maturation spontaneously. Nor does an ejaculated spermatozoon studied outside the female tract without an appropriately capacitating environment.

Capacitation is understood as the removal or modification of specific inhibitory factors deposited onto the sperm surface during transit through the male reproductive tract. While present, these factors actively suppress fertilising capacity. They presumably prevent premature activation of the acrosome reaction, or premature acquisition of full motility, before the sperm reaches the vicinity of an oocyte.

A defining feature of capacitation is substantial remodelling of the sperm plasma membrane, most importantly a significant reduction in membrane cholesterol content, which alters the fluidity and biophysical properties of the membrane overlying the acrosome. This altered membrane state is a structural prerequisite for the acrosome reaction, since the membrane fusion events that constitute the acrosome reaction cannot proceed normally across an uncapacitated, cholesterol-rich membrane in its original, pre-capacitation state.

Capacitation is also associated with redistribution and modification of specific surface proteins on the sperm head, altering the sperm's molecular recognition capacity in preparation for its subsequent interaction with and binding to the zona pellucida.

Capacitated sperm characteristically show a more vigorous pattern of flagellar movement, termed hyperactivated motility. Flagellar beating has wider amplitude and is more asymmetric than the essentially straight-line, progressive motility typical of uncapacitated sperm.

Hyperactivated motility is understood to serve two related purposes.

• It generates substantially greater mechanical force. This is believed to help the sperm penetrate the viscous cumulus matrix surrounding the oocyte, and the zona pellucida itself.
• It is thought to help detach sperm bound to the tubal epithelial reservoir. This allows their release and continued forward progression toward the ampulla at the appropriate time.

Capacitation is not itself the acrosome reaction. It is an essential, obligatory prerequisite for it. Only a capacitated spermatozoon can undergo a normal acrosome reaction on subsequent contact with the zona pellucida.

An uncapacitated spermatozoon that encounters the zona pellucida cannot trigger the acrosome reaction correctly and cannot achieve fertilisation, regardless of otherwise entirely normal structure and motility.

Capacitation therefore acts as a necessary licensing step. The acrosome reaction is an irreversible, single-use event for any individual spermatozoon. Capacitation ensures it is triggered only once the sperm has reached an appropriate stage of functional readiness and, physiologically, is in the correct location and context to encounter an oocyte.