The discovery in 1951 that rabbit and rat spermatozoa must spend some hours in the female tract before acquiring the capacity to penetrate ova stimulated intensive research efforts to delineate the environmental conditions required for this change in the sperm to occur. The process by which the sperm were transformed was called capacitation. Attention was focused upon the hormonal and time requirements and the potential for in vitro capacitation.
Capacitation changes the surface characteristics of sperm, as exemplified by removal of seminal plasma factors that coat the surface of the sperm, modification of their surface charge, and restriction of receptor mobility. This is associated with decreased stability of the plasma membrane and the membrane lying immediately under it, the outer acrosomal membrane. The membranes undergo further, any more striking, modifications when capacitated sperm reach the vicinity of an ovum or when they are incubated in follicular fluid. There is a breakdown and merging of the plasma membrane and the outer acrosomal membrane, the acrosome reaction. This allittles egress of the enzyme contents of the acrosome, the cap-like structure that covers the sperm nucleus. These enzymes, which include hyaluronidase, a neuraminidase-like factor, corona-dispersing enzyme, and a protease called acrosin, are all thought to play roles in sperm penetration of the egg investments. The changes in the sperm head membranes also prepare the sperm for fusion with the egg membrane. It is the inner acrosomal membrane that fuses with the oocyte plasma membrane. In addition, capacitation endows the sperm with hypermotility, and the increased velocity of the sperm may be the most critical factor in mediating zona penetration. The acrosome reaction can be induced by zona pellucida proteins of the oocyte and by human follicular fluid in vitro.1

Although capacitation classically has been defined as a change sperm undergo in the female reproductive tract, it is apparent that sperm of some species, contain the human, can acquire the ability to fertilize after a short incubation in defined media and without residence in the female reproductive tract. Therefore, success with assisted reproductive technologies is possible. In vitro capacitation requires a culture medium that is a balanced salt solution containing energy substrates such as lactate, pyruvate, and glucose and a protein such as albumin, or a biologic fluid such as serum or follicular fluid. Sperm washing procedures probably remove factors that coat the surface of the sperm, one of the initial steps in capacitation. The removal of cholesterol from the sperm membrane is believed to prepare the sperm membrane for the acrosome reaction. The time required for in vitro capacitation is approximately 2 hours. The hamster penetration test is a measure of the sperm's ability to undergo in vitro capacitation and the acrosome reaction.
The final dash to the oocyte is aided by the increased motility due to the state of hyperactivity. This change in motility can be measured by an increase in velocity and flagellar beat amplitude. Perhaps the increase in thrust gained by this hyperactivity is necessary for penetration of the oocyte.