SPERM CAPACITATION AND THE ACROSOME REACTION

To achieve successful fertilization under normal circumstances in vivo , mammalian spermatozoa must first undergo capacitation and then the a crosome reaction, an exocytotic event that allows cells to penetrate t he zona pellucida and fuse with the oocyte plasma membrane, These comp lex events permit spermatozoa to achieve fertilizing ability at the ri ght time in the right place, important considerations since relatively few sperm cells actually reach the site of fertilization ill vivo. Se veral mechanisms that may be involved in regulating the acquisition of fertilizing ability are considered. In vivo, selective pressures plac ed on the initial population of spermatozoa help ensure that the 'fitt est' spermatozoa are able to fertilize. Since intracytoplasmic sperm i njection bypasses this selection process, it is best used only in case s where spermatozoa are judged to be incapable of achieving normal fer tilization in vitro.