Requirements for successful mammalian sperm capacitation and fertilization

The function of sperm is to fertilize oocytes, thereby providing necessary genetic information and triggering oocyte activation and subsequent metabolic activities that are vitally necessary for successful mammalian embryonic development. Although motile when they leave the male reproductive tract, mammalian sperm are nonfertilizing; they require a further period to complete the maturation initiated in the epididymis. This final maturation phase is called capacitation and is absolutely crucial for successful sperm:oocyte interaction. Capacitation, which exhibits a species-dependent time requirement, involves loss of proteins from the sperm surface; additionally, it has been shown to be dependent on the composition of the environment. Adequate free calcium (Ca2+), sodium (Na+), and potassium (K+), and energy substrates play roles in regulating capacitation, hyperactivated motility, the acrosome reaction, and fusion with the oocyte plasma membrane. In general, conditions present within the female reproductive tract are permissive, ie, appropriate levels of the constituents are present. Therefore, the failure to achieve conception in some couples suggests that there may well be basic physiological defects in the sperm of the male partner. Current research in many laboratories is directed toward identifying such individuals and, in time, developing appropriate therapeutic treatments.