Attributes of fertile spermatozoa: an update.

The process of fertilization can be imagined as an elegant pas de deux, with its success being dependent upon the precise timing and accuracy of a well-choreographed series of steps between male (spermatozoa) and female (oocytes) gametes. Arguably, perhaps the greatest performer in the fertilization process is the spermatozoon. The spermatozoon must actively overcome numerous obstacles and undergo many dynamic changes in order to achieve its destiny, whereas the oocyte assumes a fairly passive role in events leading to and culminating in fertilization. Some of the prefertilization processes that a mammalian spermatozoon must undergo have been revealed only in the last 10-15 years (e.g., Yanagimachi, 1994; Aitken, 1997), while other aspects still remain somewhat enigmatic. In 1989, Amann published his view of parameters that are requisite for the spermatozoon to be considered fertile, and they are as follows: 1) normal structure of vital functional components; 2) fully functional metabolic pathways for production of sperm motion and for maintenance of membrane potentials, ionic microenvironment, pH, or other cellular functions; 3) motility, to penetrate the cervix and the utero-tubal junction, for departure from storage sites, and to contact and penetrate the oocyte and its vestments; 4) presence of peripheral and/or integral proteins that serve as “survival proteins” to protect the spermatozoa from the hostile environment of the female reproductive tract; 5) appropriate responses to the microenvironment and stimuli provided by the female reproductive tract; 6) proteins essential for recognition and binding of the spermatozoon to the zona pellucida and the vitelline membrane; 7) acrosomal enzymes maintained in proenzyme or inhibited form until the appropriate time for assisting in oocyte vestment penetration; 8) fusible plasma and acrosomal membranes; 9) precise tim-