The acrosome reaction in human sperm from men of proven fertility

Suspensions of motile sperm were prepared from semen samples donated by 40 men of recent, proven fertility and incubated under capacitating conditions for 24 h. At selected time-points aliquots were removed, assessed for motility, fixed and examined with the electron microscope to determine the rate of acrosome loss. Data indicate that acrosome loss increases significantly with time, but absolute values are relatively low. After 24 h a mean of 15.4% sperm had initiated the acrosome reaction; this figure included 9.7% which had completed it. The proportion of cells at intermediate stages was similar throughout incubation (approximately 5%), indicating that initiation of the acrosome reaction occurs at a fairly constant rate. In four samples motility declined over 24 h and in six, contaminating cells were observed. In the majority of these 10, acrosome loss was higher than that observed in the remaining 30 samples. Additionally, the assessment of greater than 25,000 cells during this study made it possible to evaluate specific ultrastructural features of the normal acrosome reaction in human sperm. Six stages were identified, with the intermediate ones involving loss of acrosomal matrix material while outer membranes appear to retain their integrity; this contrasts sharply with the current view of the generalized mammalian sperm acrosome reaction.