Determination of the capacity of ram epididymal and ejaculated sperm to undergo the acrosome reaction and penetrate ova.

An understanding of epididymal maturation of sperm requires descriptions of changes in membrane properties and their relation to changes in cell function. While sperm membranes have been studied in some detail in rams, few reports address associated functional changes. This report provides such data by evaluating (a) the time course of sperm acrosome reaction (AR) induction for cells from each epididymal region; (b) the capacity of epididymal sperm to penetrate ova; (c) differences in physiological AR and general sperm degeneration; and (d) acrosin release of epididymal sperm. Ram epididymal (caput, corpus, and proximal and distal cauda) and ejaculated (EJ) sperm were incubated in vitro to assess their capacity to undergo an AR. Light microscopy revealed that in sperm populations which had traversed the proximal cauda epididymidis, greater than or equal to 50% exhibited an endogenous AR in less time (less than 17 h) than did sperm isolated from more proximal regions of the epididymis (22-greater than 50 h). Heparin added to sperm did not stimulate the AR in epididymal or EJ sperm, whereas addition of a calcium ionophore (A23187) increased AR rates for cauda and EJ sperm, but not caput or corpus sperm. A second experiment evaluating percent AR, percent motile cells, and percent hamster ova penetrated revealed that sperm isolated from regions proximal to the cauda epididymidis failed to penetrate ova. When cauda or EJ sperm exhibited motility greater than 5% and AR greater than 24%, penetration of hamster eggs occurred. Comparisons of acrosomal integrity by electron or light microscopy were not different for sperm at any stage of epididymal maturation, suggesting that minimal nonspecific membrane changes occur and that light microscopy is valid for evaluating the acrosomal status of ram spermatozoa. Acrosin activity (sperm bound and dissociated) also was measured. Both total activity and release of acrosin from sperm to the medium during an 8-h incubation was greater for mature than for immature sperm. Results from these experiments are discussed in relation to the changes that must occur in sperm as they acquire the capacity to undergo an AR and penetrate hamster ova.