Fertilizing capacity of epididymal and testicular spermatozoa microinjected under the zona pellucida of the mouse oocyte.

Mouse spermatozoa were obtained from the testis and caput, corpus, and cauda epididymis incubated for 2 h in capacitation medium and a single spermatozoon from the capacitated samples microinjected into the pervitelline space of mature mouse oocytes. Spermatozoa from the testis were unable to fertilize oocytes and few spermatozoa from the caput were capable of fertilization (0-7% depending on the method of preparation). Similar fertilization rates (30-45%) were obtained with spermatozoa with forward progressive motility from the proximal and distal corpus and the cauda epididymis, but those that were vibratory or with local circular motility had significantly reduced fertilizing capacity (6-10%). The capacity of spermatozoa from the different regions of the testis and epididymis to bind to zona-free oocytes followed the same pattern as fertilization rate after microinjection. There was a progressive increase in acrosome reactions after 2 h incubation in capacitation medium in samples obtained from the testis to the cauda epididymis. Maturation of the capacity to bind to the perivitelline membrane and to develop forward progressive motility, rather than the capacity to acrosome react, appears to govern the fertilization of oocytes in which the zona has been bypassed by microinjection. These characteristics are obtained in the proximal segment of the corpus. However, there was evidence that the embryo developmental capacity of oocytes fertilized by spermatozoa from the higher segments of the epididymis is reduced, particularly in oocytes fertilized by caput spermatozoa. These observations suggest that sperm microinjection may have only a limited benefit for improving fertilization rates for men with high epididymal obstructive azoospermia.