Failure of human spermatozoa to penetrate zona free mouse and rat ova in vitro.

When incubated for 8 to 26 hours with zona-free mouse or rat ova, human spermatozoa failed to attach to or penetrate any of the ova. The ova were capable of being fertilized since both intra- and inter-species penetration of spermatozoa and formation of pronuclei occurred between rat and mouse gametes. When mouse spermatozoa were incubated for three to eight hours with rat ova, a high proportion of the ova were penetrated, formation of pronuclei occurred and in 9 out of 36 ova incubated for 40 hours after insemination, regular cleavage and formation of morphologically normal 2-cell embryos occurred. Human spermatozoa retained their morphological integrity and motility only when the culture medium contained purified bovine serum albumin (3 mg/ml) or human serum (5% v/v) and not when unpurified BSA from several different commercial sources was used as a protein source. In this latter medium, the ova of both rats and mice degenerated after 8-hour incubation in the presence of human spermatozoa but not when human spermatozoa were absent or in the presence of either rat or mouse spermatozoa. Electron microscopy indicated that the human spermatozoa incubated for eight hours in medium containing purified BSA had undergone an acrosome reaction. These spermatozoa also attached to and penetrated human oocytes which had been matured in vitro.