Analysis of Human Spermatozoal Fertilizing Ability Using Zona-Free Ova*†

An in vitro fertilization assay employing zona-free hamster eggs was used to analyze human spermatozoal fertilizing ability. Human spermatozoa were preincubated for 18 to 20 hours in Biggers, Whitten, and Whittingham’s medium (1971) at a concentration of 1 x 107sperm/ml prior to the addition of zona-free superovulated hamster eggs. Eggs were examined microscopically 2 hours later for evidence of swelling or decondensing sperm heads in the cytoplasm. A total of 6266 eggs were examined in assays for both suspected fertile and infertile donors; 50 eggs/sample were examined. The percentage fertilization was found to range from 14% to 100% in the suspected fertile group with an average of 56.3%. The sperm concentration in this fertile group ranged from 22 to 303 million/ml with an average of 114. The suspected infertile samples yielded fertilization rates of 10% or less and an average count of 50.6 million/ml. These data suggest that human spermatozoa fuse with the vitelline membrane of zona-free hamster eggs and decondense with varying efficiencies. The percentage of fertilization in this cross-species system did not show a significant correlation with sperm concentration or motility. However, suspected infertile samples always yielded 10% or less fertilization in this assay. This method may have potential value as a diagnostic tool in evaluating human spermatozoal fertilizing capacity which avoids the ethical and logistical problems associated with fertilization of human eggs in vitro.