A retrospective analysis of unfertilized and presumed parthenogentically activated human oocytes demonstrates a high frequency of sperm penetration.

A total of 518 normal-appearing, meiotically mature human oocytes that were judged unfertilized after insemination in vitro were examined for sperm penetration by conventional fluorescence and laser scanning confocal microscopy with DNA-specific probes. A similar analysis was performed on 29 single pronuclear oocytes that were presumed to originate by spontaneous (parthenogenetic) activation. The results demonstrate that 22% of the unfertilized oocytes and 52% of the presumed parthenogenetic oocytes were actually penetrated. Sperm penetration occurred in both normozoospermic and male factor cases. The findings indicate the importance of penetration analysis in determining the causes of fertilization failure that may reside with the male or female gamete, especially when assessing the utility of and necessity for assisted fertilization in subsequent attempts. The results also suggest that the cytoplasmic capacity to decondense sperm DNA may decline more rapidly than the ability of the oocyte to be penetrated and to mount an effective block to polyspermy.