Monospermy and polyspermy after partial zona dissection of reinseminated human oocytes

Partial zona dissection (PZD), a zona drilling method that uses mechanical force to open the zona pellucida while the oocyte is shrunken in a sucrose solution, was applied to 121 unfertilized 1-day-old mature human oocytes prior to reinsemination. The 115 surviving oocytes were divided into three groups in which the duration between sucrose addition and reinsemination was varied: I) Less than 20 minutes, II) 21 to 45 minutes, and III) longer than 45 minutes. There was a trend toward a reduced fertilization and polyspermy rate as the time between sucrose exposure and insemination in sucrose-free medium increased. Moreover, there was a statistically significant reduction in the number of oocytes penetrated by more than four sperm in group III (0/41) versus group I (7/34), and in group III, parthenogenetic development was observed. The incidence of polyspermy was also increased in oocytes manipulated more than 25 hours after retrieval compared with those manipulated 21-24 hours after recovery, supporting the idea that aged oocytes have a reduced ability to block polyspermy. Oocyte contraction in sucrose occurred in three different patterns: spherical, pear-shaped, and crenated. Both the fertilization and polyspermy rates were significantly higher in the crenated group. These results indicate that changes resembling activation occur following sucrose exposure and that sucrose activation can be used to reduce the risk of polyspermic fertilization in zona drilling procedures. In addition, the pattern of shrinkage in sucrose can be used as an indicator of oocyte receptivity to sperm penetration.