Failed fertilization after intracytoplasmic sperm injection: The extent of paternal and maternal chromatin decondensation

Objective: To determine the extent of paternal and maternal chromatin decondensation in unfertilized eggs after intracytoplasmic sperm injection (ICSI). Design: Eggs that failed to show two pronuclei (2-PN) 48 hours after ICSI were studied at two different time intervals: at ICSI program inception (group A) and after 8 months (group B). Patient(s): Forty-nine patients undergoing IVF cycles. Main Outcome Measure(s): The unfertilized eggs were studied by chromatin staining. Result(s): The average fertilization rate from all ICSI cycles in these two groups was 45%. The fertilization rates in groups A and B were 35% and 59%, respectively. In group A, 65% of the unfertilized eggs were characterized by condensed sperm chromatin with 11% showing partial decondensation. In group B, only 28% of the unfertilized eggs demonstrated condensed sperm chromatin, whereas 45% were partially decondensed. In these two groups, no sperm chromatin was detected in 24% of the unfertilized eggs. The maternal chromatin remained at metephase II in 84% of all unfertilized eggs analyzed. Conclusion(s): These observations suggest that the technical problem of deposition of the sperm inside the egg is not the major cause of failure of fertilization rates in ICSI cycles. Rather, it is likely to be the failure to complete both the maternal and paternal chromatin transitions that occur with normal fertilization.