Gamete-specific DNA fragmentation in unfertilized human oocytes after intracytoplasmic sperm injection

The objective of the present study was to assess the integrity of maternal and/or paternal chromatin in injected oocytes that remained unfertilized after intracytoplasmic sperm injection (ICSI). The study was performed on 102 oocytes that failed to show pronuclear formation 18-20 h after ICSI. We used chromatin labelling with 4,6-diamidino-2-phenylindole (DAPI) to identify maternal and paternal chromatin, coupled with biotin-mediated end-labelling to assess DNA fragmentation in each gamete. It was shown that 50% of oocytes without pronuclear formation following ICSI contained chromatin with damaged DNA, and that the source of the DNA fragmentation was equally divided between the spermatozoon (25.8%) and the oocyte (24.4%). A significantly greater proportion of condensed spermatozoa in human oocytes had damaged DNA, compared to decondensed spermatozoa (24.7 compared to 5.9%, P=0.002). There was a significant increase in the incidence of DNA fragmentation in oocytes from patients older than 35 years (65+/-1.2%) compared to those from younger patients (36+/-1.0%) (P < 0.05). Further, 17% of unfertilized oocytes contained no paternal chromatin. Thus, DNA fragmentation in both spermatozoa and oocytes is associated with failure of fertilization in ICSI. In some cases of severe male factor infertility, a significant proportion of spermatozoa injected into oocytes may contain fragmented DNA. Injection of oocytes with spermatozoa containing abnormal chromatin will probably result in failure of sperm decondensation and fertilization. In older women, a significant proportion of oocytes injected may contain fragmented DNA. These observations may explain the consistent inability of most clinics to achieve fertilization rates higher than 65% with ICSI.