Fluorescent study of chromatin and tubulin in apparently unfertilized human oocytes following ICSI

In this study we examined 138 oocytes which were meiotically mature and, on light microscopic examination, contained either no or one pronucleus follo wing intracytoplasmic sperm injection (ICSI). Oocytes were fixed and simult aneously stained for chromatin (Hoechst 33258) and the spindle (alpha-tubul in antibody). In nine oocytes, no sperm nucleus was observed. The remaining oocytes were separated into two groups following staining; (i) oocytes whi ch had remained at metaphase II after ICSI (n = 74); and (ii) oocytes in wh ich resumption of meiosis was observed after ICSI (n = 55). In all oocytes in which sperm chromatin was absent no resumption of meiosis had occurred a nd therefore parthenogenetic activation by the process of ICSI seems to be a rare event. In 17 out of 74 (23%) oocytes which remained at metaphase II, staining identified premature chromosome condensation (PCC) of the sperm c hromatin (G(1)-PCC). Sperm nuclear decondensation or further transformation of the sperm chromatin was observed in 56 out of 74 (76%) oocytes which re mained at metaphase II after ICSI and in 46 out of 55 (84%) oocytes which h ad resumed meiosis, indicating that initiation of sperm decondensation is i ndependent of the resumption of meiosis in the oocyte. In contrast, transit ion of the sperm nucleus beyond the decondensed stage only occurred in asso ciation with resumption of meiosis in the oocyte (no pronuclei in metaphase II oocytes). The presence of both male and female pronuclei in 53% of oocy tes which had resumed meiosis indicates that changes in sperm chromatin bey ond the initial decondensation stage are dependent on cytoplasmic condition s which also permit female pronuclear formation.