Behaviour of spermatozoa in human oocytes displaying no or one pronucleus after intracytoplasmic sperm injection

The behaviour of sperm cells after intracytoplasmic sperm injection (ICSI) was investigated by analysing 192 unfertilized and 37 one-pronuclear (1PN) oocytes following ICSI. Eighty-two unfertilized oocytes were directly fixed whereas 110 were first parthenogenetically activated by puromycin. In contrast to the findings in unfertilized oocytes after in-vitro fertilization, most unfertilized oocytes after ICSI (n = 76) contained evidence of the presence of spermatozoa in the cytoplasm. Few oocytes (n = 6) contained prematurely condensed sperm chromosomes (PCC), whereas the majority contained either intact sperm heads (n = 31) or swollen sperm nuclei (n = 39) along with metaphase II chromosomes of the oocyte. Following activation by puromycin, swollen sperm nuclei and PCC were no longer observed, whereas unchanged sperm heads persisted in 12 oocytes displaying a single pronucleus. A non-decondensed sperm nucleus along with decondensed maternal chromatin were also discovered in 32 out of 37 oocytes displaying a single pronucleus after ICSI. The findings in unfertilized and 1PN oocytes after ICSI indicate that successful sperm injection, even followed by oocyte activation, is not sufficient to guarantee normal fertilization. It seems that partial sperm membrane damage prior to injection is also required to ensure normal sperm decondensation.