Fertilization and early embryology: 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 37one-pronuclear (1PN) oocytes following ICSI. Eighty-two unfertilizedoocytes were directly fixed whereas 110 were first parthenogeneticallyactivated by puromycin. In contrast to the findings in unfertilizedoocytes after in-vitro fertilization, most unfertilized oocytesafter ICSI (n = 76) contained evidence of the presence of spermatozoain the cytoplasm. Few oocytes (n = 6) contained prematurelycondensed sperm chromosomes (PCC), whereas the majority containedeither intact sperm heads (n = 31) or swollen sperm nuclei (n= 39) along with metaphase II chromosomes of the oocyte. Followingactivation by puromycin, swollen sperm nuclei and PCC were nolonger observed, whereas unchanged sperm heads persisted in12 oocytes displaying a single pronucleus. A non-decondensedsperm nucleus along with decondensed maternal chromatin werealso discovered in 32 out of 37 oocytes displaying a singlepronucleus after ICSI. The findings in unfertilized and 1PNoocytes after ICSI indicate that successful sperm injection,even followed by oocyte activation, is not sufficient to guaranteenormal fertilization. It seems that partial sperm membrane damageprior to injection is also required to ensure normal sperm decondensation.