Human oocyte activation following intracytoplasmic injection: the role of the sperm cell

The aim of this study was to investigate whether the human spermatozoon participates in the activation of human oocytes following intracytoplasmic sperm injection (ICSI) and if so, by what mechanism. In the first series of experiments, we randomized human oocytes which had remained unfertilized after in-vitro fertilization (IVF) or ICSI, for intracytoplasmic injection with live spermatozoa, spermatozoa presumed to be dead and no spermatozoa. Secondly, unfertilized human oocytes and freshly ovulated mouse oocytes were randomized for intracytoplasmic and sub-zonal injection with human sperm cytosolic fraction (CF) before and after heat treatment. We found that oocyte injection with initially motile spermatozoa induces human oocyte activation at a significantly higher rate than injection with dead spermatozoa (61 versus 0%; P < 0.001) or injection without a spermatozoon (61 versus 14%; P < 0.001). Intracytoplasmic injection of CF activated both human and mouse oocytes at the same rate as sperm injection of human oocytes (activation rates of 70 and 65% respectively). This effect was greatly reduced by heat treatment of the CF. From these experiments we conclude firstly that the human spermatozoon injected intracytoplasmically contributes to human oocyte activation and secondly that the spermatozoon releases into the oocyte a heat-sensitive, intracellularly active factor, which is not species-specific.