HUMAN OOCYTE ACTIVATION FOLLOWING INTRACYTOPLASMIC INJECTION - THE ROLE OF THE SPERM CELL

The aim of this study was to investigate whether the human spermatozoo n participates in the activation of human oocytes following intracytop lasmic sperm injection (ICSI) and if so, by what mechanism. In the fir st series of experiments, we randomized human oocytes which had remain ed unfertilized after in-vitro fertilization (IVF) or ICSI, for intrac ytoplasmic injection with live spermatozoa, spermatozoa presumed to be dead and no spermatozoa. Secondly unfertilized human oocytes and fres hly ovulated mouse oocytes were randomized for intracytoplasmic and su b-zonal injection with human sperm cytosolic fraction (CF) before and after heat treatment. We found that oocyte injection with initially mo tile spermatozoa induces human oocyte activation at a significantly hi gher rate than injection with dead spermatozoa (61 versus 0%; P < 0.00 1) or injection without a spermatozoon (61 versus 14%; P < 0.001). Int racytoplasmic injection of CF activated both human and mouse oocytes a t the same rate as sperm injection of human oocytes (activation rates of 70 and 65% respectively). This effect was greatly reduced by heat t reatment of the CF. From these experiments we conclude firstly that th e human spermatozoon injected intracytoplasmically contributes to huma n oocyte activation and secondly that the spermatozoon releases into t he oocyte a heat-sensitive, intracellularly active factor, which is no t species-specific.