THE DEVELOPMENTAL ABILITY OF HUMAN OOCYTES PENETRATED AT THE GERMINALVESICLE STAGE AFTER INSEMINATION IN-VITRO

This study demonstrates that sperm penetration into the ooplasm occurs at high frequency in germinal vesicle (GV) stage human oocytes which failed to resume meiosis after ovulation induction in cycles of ovaria n hyperstimulation for in-vitro fertilization. The capacity of the imm ature human oocyte to prevent polyspermic penetration at the cell surf ace level was suggested by the finding that despite the presence of nu merous spermatozoa within the zona pellucida and on the oocyte surface within 3 h after insemination, all normal-appearing GV stage oocytes examined in this study were penetrated by a single spermatozoon. This notion was also supported by scanning confocal microscopic analysis of oocytes double-stained for DNA and cortical granules which showed hig hly localized regions of cortical granule-free cytoplasm in proximity to the penetrated spermatozoon. The developmental ability of these ooc ytes was assessed by culture in vitro. The results show that oocytes p enetrated by a spermatozoon at the GV stage resume meiosis, develop th e capacity to decondense sperm DNA, abstrict both first and second pol ar bodies, and form a male pronucleus from the spermatozoon which ente rs the oocyte prior to the resumption of meiotic maturation. After pen etration, sperm nuclei rapidly migrate to the centre of the oocyte and become juxtaposed ,vith the germinal vesicle, suggesting the presence of a cellular mechanism which permits directed movement within the cy toplasm. The developmental ability of these oocytes and the normality of the resulting embryos are discussed.