Early Penetration of Human Sperm Through the Vestments of Human Eggs in Vitro

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Abstract
Penetration of human sperm through the vestments of human oocytes during the first 3 h after insemination was investigated to determine the time taken for sperm capacitation, which precedes the acrosome reaction and fertilization. Twelve oocytes from women who became pregnant by IVF, recovered at laparoscopy after appropriate stimulation, were examined by electron microscopy. Follicular development was controlled by administration of clomiphene citrate, hMG, and hCG. Oocytes were cultured in Whittingham''s T6 medium for 7-14 h before insemination and were fixed 1-3 h after insemination with preincubated sperm obtained from fertile men. All oocytes had matured and eight were normally fertilized 2-3 h after insemination. The acrosome reaction had already begun 1 h after insemination when 10-30% of sperm had reacted. Sperm-oocyte membrane fusion occurred 2 h after insemination, and sperm decondensation and pronuclear formation were in progress 3 h after insemination. Variable numbers of sperm (30-60%) had reacted acrosomes after 2-3 h of insemination. Many sperm pentrating the cumulus were intact or had partially reacted acrosomes. Intact partially and fully reacted sperm were found at the surface of the zona. Intact sperm were bound to the zona by their plasma membranes. Sperm penetrating the zona had reacting or reacted acrosomes exposing their inner acrosome membranes. Those approaching the perivitelline space had a persistent equatorial vestige of the acrosome with intact plasma membrane. The acrosome reaction involved multiple fusions of the sperm plasma and outer acrosome membranes, resulting in vesiculation. This study shows that human sperm could complete capacitation and initiate the acrosome reaction within 1 h of insemination in vitro.