Age-dependent losses in the penetrability of mouse eggs

The fertilizable life of mammalian eggs retained in the oviduct is limited; in the mouse, estimates vary from 10\p=n-\15hr (for review, see Iwamatsu & Chang, 1971), but the effect of egg age on the kinetics of sperm penetration has not been examined thoroughly. In considering age-dependent losses in egg fertilizability, both in vivo and in vitro, we were interested in differentiating between changes occurring at the zonal level versus those affecting the sperm\p=n-\eggfusion process, i.e. vitellus-related. Accordingly, sperm penetration of zona-intact and zona-free eggs was investigated. Swiss mice (6-10 weeks of age) were induced to superovulate by i.p. injections of PMSG (7-5 i.u.; Organon) and 48 hr later HCG (7-5 i.u.; Sigma). The unfertilized eggs were recovered and handled as described previously (Inoue & Wolf, 1975; Wolf & Inoue, 1976). Animals were killed approximately 14 hr after HCG for the recovery of oocytes to be aged in vitro. Oocytes aged in vivo (3 and 10 hr old) were recovered from animals killed 17 and 24 hr after the HCG injection, since ovulation was con¬ sidered to be completed by 14 hr after HCG (Edwards & Bates, 1959). Eggs recovered at these times are known to be highly and poorly fertilizable, respectively (Iwamatsu & Chang, 1971). Cumulus cells around the 3-hr-old eggs were removed in culture medium containing 0-1 % hyaluronidase (Sigma), and zonae pellucidae were removed mechanically by aspiration through micropipettes (Wolf