REQUIREMENT OF CAPACITATION FOR SPERM PENETRATION OF ZONA-FREE RAT EGGS

Worcester Foundation for Experimental Biology, Shrewsbury, Massachusetts 01545, U.S.A. (Received 23rd December 1974) After dissolution of the zona pellucida by chymotrypsin, rat eggs can be penetrated by uterine (capacitated) and epididymal (uncapacitated) spermatozoa (Toyoda & Chang, 1968). Yanagimachi & Noda (1970) stated that "capacitated spermatozoa quickly fused and became incorporated with the [zonafree hamster] eggs, while uncapacitated spermatozoa were never fused with eggs." Uncapacitated guinea-pig spermatozoa also failed to enter the zona-free eggs (Yanagimachi, 1972). In a study of the role of cumulus cells and the zona pellucida in the fertilization of mouse eggs in vitro, Pavlok & McLaren (1972) concluded that "capacitation of mouse spermatozoa appears to be required for the penetration of the zona pellucida only, not for entry into the vitellus." Recently, we have found that the capacitation of rat spermatozoa can be achieved by preincubation of epididymal spermatozoa in a diluted rather than a concentrated form