The cholesterol/phospholipid (C/PL) ratio was determined for spermatozoa from eight men with normal semen parameters. There was a close correlation between the C/PL ratio and the rate of sperm capacitation as assessed by the hamster egg penetration test. A lower C/PL ratio correlated with a faster capacitation time. This supports the notion that the loss or reduction of membrane cholesterol constitutes an important step of capacitation in human spermatozoa.

Zonae pellucidae of unfertilized hamster and human eggs remain penetrable by homologous spermatozoa when they are stored in highly concentrated salt solutions [Yanagimachi et al., 1979]. Neutralization of salt solutions with the appropriate pH buffers is recommended for long-term storage of the eggs (zonae), because zonae may gradually swell or even dissolve in acidic or alkaline solutions. At least for the hamster, 2 M ammonium sulfate solution containing 40 mM Hepes and 0.5% dextran (pH 7.0) seems to serve the purpose satisfactorily. Thorough rinsing of salt-stored zonae prior to insemination is important, but inclusion of protease inhibitors in the rinsing medium does not appear to be essential. A dialysis method is recommended when a large number of salt-stored eggs must be rinsed at one time. Salt-stored hamster zonae, like fresh (control) zonae, permit attachment and penetration of hamster spermatozoa, but not human spermatozoa. Salt-stored human zonae, on the other hand, allow penetration by not only human spermatozoa, but also hamster spermatozoa.

It is well established that the zonae pellucidae of mature unfertilized eggs have the ability to induce the acrosome reaction of capacitated spermatozoa. To determine if this capacity of the zona is species-specific, hamster spermatozoa were allowed to attach to the zonae of homologous and heterologous eggs and examined for the acrosome reaction. The zonae of eggs from six different species were tested and the zona of hamster egg was found to have the strongest capacity to induce the acrosome reaction of hamster spermatozoa, followed by human and rat zonae. The zonae and mouse, guinea pig, and domestic fowl eggs were incapable of inducing the acrosome reaction of hamster spermatozoa. The acrosome reaction-inducing ability of the hamster zona was found to increase during maturation in the ovary. The zona of mature unfertilized hamster eggs maintained their acrosome reaction-inducing ability even after aldehyde fixation or storage in a highly concentrated solution of ammonium sulfate.

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