Observations on the Penetration of the Sperm into the Mammalian Egg

Abstract

A review of the literature suggests that the evidence so far put forward for fertilization in vitro of mammalian eggs is inconclusive. Observations on eggs recovered at intervals after induced ovulation in mated rats indicate that sperm penetration of the zona pellucida occurs very rapidly and, generally, very soon after ovulation. As a rule, the sperm enters the vitellus immediately after passing through the zona, but quite often it remains for a period in the perivitelline space before entering the vitellus. The slit or potential hole the sperm makes in penetrating the zona persists and may be demonstrated at later stages. Sperm entry into the vitellus has been observed in vitro; the process appears to be largely a function of the vitellus as the sperm is often motionless at the time. When sperms were introduced into the fallopian tube of the rabbit before ovulation, most of the eggs subsequently recovered were fertilized. However, if the sperms were introduced shortly after ovulation the eggs rarely showed signs of penetration. When sperms were introduced into the peri-ovarian sac of the rat shortly after ovulation, sperm penetration did not occur until 4 or more hours later, although sperms were regularly found about the eggs at 2 hours and later. It appears therefore that the sperm must spend some time in the female tract before it is capable of penetrating the zona. These results and observations are discussed with the object of deriving a working hypothesis on the mechanism of sperm penetration through the zona pellucida.