Fertilization of the Mouse Egg and the Effect of Delayed Coitus and of Hot-Shock Treatment

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Abstract
A total of 329 mated adult mice was used. The mean number of eggs per mouse recovered from 171 mice killed 3-15 hours after ovulation was 8.10 (S.D. 1.94). In mice maintained under conditions of controlled illumination (light from 3 to 5 a.m., dark from 5 a.m. to 3 p.m.) ovulation began about 11 a.m. and was virtually complete by 4 p.m. The average interval between ovulation and sperm penetration of an egg was about 5 hours. Sperms spent a mean time of about 35 minutes in the perivitelline space before entering the vitellus; after a further mean interval of about 2.75 hours the early pronuclei were seen. The mean delay between the beginning of ovulation and the beginning of sperm penetration, in any one mouse, was 2.5 hours. The mean time required for the majority ([greater than or equal to] 3/4) of the eggs in any one mouse to be penetrated by sperms, once penetration had begun, was 3.5 hours. A mean number of 18.6 sperms per tube was found at the site of fertilization shortly after ovulation. Mice kept under natural lighting conditions were not permitted coitus until 8-8.45 a.m. Egg penetration began between 9 and 930 a.m. Once it had begun, the average period required for penetration of at least 3/44 of the eggs in any 1 mouse was about 2 hours. The mean number of sperms at the site of fertilization at 9, 9:30, and 10 a.m. was 6.3, 7.9, and 20.6 sperms per tube, respectively. These findings are discussed in relation to the concept that sperms need to spend a period in the female tract to capacitate them for penetration. The incidence of polyspermy was not increased by delaying coitus until 3-7 hrs. after ovulation. Hot-shock treatment applied to the eggs 3 hours after mating increased the incidence of polyspermy (dispermy) from 0.3 to 3.8% and of eggs exhibiting suppression of the second polar body from 0.5 to 12.4%. Several other forms of abnormal fertilization were also observed. The triploidy reported in mouse embryos following hot-shock treatment of the eggs probably arose in most instances through suppression of second polar body formation, and only occasionally through polyspermic fertilization.

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