Fertilization in vitro of Eggs and First Cleavage of Embryos in Different Strains of Mice

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Abstract
Fertilization in vitro in a modified Krebs-Ringer bicarbonate solution was studied in four inbred strains of mice and two closed colonies. When the eggs were inseminated with epididymal spermatozoa preincubated for 1–1.25 h, sperm penetration was completed within 1 h in I-tester, NC, C57BL/6, ICR-JCL and ddY-F mice and within 2 h in C3H mice. Penetration rates 5 h after insemination were higher in I-tester (100%), NC (96%), ICR-JCL (99%) and ddY-F (98%) than in C57BL/6 (67%) and C3H (84%) mice. The proportions of eggs penetrated with more than one sperm and of polyspermy were also extremely variable among the strains. In general, the block against penetration by more than one sperm was more effective at the level of the vitelline membrane than at that of the zona pellucida. Among the 4 strains of mice tested, there were significant differences in the proportion of eggs cleaved 20 h after insemination, indicating that strain differences are also in existence at the time of first cleavage.