Ability of (BALB/c •~ C57BL/6) F1 and ICR Mouse Embryos Fertilized In Vitro and Their Chromosome at the First Cleavage Division

Superovulated eggs in (BALB/cxC57BL/6) F1 and ICR (outbred in a closed colony) female mice were fertilized in vitro with spermatozoa obtained from caudal epididymides of ICR males. Air-dried chromosome preparations were made from colcemid-primed 1-cell eggs and stained by the C-banding method. The fertilization rate was lower in F1 eggs than in ICR eggs (88 vs. 92%, P<0.02). The incidence of metaphase ("syngamy") eggs was higher in F1 eggs, and the incidence of pronuclear and late prometaphase ("pre-syngamy") eggs was higher in ICR eggs (P<0.001, P<0.005), showing delayed progress of the first cleavage in the ICR eggs. Developmental rates from 2-cell to 4-cell stage and from morula to blastocyst stage were significantly lower in ICR eggs (P<0.001) than in F1 eggs. These results show that a delay of embryo development had already appeared before male and female genomes fused (pre-syngamy). The incidence of triploidy, which might be caused by dispermy, was higher in F1 eggs, in both the pronuclear and mitotic stages (P<0.001). A little aneuploidy and structural aberration of chromosomes occurred in both F1 and ICR eggs, and no significant difference was observed between F1 and ICR eggs. The sex ratio at the first cleavage stage in both F1 and ICR eggs showed no significant deviation from equality.