Transient retardation in embryo growth in normal female mice made pregnant by males whose testes had been heated

In three separate experiments, using three different strains of mice, when normal females were mated by males whose testes had been heated once to 42 degrees C for 20 min, the embryos at 10.5 days post-coitum were approximately 20% smaller than control embryos. In one experiment, the difference was still present, although proportionately less, at 15.5 and 18.5 days and, in another experiment, a difference could be seen in 11.5 and 13.5 day old embryos but not in 12.5, 14.5, 15.5 or 18.5 day old embryos. The frequency of mating and pregnancy rates were unaffected. In one experiment, the time available for mating was restricted to 4 h instead of overnight, without effect on the result. In another experiment, other males were heated for 30 min, and these showed a period of infertility from 10 to 32 days later, preceded and followed by the production of smaller than normal embryos; litter size was also reduced in the period after the return of fertility in these animals. The yolk sacs and the trophoblasts of the embryos sired by the heated males were also slightly smaller than those sired by the controls in the two experiments in which these were measured. The pattern of weight reduction is thus different from that seen in gynogenetic embryos or when the gene for insulin-like growth factor (IGF)-II is disrupted, and suggests a reduction in embryo growth at the earlier stages, with compensatory growth occurring later in pregnancy.