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 o nce to 42 degrees C for 20 min, the embryos at 10.5 days post-coitum w ere similar to 20% smaller than control embryos. In one experiment, th e 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 d ay old embryos, The frequency of mating and pregnancy rates were unaff ected, 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 pe riod of infertility from 10 to 32 days later, preceded and followed by the production of smaller than normal embryos; litter size was also r educed in the period after the return of fertility in these animals, T he yolk sacs and the trophoblasts of the embryos sired by the heated m ales were also slightly smaller than those sired by the controls in th e two experiments in which these were measured, The pattern of weight reduction is thus different from that seen in gynogenetic embryos or w hen the gene for insulin-like growth factor (IGF)-II is disrupted, and suggests a reduction in embryo growth at the earlier stages, with com pensatory growth occurring later in pregnancy.