Effects of paternal heat stress on the in vivo development of preimplantation embryos in the mouse

The objective of this study was to examine the effect of paternal heat stress on the in vivo development of preimplantation embryos in the mouse. Synchronised B6CBF1 female mice were mated either to a control male mouse or to one that had been exposed at 7, 21 or 35 days previously, for 24 h to an ambient temperature of 36+/-0.3degreesC and 66+/-5.6% relative humidity. Embryos were collected from the oviducts of mice at 14-16 h, 34-39 h or 61-65 h after mating or from the uterus at 85-90 h after mating and their developmental status was evaluated morphologically. The number of cells within blastocysts was also determined using bisbenzimide-propidium iodide staining. Paternal heat stress 7 days before mating reduced the proportion of embryos developing from 4-cell (4-C) to morulae (M), hatched blastocysts, total blastocysts and the number of inner cell mass (ICM) and trophectoderm (TE) cells in the blastocyst. Paternal heat stress 21 days prior to mating reduced the proportion of 2-C and 4-C to M embryos with no embryos developing to blastocysts. There were also increases in the number of 1-C and abnormal embryos recorded at this time. Paternal heat stress 35 days before mating decreased the proportion of 2-C embryos, expanded blastocysts and ICM and TE cells in the blastocyst. These results support previous work demonstrating that both the sperm in the epididymis and germ cells in the testis are susceptible to damage by environmental heat stress, with spermatocytes being the most vulnerable. This study also demonstrates that subtle effects on the male such as a short exposure to elevated environmental temperatures can translate to quite profound paternal impacts on early embryo development.