Effect of paternal heat stress on the development in vitro of preimplantation embryos in the mouse

Summary.  This study was conducted to investigate the effects of paternal heat stress on the development in vitro of preimplantation embryos in the mouse. Female C57/CBA mice were superovulated using eCG/hCG and mated either to an untreated (control) male mouse or to one that had been exposed for 24 h to an ambient temperature of 36 ± 0.1 °C and 62 ± 0.4% relative humidity, between 3 and 42 days previously. Putative zygotes were collected from the oviducts of mated mice, 25–28 h after hCG injection, and cultured in vitro. Embryo development was evaluated at 24-h intervals for up to 120 h. Paternal heat stress significantly reduced the proportion of embryos that developed normally during 24–120 h of in vitro culture, when zygotes were sired by males which had been heat stressed between 7 and 35 days prior to mating. Maximum impairment to development (including nondevelopment, abnormal and dying/dead embryos) occurred in those embryos sired by males at days 14 and 21 after heating. Embryo development returned to control levels by day 42 after heat stress. Furthermore, whilst all stages of embryo development were affected by paternal heat stress, the proportion of embryos at the two-cell stage appeared to be most severely affected. Four-cell to morula stages and the morula to blastocyst stage also demonstrated impairment at days 14, 21, 28 and 35 after heating. These results demonstrate that a single episode of paternal heat stress significantly reduces the development of preimplantation embryos, and this is not recovered until day 42 after heating. The present results also support previous work demonstrating that sperm from the epididymis as well as germ cells in the testis are susceptible to damage by heat stress, with both spermatids and spermatocytes being the most vulnerable.