Development of cultured bovine embryos after exposure to high temperatures in the physiological range

Embryonic development is inhibited by exposure of cultured embryos to high temperatures. However, culture temperatures used to demonstrate the effects of heat on development have been higher than the body temperatures experienced typically by heat-stressed cows. The aim of this study was to determine whether exposing bovine oocytes and embryos to temperatures characteristic of body temperatures of heat-stressed cows would affect embryonic development in vitro. The CO2 percentage of the gas phase was adjusted in all experiments to prevent pH changes in the medium caused by decreased solubility of CO2 at high temperatures. Fertilization of oocytes at 41.0 degreesC reduced cleavage rate and the percentage of oocytes that became blastocysts compared with at 38.5 degreesC. There was no deleterious effect of fertilization at 40.0 degreesC. When putative zygotes and two-cell embryos were exposed to a range of temperatures from 38.5 to 41.0 degreesC for 3, 6, 9 or 12 h, heat shock reduced the number that developed to the blastocyst stage but only after exposure to 41.0 degreesC for 9 or 12 h, In addition, it was tested whether low O-2 tension would reduce the detrimental effects of heat shock. The deleterious effect of 41.0 degreesC was not dependent upon oxygen content or the gas mixture used for culture (5% versus 20.35% O-2), indicating that the deleterious effects of heat shock did not depend upon a high O-2 environment. In the final experiment, embryos were exposed to 24h fluctuations in temperature designed to mimic the rectal temperatures of cows exposed to heat stress. Exposure of embryos to this pattern of temperatures starting after fertilization reduced development when embryos were exposed to this environment for 8 days but not when embryos were exposed for 1 day only. These findings indicate that embryonic development can be disrupted by a short-term severe or a prolonged mild heat shock and that the effects of heat shock are not artefacts of changes in pH or high oxygen tension.