Development of cultured bovine embryos after exposure to high temperaturesin 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 experie nced typically by heat-stressed cows. The aim of this study was to determin e whether exposing bovine oocytes and embryos to temperatures characteristi c of body temperatures of heat-stressed cows would affect embryonic develop ment in vitro. The CO2 percentage of the gas phase was adjusted in all expe riments to prevent pH changes in the medium caused by decreased solubility of CO2 at high temperatures. Fertilization of oocytes at 41.0 degreesC redu ced cleavage rate and the percentage of oocytes that became blastocysts com pared with at 38.5 degreesC. There was no deleterious effect of fertilizati on at 40.0 degreesC. When putative zygotes and two-cell embryos were expose d 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 tes ted whether low O-2 tension would reduce the detrimental effects of heat sh ock. 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), indicat ing 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 fluctu ations in temperature designed to mimic the rectal temperatures of cows exp osed to heat stress. Exposure of embryos to this pattern of temperatures st arting after fertilization reduced development when embryos were exposed to this environment for 8 days but not when embryos were exposed for 1 day on ly. 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.