Determinants of embryonic resistance to heat shock during the preimplantation period in cattle.

摘要

Preimplantation embryos acquire resistance to elevated temperature as they advance in development. The goal of this dissertation was to identify determinants of embryonic resistance to elevated temperature in preimplantation bovine embryos.; Studies were performed to determine the role of heat-induced apoptosis in bovine embryos. Exposure of bovine embryos >16 cells to heat shock increased the percentage of TUNEL-positive cells and group II caspase activity. However, heat shock did not induce apoptosis in 2- or 4-cell embryos or increase group II caspase activity in 2-cell embryos. Treating embryos with an inhibitor of group II caspases (z-DEVD-fmk) blocked heat-induced increase in caspase activity and TUNEL-positive cells. Exposure of embryos to heat shock reduced development to the blastocyst stage and this deleterious effect of heat shock was magnified in the presence of z-DEVD-fmk, suggesting that blocking apoptosis has a detrimental effect on embryonic resistance to heat shock.; The detrimental effects of heat stress on fertility in cattle are less pronounced in heat-tolerant breeds than heat-sensitive breeds. Exposure of bovine embryos to heat shock decreased development to the blastocyst stage and embryo cell number. However, the deleterious effects of heat shock was less pronounced for Brahman than for Holstein and Angus. In contrast to the sensitivity of embryos to heat shock, there was no effect of heat shock on [3H]leucine incorporation into proteins secreted by oviductal or endometrial explants. Brahman and Senepol lymphocytes were more resistant to heat-induced apoptosis than the other breeds.; In the last study we demonstrated that multiple intramuscular injections of vitamin E and selenium did not improve postpartum reproductive function or milk yield in lactating dairy cows. In vitro studies showed that vitamin E did not alleviate the deleterious effects of heat shock on development to the blastocyst stage.; Taken together, results suggest that the ability to undergo heat-induced apoptosis to remove damaged blastomeres and embryo genotype are important determinants of embryonic resistance to heat shock. In contrast, supplemental antioxidants treatment was not able to improve pregnancy rates in lactating dairy cows or development of heat-shocked embryos.