Apoptosis is a form of cell death that can function to eliminate cells damaged by environmental stress. One stress that can compromise embryonic development is elevated temperature (i.e., heat shock). For the current studies, we hypothesized that heat shock induces apoptosis in bovine embryos in a developmentally regulated manner. Studies were performed to 1) determine whether heat shock can induce apoptosis in preimplantation embryos, 2) test whether heat-induced apoptosis is developmentally regulated, 3) evaluate whether heat shock-induced changes in caspase activity parallel patterns of apoptosis, and 4) ascertain whether exposure to a mild heat shock can protect embryos from heat-induced apoptosis. As determined by TUNEL reaction, exposure of bovine embryos ≥16 cells on Day 5 after insemination to 41 or 42°C for 9 h increased the percentage of cells undergoing apoptosis. In addition, there was a duration-dependent increase in the proportion of blastomeres that were apoptotic when embryos were exposed to temperatures of 40 or 41°C, which are more characteristic of temperatures experienced by heat-stressed cows. Heat shock also increased caspase activity in Day 5 embryos. However, heat shock did not induce apoptosis in 2- or 4-cell embryos, nor did it increase caspase activity in 2-cell embryos. The apoptotic response of 8- to 16-cell-stage bovine embryos to heat shock depended upon the day after insemination that heat shock occurred. When 8- to 16-cell embryos were collected on Day 3 after insemination, heat shock of 41°C for 9 h did not induce apoptosis. In contrast, when 8- to 16-cell embryos were collected on Day 4 after insemination and exposed to heat shock, there was an increase in the percentage of cells undergoing apoptosis. Exposure of 8- to 16-cell embryos at Day 4 to a mild heat shock of 40°C for 80 min blocked the apoptotic response to a subsequent, more-severe heat shock of 41°C for 9 h. In conclusion, apoptosis is a developmentally acquired phenomenon that occurs in embryos exposed to elevated temperature, and it can be prevented by induced thermotolerance.
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