Heat shock also compromised embryonic viability as determined by the total number of cells per embryo at 24 h after the initiation of heat shock. The deleterious effect of heat shock on embryo cell number depended upon the magnitude of heat shock and stage of embryonic development. Thus, heat shock at the 2- to 4-cell stage caused a larger reduction in embryo cell number than heat shock at the morula stage. This finding is in agreement with earlier studies in which heat shock caused a greater reduction in development when applied at the 2-cell stage than the morula stage or at Day 3 after fertilization than at Day 4. Therefore, those embryos that were at stages of development that were capable of heat-induced apoptosis were more resistant to the deleterious effect of heat shock on development.
One possibility is that the sensitivity of the early embryo to heat shock is a reflection, at least in part, of the failure of the embryos to undergo heat-induced apoptosis to remove damaged cells from the embryonic lineage. There is some evidence that induction of limited ap-optosis by heat shock does not necessarily inhibit embryonic development because embryos >16 cells that experienced apoptosis induced by 40 or 41 °C did not have a reduced cell number 24 h after heat shock. Further studies evaluating other indices of development as well as the effects of inhibitors of apoptosis on development should add insight into the implications of stress-induced apoptosis for embryonic survival.