Cellular, subcellular, and developmental responses of two-cell bovine embryos to a physiologically relevant heat shock

Exposure of preimplantation embryos to elevated temperatures blocks development. Temperatures used to demonstrate heat shock effects on development have been higher, however, than body temperatures typically experienced by heat-stressed cows. The goal of this dissertation was to determine the mechanism by which heat shock in the physiological range blocks development.;Initial findings demonstrated that development was disrupted by a short-term heat shock of 41.0°C or by continuous exposure to a 24-h pattern of temperatures similar to those experienced by heat-stressed cows. Ultrastructural changes induced in embryos by exposure to 41.0 or 43.0°C for 6 h were examined to determine mechanisms for inhibition of development. Heat shock of 41.0°C caused movement of organelles towards the center of the blastomere and an increase in the percent of mitochondria exhibiting a swollen morphology. Exposure to 43.0°C (a non-physiological temperature) caused more widespread ultrastructural damage such as chromatin precipitation.;Experiments evaluated consequences of mitochondrial disruption caused by heat shock. Exposure to 41.0°C appears not to have an immediate effect on oxidative phosphorylation because heat shock did not immediately alter oxygen consumption or ATP content. It was observed that heat shock at the two-cell stage first caused a block in development at the eight-cell stage. Thus, damage to mitochondria or other structures at the two cell-stage first become critical to survival as the embryo enters the 8--16 cell stage.;An experiment was designed to test whether the heat shock-induced movement of organelles is a result of cytoskeletal rearrangement. At 41.0°C, movement of organelles from the periphery of the cell was blocked by rhizoxin, an agent causing microtubule depolymerization, and latrunculin B, a microfilament depolymerizer. Thus, disruption of both microfilaments and microtubules is involved in organellar movement. Only rhizoxin blocked organellar movement at 43.0°C, suggesting that microtubule damage at 43.0°C causes redistribution of organelles even when microfilaments are depolymerized. Microtubules are also involved in mitochondrial swelling because rhizoxin blocked effects of heat shock on mitochondrial swelling.;In conclusion, physiologically-relevant heat shock disrupts development of two-cell embryos and this action is mediated, at least in part, by disruptions of the mitochondria and cytoskeleton. Heat shock does not cause an immediate block to development in the two-cell embryo but prevents development past the eight-cell stage.