Alterations in ultrastructural morphology of two-cell bovine embryos produced in vitro and in vivo following a physiologically relevant heat shock. Biol. Reprod

Exposure of cultured preimplantation embryos to tempera-tures similar to those experienced by heat-stressed cows inhibits subsequent development. In this study, the effects of heat shock on the ultrastructure of two-cell bovine embryos were examined to determine mechanisms for inhibition of development. Two-cell embryos produced in vitro were harvested at;28 h postin-semination and cultured for 6 h at one of three temperatures: 38.58C (cow body temperature), 41.08C (characteristic temper-ature for heat-stressed cows), or 43.08C (severe heat shock). Ul-trastructural examinations revealed that both heat shocks re-sulted in the movement of organelles towards the center of the blastomere. In addition, heat shock increased the percentage of mitochondria exhibiting a swollen morphology. Distance be-tween the membranes comprising the nuclear envelope was in-creased but only when embryos were treated at 43.08C. To de-termine whether ultrastructural responses to heat shock in cul-ture were similar for embryos produced in vitro and in vivo, two-cell embryos were collected from superovulated Angus cows 48 h postinsemination and treated ex vivo for 6 h at 38.58C or 41.08C. Again, heat shock caused an increase in number of swollen mitochondria and movement of organelles away from the periphery of the blastomere. Exposure of two-cell bovine embryos to physiologically relevant elevated temperatures caus-es disruption in ultrastructural morphology that is inimical to development. The observation that overall morphology and re-sponse to heat was similar for embryos produced in vitro and in vivo implies that the former can be a good model for under-standing embryonic responses to heat shock. embryo, environmen