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Alterations in Ultrastructural Morphology of Two-Cell Bovine Embryos Produced In Vitro and In Vivo Following a Physiologically Relevant Heat Shock¹

Department of Animal Sciences³ Interdisciplinary Center for Biotechnology Research,⁴ Electron Microscopy Core Laboratory, University of Florida, Gainesville, Florida 32611

Exposure of cultured preimplantation embryos to temperatures 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 postinsemination and cultured for 6 h at one of three temperatures: 38.5°C (cow body temperature), 41.0°C (characteristic temperature for heat-stressed cows), or 43.0°C (severe heat shock). Ultrastructural examinations revealed that both heat shocks resulted 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 between the membranes comprising the nuclear envelope was increased but only when embryos were treated at 43.0°C. To determine whether ultrastructural responses to heat shock in culture 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 38.5°C or 41.0°C. 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 causes disruption in ultrastructural morphology that is inimical to development. The observation that overall morphology and response to heat was similar for embryos produced in vitro and in vivo implies that the former can be a good model for understanding embryonic responses to heat shock.

¹ This research was supported in part by grants from the USDA: IFAFS 2001-52101-11318 and TSTAR 2001-34135-11150. This is Journal Series no. R-09573 of the Florida Agricultural Experiment Station. Portions of this research were presented at the 2002 Southeastern Electron Microscopy Society meeting in Athens, Georgia.

² Correspondence: Peter J. Hansen, Department of Animal Sciences, University of Florida, P.O. Box 110910, Gainesville, FL 32611-0910. FAX: 352 392 5595; hansen{at}animal.ufl.edu

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