Date of Award

12-2007

Degree Type

Thesis

Degree Name

Master of Science

Major

Animal Science

Major Professor

J. Lannett Edwards

Committee Members

Frank Neal Schrick, Michael O. Smith

Abstract

Objectives were to 1) examine effects of heat stress on maturing oocytes to alter the ability of resulting embryos to develop after fertilization, 2) evaluate blastocyst development of compact morulae derived from control or heatstressed oocytes after culture at 38.5°C or 41.0°C, and 3) evaluate effects of heat stress on compact morulae to alter sex ratio. Culture of cumulus oocytes at 41.0°C did not alter ability of presumptive zygotes (PZ) to cleave; however, the proportion of embryos that cleaved to the 8- to 16-cell stage was lower (P = 0.01). Also, the proportion of 8- to 16-cell embryos derived from heat-stressed oocytes undergoing compaction was lower (P = 0.01). Effects of heat stress to reduce compaction was without consequence on morphology, as quality scores of embryos derived from heat-stressed oocytes were similar to those derived from nonheat-stressed oocytes. Application of heat stress to compact morulae derived from nonheat-stressed oocytes did not affect blastocyst development. Also, blastocyst development of compact morulae derived from heat-stressed oocytes was similar to those derived from nonheat-stressed controls. However, if compact morulae derived from heat-stressed oocytes were exposed to 41.0°C, blastocyst development was reduced (P = 0.007). This effect was coincident with an increase in the proportion of degenerate embryos (P = 0.02). Temperature at which oocytes were matured did not alter sex ratio of resulting blastocysts. Rather, there was a tendency for embryo temperature to increase the proportion of male embryos that survived (P < 0.06). Results described herein are significant as they not only clarify the extent to which heat stress during maturation reduces embryonic development after fertilization but are also informative of heat-induced perturbations that carry over to increase the susceptibility of resulting embryos to heat stress. With this in mind, results raise possible concerns about current practices of utilizing MOET in heat-stressed cattle with the intent of obtaining “developmentally-competent” embryos. Additionally, since it was also demonstrated that developmental competence of oocytes has an impact on the responsiveness of resulting embryos to heat stress, effort to clarify developmental competence of oocytes before utilizing in vitro derived embryos to improve fertility of heat-stressed dairy cows is needed.

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