PARENT SESSION

(M322) DIFFERENTIAL RESPONSE OF PORCINE PREIMPLANTATION EMBRYOS TO TIMING VARIATIONS OF AN EXOGENOUSLY APPLIED HEAT STRESS.

Isom, S. Clay¹, Alinea, Corinne¹, Dillender, Jessica¹, Prather, Randall¹, Rucker III, Edmund¹, ¹ University of Missouri-Columbia, Columbia, MO

ABSTRACT- Recent efforts in our laboratory have been directed towards explaining the biochemistry and molecular biology behind the apparent effects of heat stress on in vitro produced parthenogenetic porcine embryos. Very surprisingly, we have determined that embryos can have very different responses to heat stress episodes depending on the timing of stressor application relative to the time of oocyte activation. Embryos were exposed to a 9-hour heat stress of 42° C at one of three periods of time relative to egg activation: immediately after egg activation (0-9HS), beginning 13 hours after egg activation (13-22HS), or from 22 to 31 hours after activation (22-31 HS). Control embryos (NHS) were maintained at 39° C. Embryos were then characterized by using a variety of criteria to evaluate embryo quality: percentage of embryos cleaved at 24 hours post-activation (hpa), day 5 cell number, day 5 apoptosis, day 7 blastocyst rate, and day 7 cell number. Embryos from the 13-22HS and 22-31 HS groups tended to perform statistically the same as, or slightly worse than did the NHS embryos. In stark contrast, the 0-9HS embryos consistently performed remarkably better than did the embryos from any of the other groups. For example, 63% of 0-9HS embryos cleaved by 24 hpa, but for the NHS, 13-22HS, and 22-31HS embryos, the rate was less than 30%. Average day 5 cell numbers were 5.4, 10.5, 6.5, and 5.1 for NHS, 0-9HS, 13-22HS, and 22-31HS, respectively. The percentage of blastomeres undergoing apoptosis on day 5 was 5.4% for 0-9HS embryos, and at least 12% in all other groups. On day 7, 38.0% of the 0-9HS embryos had developed to blastocyst, whereas only 18.5% of NHS, 12.4% of 13-22HS, and 14.6% of 22-31HS embryos were successful in developing to that stage. The average cell number of day 7 embryos was approximately 2.5 times higher in the 0-9HS embryos than in any of the other groups. These results indicate that the response of in vitro produced embryos to an applied heat stress depends heavily on the timing of stressor application. A significant heat stress episode applied immediately after oocyte activation was shown to have an apparently beneficial effect on embryo development. The effect of this early heat stress on these embryos is astonishingly dramatic and entirely unexpected. Efforts are currently underway to explain the mechanism(s) behind this phenomenon.

KEY WORDS: parthenogenesis, heat stress, preimplantation embryo development, in vitro embryo production