239. Sensitivity of bovine morulae and blastocysts to heat shock in vitro

Abstract

Thermotolerance of blastocysts and morulae is greater than that of zygotes. However, lower rates of pregnancy have been recorded when morulae rather than blastocysts were transferred. The aim of this study was to determine if morulae displayed lower thermotolerance than blastocysts in vitro. Embryos were produced from oocytes collected from abattoir-sourced ovaries. On Day 7 post fertilisation, embryos were classified as morulae or blasotcysts and subjected to either an increase in temperature from 39ºC to 41.5ºC over 1 h followed by a decrease to 39ºC over 2 h (HS) or maintained at 39ºC (NHS), using a water jacketed CO₂ incubator. The number of embryos progressing to expanded and hatched blastocysts was recorded after 48 h further culture at 39ºC. After arcsine transformation, the proportions progressing were submitted to a general linear model using adjusted sum of squares for tests of difference. Factors were embryonic stage, treatment and sire and the interaction terms of stage and sire with treatment. The Kruskall-Wallis test was also applied to the untransformed, non-parameteric data set. Non-parameteric , univariant analysis indicated non-significant effects of treatment (NHS median proportion progressing =72%, HS = 64%, P = 0.12) and of sire (NHS = 78%, HS = 64%, P = 0.168), while the effect of stage was highly significant (morulae = 35%, blastocysts = 79%, P = 0.0000). However, according to the general linear model, treatment and stage were significant factors (F = 5.39 and 38.3, respectively, and P = 0.032 and 0.000, respectively) and sire approached significance (F = 4.09, P = 0.058). Neither of the interaction terms was significant. It was concluded that embryos, which were morulae on Day 7, were less likely to progress to expanded or hatched blastocysts and that heat shock reduced developmental progression.