Studies On Effects Of Heat Shock On Bovine Oocytes And Embryos Cultured In Vitro

Effects of elevated temperature on the early bovine embryo in vitro were analyzed in order to determine its impact on the development rate, cell lineage allocation, survival rate based on genetic sex, and on expression of the heat shock protein 70 (hsp70) gene. Exposure of bovine embryos on day 3 of development to 42 for 0.5h, 2.Oh, and 4.0h lowered their potential to reach blastocyst stage on day 9 of development compared to controls(39.0). However, cell counts and inner cell mass to trophectoderm ratios of the embryos exposed to elevated temperature were not different from controls, suggesting that the developmental potential of the embryos which survive to blastocyst stage are not overtly impaired. Analysis of the genetic sex of control embryos which developed to the hatched blastocyst stage in spite of exposure to 42.0'C on day 3 of development showed a shift in sex ratio in favour of females, indicating that XX embryos may be better able to withstand heat stress.In vitro matured bovine oocytes, two-cell and eight-cell embryos, and day-9 hatched blastocysts subjected to control and elevated temperature conditions were analyzed by semi-quantitative reverse transcription polymerase chain reaction methods for hsp70 mRNA expression. Results revealed the expression of hsp70 mRNA under control conditions and that early embryos can respond to heat stress by transcribing hsp70 mRNA which increase as development proceeds.Confocal laser scanning microscopy used to localize the hsp70 protein in oocytes and embryos revealed that the distribution of hsp70 in the ooplasm of immature and mature oocytes is unaffected by exposure to decreased or elevated temperatures and that this protein was closely associated with the meiotic spindle indicating its possible role in stabilizing this structure, hi eight-cell embryos derived under control conditions, hsp70 was evenly distributed in the cytoplasm but appeared as aggregates in some embryos exposed to elevated temperature. In heat stressed hatched blastocysts, a more even distribution was noted following heat stress relative to corresponding controls, indicating their competence to respond to elevated temperature.