Heat shock on bovine embryos from day 2.5–3 selects the most competent for progression to the blastocyst stage

Heat shock can impair embryo formation, while growth factors, such as colony-stimulating factor 2 (CSF2), modulate embryonic development. This study evaluated the effect of heat shock between days 2.5 and 3, as well as the impact of CSF2 at day 5 on bovine embryos cultured in a serum-free in vitro medium. The focus was on blastocyst development, the number of blastomeres, DNA fragmentation (TUNEL-positive cells), and mitochondrial activity. Heat shock reduced the proportion of cleaved embryos that developed into blastocysts (P = 0.0603). The resultant blastocysts exhibited a reduced number and proportion of TUNEL-positive cells in the trophectoderm (P = 0.0270 and P = 0.0240, respectively) and in the entire embryo (P = 0.0029 and P = 0.0031, respectively). Additionally, mitochondrial activity was lower in blastocysts derived from heat-shocked embryos (P = 0.0150) and further reduced in embryos exposed to both heat shock and CSF2 (P = 0.0415). In conclusion, the exposure of cleaved embryos to heat shock reduced their development to the blastocyst stage. However, the resulting blastocysts showed decreased DNA fragmentation and mitochondrial activity.