Heat shock reduces developmental competence and alters spindle configuration of bovine oocytes

Heat shock may enhance the thermotolerance of, or cause detrimental effects on, a variety of cell types or organisms, depending on the duration and intensity of the thermal challenge. Experiments were designed to investigate the effect of heat shock on the developmental competence and cytoskeletal structures of bovine oocytes following IVF. In Experiment 1, bovine cumulus-oocyte complexes (COCs) were subjected to standard IVM culture conditions for 20 h and were then randomly allocated to groups for heat shock at 42 °C for 0 (control), 1, 2, or 4 h. The oocytes were fertilized after heat shock and followed by culture in KSOM for 8 d. There were no significant differences in cleavage rates, but blastocyst formation (27% versus 44%) and total cell number per blastocyst (82 ± 21 versus 108 ± 36; mean ± S.D.) were lower in the 4-h heat shock group compared to the control (P < 0.05). Trophectoderm, but not ICM, cell numbers were decreased (P < 0.05) in the 4-h heat shock group compared to the control. Alterations in the meiotic spindle of IVM oocytes (n = 120-126) were examined after 1 to 4-h of heat shock in Experiments 2 and 3. The metaphase spindle became elongated or aberrant and smaller following heat shock, compared to the non-heat shock oocytes (P < 0.05). The basis for changes in spindle configuration and the differential decrease in trophectoderm cell numbers after heat shock are not clear, but may lead to reduced embryonic development and perhaps the low pregnancy rate of domestic animals during hot seasons.