Block et al. (2002) tested whether the ability of Brahman embryos (B. t. indicus) to resist the deleterious
effects of heat shock was a result of the genetic and cellular contributions from the oocyte,
spermatozoa, or a combination of both. Embryos at ≥9 cell stage at day 4 after fertilization (96 hpi)
were more affected by heat shock (41 ◦C for 6 h) if produced using Holstein oocytes (55.6 compared
with 29.8% blastocyst, for 38.5 and 41 ◦C, respectively) than if using Brahman oocytes (42.1 compared
with 55.5% for 38.5 and 41 ◦C). In contrast, the breed of sire had no effect on the thermal resistance of
embryos produced using Angus and Brahman spermatozoa to fertilize Holstein oocytes. These results
indicate that the contribution of the oocytes has a more crucial role in the ability of the embryo to resist
effects of heat shock than indicated in the present research using Nelore or Holstein oocytes fertilized
with Nelore or Angus semen, i.e., the deleterious effect of heat shock (decline in blastocyst rate) was
less pronounced in Nelore as compared to Holstein or crossbred embryos (indicus vs. taurus). However,
in contrast to the previous report (Block et al., 2002), oocytes from Holstein cows were more resistant
to heat shock when fertilized with Nelore as compared with Angus semen, suggesting contribution of
the spermatozoa. Similar results regarding the contribution of oocyte were that the breed of the sire
(indicus) influenced the tolerance of the embryo to heat shock.