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1 Department of Animal Science, The University of Tennessee Institute of Agriculture, and Tennessee Agricultural Experiment Station, Knoxville 37996-4574
2 Program in Microscopy, Division of Biology, University of Tennessee, Knoxville 37996
Corresponding author: J. Lannett Edwards; e-mail: jedwards{at}utk.edu.
The objective of this study was to evaluate nuclear (progression to metaphase II) and cytoplasmic (translocation of cortical granules to the oolemma) maturation in control (38.5° C) and heat-stressed (41.0° C) oocytes. Hoechst staining indicated that a similar proportion of control and heat-stressed oocytes progressed to meta-phase II. More heat-stressed oocytes had type III cortical granule distribution suggesting that heat stress accelerated cytoplasmic maturation. The kinetics of nuclear maturation was examined in a second experiment in which a higher proportion of heat-stressed oocytes progressed to metaphase I by 8 h and arrested at meta-phase II at 16 and 18 h after placement into maturation medium. However, differences related to maturation temperature were no longer apparent by 21 h. Heat-induced alterations in kinetics of nuclear and cytoplasmic maturation prompted a third experiment to evaluate if earlier insemination of heat-stressed oocytes ameliorates heat-induced reductions in development. A significant temperature x insemination time interaction was noted when evaluating blastocyst development. Blastocyst development was reduced when heat-stressed oocytes were inseminated with sperm 24 h after placement into maturation medium compared with controls. In contrast, blastocyst development was similar to controls when heat-stressed oocytes were inseminated at 19 h. Based on this interaction, earlier insemination in vitro prevented heat-induced reductions in oocyte development. Collectively, these studies suggest a cumulative effect of heat stress to hasten in vitro maturation in bovine oocytes.
Key Words: heat stress • oocyte • maturation
Abbreviation key: COC = cumulus-oocyte complexes, GV = germinal vesicle, HS = heat stress, IVF = in vitro fertilization, IVM = in vitro maturation, M199 = tissue culture medium-199, MI = metaphase I, MII = meta-phase II, OMM = oocyte maturation medium, PZ = putative zygotes.
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