Abstract
Oocyte maturation is a key issue in current animal biotechnology. This study was designed to examine effects of incubation temperature and cumulus investment on in vitro maturation (IVM) of porcine oocytes. Cumulus-oocyte complexes were recovered from slaughterhouse ovaries. The oocytes, surrounded completely or partially with cumulus cells (completely- and partially-enclosed groups, respectively), were incubated for 44 h at either 37°C or 39°C, and progression of meiosis and changes in the cytoskeleton distribution were evaluated by fluorescence staining. Prior to maturation culture (0 h), 94-100% of the oocytes were at the germinal vesicle (GV) stage. At 24 h IVM, a significantly greater number of the oocytes showed signs of GV breakdown (GVBD) when incubated at 39°C than at 37°C (96% vs. 61% and 92% vs. 40% for completely- and partially-enclosed groups, p<0.01, respectively). Percentages of the oocytes which reached metaphase-II (M-II) at 36 h IVM was higher at 39°C than at 37°C (p<0.01), whereas no significant difference was found in the maturation rate at 44 h IVM. The completely-enclosed oocytes had a significantly higher maturation rate than did the partially-enclosed oocytes (90% vs. 68% for 37°C and 91% vs. 53% for 39°C, p<0.01 respectively). Fluorescence staining showed that transzonal microfilaments were abundant at the GV and M-I stages (0 to 24 h), but decreased in number at the M-II stage (36 to 44 h) and that there was no remarkable difference in the distribution of microtubules and microfilaments within the ooplasm, irrespective of the incubation temperature or the condition of cumulus attachment. These observations suggest that GVBD and meiotic progression may be drastically delayed by a 2°C decrease in incubation temperature and that the cumulus condition may affect porcine oocyte maturation.
