Maturation of Bovine Oocytes Cultured in vitro: Relation to Ovarian Activity, Follicular Size and the Presence or Absence of Cumulus Cells1

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Abstract
Two experiments were conducted to study maturation of bovine follicular oocytes. In experiment 1, the proportions of oocytes with or without cumulus cells relating to follicular sizes and ovarian activity were investigated without culture. Experiment 2 investigated maturation of bovine oocytes cultured in vitro and its relation to ovarian activity (with or without corpora lutea), follicular sizes (≤5 mm, 6–10 mm and 11–20 mm) and the presence or absence of cumulus cells. Oocytes collected from follicles were placed in a test tube (60 mm long, 10 mm in diameter) containing 1 ml medium (TCM 199 + BSA 3 mg/ml + antibiotics). The tubes were gassed with 5% CO2 in air for 1 min and then kept at 37–38°C for 24–33 h in culture. Experiment 1 showed that the proportion of oocytes without cumulus cells was increased by∼ 30% in the oocytes collected from the larger follicles compared with that of the smaller (≤5 mm) follicles. In experiment 2, there was no significant difference in the maturation rate of oocytes collected from ovaries with or without corpora lutea (49.6% and 45.9%, respectively). It was also found that there was no significant difference in maturation rate among 3 different sizes of follicles, although it has been expected that oocytes collected from the larger follicles would have higher maturation rates than those from the smaller follicles. A highly significant difference was found for oocyte maturation rates between oocytes with cumulus cells and those without cumulus cells (P<0.001). It appears that the presence of cumulus cells was the most important factor studied for in vitro maturation of bovine follicular oocytes.