Study of evaluation criteria used for in vitro bovine oocyte selection and maturation

Production of bovine preimplantation embryos in vitro requires beneficial maturation conditions and high quality oocytes at the germinal vesicle stage. The current classification of oocytes is based on character of the cumulus cell investment around the oocyte. We wished to study the nuclear stage of immature oocytes selected for in vitro maturation according to cumulus cell character and, in the other hand, to compare the relationship among 3 parameters utilized to evaluate in vitro maturation of bovine oocytes (degree of cumulus expansion, meiotic maturation rate and in vitro fertilization rate) when fetal calf serum, steer serum and bovine follicular fluid supplementation were used. Ovaries were collected at an abattoir and the oocytes harvested. As regards selection criteria, immature oocytes were classified as Class A, B, C and D according to the character of the cumulus cells. A high percentage of Class A oocytes (87.7%) were in the germinal vesicle stage with respect to the other classes (p < 0.05). Significant differences were found in the meiotic maturation rate in Class A oocytes (76.5%) versus those of the other classes (p < 0.05). The meiotic maturation rate diminished to 47.5% when Class A oocytes were denuded and then matured in vitro (p < 0.05). As regards maturation criteria, there was no cumulus expansion when oocytes were matured in TCM-199 without supplementation, partial expansion with the addition of fetal calf serum and full expansion when supplemented with steer serum or bovine follicular fluid. No significant differences were found in the meiotic maturation rate for the various treatments. In vitro fertilization rate was significantly lower in media without supplementation versus supplemented media (p < 0.05), but no significant differences were found between the supplemented media inter se. There is no direct relationship between the three studied parameters to evaluate in vitro maturation. Class A oocytes are the most likely to mature in vitro as they not only have a close association with their surrounding cumulus cells, but are also very numerous in the germinal vesicle stage. The degree of cumulus expansion and the meiotic maturation rate have a relative importance in evaluating in vitro maturation, as oocyte maturation implies not only nuclear events but also at other cellular levels, as evaluated by in vitro fertilization.