The paracrine effects of oocyte secretions on the developmental competence of oocyte are still under investigation. Oocyte-secreted factors (OSFs) seem to be involved in the maturation of bovine oocytes. Therefore, this study was conducted to investigate the effects of different concentrations of endogenous OSFs on the in vitro maturation of bovine cumulus-oocyte complexes (COCs). For this purpose, COCs (n=1211) were aspirated from antral follicles of ovaries collected from a local abattoir. Only oocytes surrounded by complete and compact layers of cumulus cells with homogenous ooplasm were defined as well-qualified COCs (n=353), whereas other COCs (n=858) with low quality were treated to obtain denuded oocytes (DOs). The selected COCs were randomly cultured in four groups: I) COCs cultured alone as control; II) COCs co-cultured with DOs as 1:1 ratio; III) COCs co-cultured with DOs as 1:3 ratio, and IV) COCs co-cultured with DOs as 1:6 ratio. After maturation, the two variables of cumulus expansion and nuclear status were assessed. The complete cumulus expansion rate was 80% for the control group, and 75.27%, 72.63% and 76.25% for groups of II, III, and IV, respectively. The percentage of metaphase II (MII) stage oocytes was 65.71% for the control group, and 62.67%, 64.86% and 67.60%, for groups of II, III, and IV, respectively. Data analysis revealed that the rate of complete expansion of cumulus cells and the development of MII stage oocytes in experimental groups did not differ significantly as compared to the control group. In conclusion, the different concentration of native OSFs does not affect the rate of nuclear maturation and cumulus expansion of bovine COCs. However, group size of co-cultured COCs can be of important criterion for co-culturing systems.

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