Background: In vitro maturation is a good method to decrease cancer risk of superovulation by gonadotropin hormones. A paracrine effect of oocyte secretions on oocyte developmental competence is under investigation. Apart from oocyte maturation, ovulation in vivo requires a precise control of extracellular matrix modification. Cumulus cells secrete hyaluronan to form a muco-elastic extracellular matirx with proteins derived from the serum and the follicle (cumulus expansion). This matrix structure is of importance for oocyte extrusion from the follicle and for pick-up by the fimbria. In addition, a function of selective barrier for sperm has also been reported for this matrix structure. Oocyte secreted factors (OSFs) may be involved in cumulus expansion of oocytes. This study was conducted to identify if the increasing amount of native OSFs improve the rate of cumulus expansion of bovine and ovine oocytes. Materials and Methods: Collected ovaries from the local abattoir were transported to the laboratory in PBS at 30-35°C. The cumulus - oocyte complexes (COCs) of follicles were recovered by aspiration. Some COCs were denuded using hyaluronidase and vortexing. Then COCs surrounded with at least three layers of cumulus cells were co-cultured with denuded oocytes (Dos) in a 50 μl droplet of oocyte cultue medium (OCM). The selected COCs were randomly co-cultured in four groups (COCs cultured alone, with Dos in 1:1, 1:3 and 1:6 ratios. After an incubation period (24 hours), cumulus expansion was assessad. Results: In bovine COCs, complete cumulus expansion rate was 80% (control), 75%, 72% and 76%, respectively. Conclusion: The analysis of data showed that the rate of complete expanded cumulus in treatment groups was not significant when compared with control group. Thus, although these results show that increasing amount of native OSFs does not improve cumulus expansion of bovine COCs, the other aspects of oocyte maturation at molecular level as well as fertilization and preimplantation embryogenes should be more investigated for better understanding the effects of OSFs in cattle

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