Role of eNOS in acquisition of oocyte developmental competence in low antral follicle count bovine ovaries

Numerous angiogenic factors, including endothelial Nitric Oxide Synthase (eNOS), are present in ovarian follicles of several species. Their role is to enhance the recruitment of vascular supply, to keep follicles healthy and to improve oocyte quality. Recently, it has been demonstrated that in single ovulating species, like humans and cattle, the amount of antral follicles is positively associated to the number of morphologically healthy oocytes and follicles in ovaries. In cow, we previously established that ovaries with low antral follicle count (<10) generate oocytes with poor developmental competence. Aim of this work is to test the hypothesis that follicles depletion and reduced oocyte quality in low antral follicle count ovaries can be related to a reduced expression of eNOS in bovine ovary. RT-PCR analysis shows that eNOS is present in all the compartments of bovine antral follicles. Immunohistochemistry reveals that the protein is produced during all the stages of folliculogenesis. However, in healthy medium antral follicles, eNOS concentration, as quantified by image analysis, is significantly lower (P<0.05, Student’s t-test) in ovaries with low antral follicle count. This different concentration suggests that eNOS may be involved in the faulty developmental competence of oocyte isolated from low antral follicle count ovaries. To test this hypothesis, S-nitroso acetyl penicillamine, a Nitric Oxide donor, was added during oocyte in vitro maturation. However this molecule does lack in significantly improving embryo developmental capability. Advanced studies are in progress to assess the biological role of eNOS as angiogenic factor during the acquisition of oocyte developmental competence in low antral follicle count ovaries. Low follicle vascularity may reduce exchange of nutrients, gonadotropins and growth factors and this can account for a decrease in oocyte quality. Our findings further support the use of cow as animal model to study human fertility.