The endothelial nitric oxide synthase/nitric oxide system is involved in the defective quality of bovine oocytes from low mid-antral follicle count ovaries

In a previous survey concerning cows of reproductive age, we demonstrated that oocytes isolated from ovaries with < 10 medium antral follicles of 2 to 6 mm in diameter (Low ovaries, Lo) show a lower developmental competence than oocytes collected from ovaries with > 10 medium antral follicles (High ovaries, Hi). The aim of the present study was to evaluate whether a defective endothelial nitric oxide synthase/nitric oxide (eNOS/NO) system and vasculature in healthy medium antral follicles is likely to reduce oocyte competence from Lo ovaries. Thus, experiments were conducted to: (1) immunolocalize eNOS protein during folliculogenesis; (2) quantify eNOS protein/vasculature in the follicle wall; and (3) verify if NO donor, S-nitroso acetyl penicillamine (SNAP) administration during in vitro maturation affects developmental competence of oocytes isolated from Lo ovaries. Endothelial-NOS protein was detected in granulosa and theca cells, as well as in blood vessels from primordial to antral follicles. Quantitative analysis indicated that in medium antral follicles from Lo ovaries, eNOS protein expression and vasculature were reduced (P < 0.05). The addition of SNAP improved blastocyst and hatching rates of oocytes from Lo ovaries, promoting a percentage similar to oocytes from Hi ovaries, and reduced the percentage of apoptotic nuclei in in vitro-produced blastocysts (P < 0.05). Results from our study suggest that in bovine ovaries with low mid antral follicle number, a defective eNOS/NO system is related to a reduced follicle vasculature and may impact oocyte quality, thus inducing a premature decline of fertility.