We investigated the ability of reactive oxygen species (ROS), such as hydrogen peroxide (H2O2), hydroxyl radical (OH),
and hypochlorous acid (HOCl), to overcome the defensive capacity of cumulus cells and elucidate the mechanism through
which ROS differentially deteriorate oocyte quality. Metaphase II mouse oocytes with (n ¼ 1634) and without cumulus cells
(n ¼ 1633) were treated with increasing concentration of ROS, and the deterioration in oocyte quality was assessed by the
changes in the microtubule morphology and chromosomal alignment. Oocyte and cumulus cell viability and cumulus cell
number were assessed by indirect immunofluorescence, staining of gap junction protein, and trypan blue staining. The
treated oocytes showed decreased quality as a function of increasing concentrations of ROS when compared to controls.
Cumulus cells show protection against H2O2 and OH insult at lower concentrations, but this protection was lost at higher
concentrations (>50 mmol/L). At higher H2O2 concentrations, treatment dramatically influenced the cumulus cell number
and viability with resulting reduction in the antioxidant capacity making the oocyte more susceptible to oxidative damage.
However, cumulus cells offered no significant protection against HOCl at any concentration used. In all circumstances in
which cumulus cells did not offer protection to the oocyte, both cumulus cell number and viability were decreased.
Therefore, the deterioration in oocyte quality may be caused by one or more of the following: a decrease in the antioxidant
machinery by the loss of cumulus cells, the lack of scavengers for specific ROS, and/or the ability of the ROS to overcome