Excessive expression and activity of cathepsin B in sheep cumulus cells compromises oocyte developmental competence

•The active form of cathepsin B in sheep cumulus oocyte complexes is predominantly located in cumulus cells instead of oocytes.•Activity and expression of Cathepsin B and apoptosis rate in cumulus cells are correlated with the developmental competence of sheep oocytes.•The addition of E-64 to the IVM medium for sheep oocytes can improve the developmental competence of oocytes and decrease activity and expression of cathepsin B and apoptosis rate in cumulus cells