Immunoreactivity patterns of melatonin receptor MT1, MT2 and NQO2 in bovine oocyte. MT1, MT2 melatonin receptors and NQO2 are expressed by the matured oocyte. The upper-left panel shows localized intense MT1 staining suggesting that MT1 is expressed by polar bodies. The upper-right panel shows a cluster-like distribution of MT2 throughout the matured oocyte. The bottom-left panel shows a diffuse expression of NQO2 enzyme, and the bottom-right shows the negative control. (Scale bar, 30 μm).