Mitochondrial inner membrane potential (Δψm) in oocytes (Mitotracker Red CMXRos). (A) Small oocytes show a less proportion of red-stained mitochondria, than (B) mature eggs (around 100 μm), in which all the mitochondrial mass is red-stained pointing to an overall high membrane potential. (C) Magnification of a mature oocyte inside the acinus (from B). (D) Spawned eggs as well show an overall high mitochondrial membrane potential. Nuclei of cells surrounding the acinus are visible in blue (TO-PRO 3 nuclear dye).