Histochemical staining for the activity of mitochondrial succinate dehydrogenase (complex II) and cytochrome c oxidase (complex IV) in ovaries was performed as previously described, with modifications (Wang et al., 2019 (link)). Ovaries were dissected in PBS and incubated in cytochrome c oxidase assay medium (100 µM cytochrome c, 4 mM diaminobenzidine tetrahydrochloride, and 20 µg/ml catalase in 0.2 M phosphate buffer, pH 7.0) at room temperature for 30 min. The ovaries were then washed in PBS three times and incubated in succinate dehydrogenase assay medium (130 mM sodium succinate, 200 µM phenazine methosulphate, 1 mM sodium azide, and 1.5 mM nitroblue tetrazolium in 0.2 M phosphate buffer, pH 7.0) at room temperature for 10 min. After washing in PBS three times, ovaries were dehydrated in a graded ethanol series (70%, 95%, 2 × 100%), cleared in Histoclear (National Diagnostics) for 15 min, and mounted in dibutyl phthalate in xylene (VWR) overnight. Bright-field images were collected on a Zeiss Axio Observer Z1 microscope (Objective C-Apochromat 40×/1.1 W Corr, Leica LAS EZ software, Leica EC4 Digital Camera, water as immersion medium).