Cell culture To determine whether exogenous donor mitochondria can be taken up into neurons growing in culture, we co-cultured exogenous mitochondria extracted from the brain or muscle tissues of donor rats with neural cell cultures. Both donor mitochondria and recipient neural cell cultures were derived from male Sprague–Dawley rats (12 weeks old; Charles River Laboratories, Wilmington, MA, USA). Neural cells were isolated using the Adult Brain Dissociation kit (Miltenyi Biotec, Inc., Somerville, MA, USA). Cells were seeded at a density of 1 × 10 5 cells/cm 2 onto glass coverslips coated with poly- d -lysine (0.1 mg/mL). The endogenous (native) mitochondria of the cells were separately labeled 24 h before mitochondrial transfer with MitoTracker dye (Thermo Fisher Scientific, Waltham, MA, USA) according to the manufacturer's instructions. Briefly, the cells were suspended in a prewarmed (37 °C) staining solution containing the MitoTracker Green