Figures 222 (upper) and 223 (lower) from Chapter 7 (Mitochondria) of 'The Cell, 2nd Ed.' by Don W. Fawcett M.D. Mitochondrial matrix granules are located within the space bounded by the inner mitochondrial membrane, and are apparent as spherical, dark inclusions in EM preparations because they are osmiophilic. Upper, mitochondria from cat ventricuar cardiac muscle with numberous matrix granules; lower, mitochondrion of bat pancreas. A PDF copy of the accompanying chapter is available on the ASCB’s BioEDUCATE website.