A, Normal muscle biopsy specimen (except for one small angular fiber). Note polygonal shape of myofibrils, normal distribution of type I and type II fibers, and normal connective tissue of endomysium (nicotinamide adenine dinucleotide-tetrazolium reductase [NADH-TR] stain, ×125). B, Muscular dystrophy. Fibers are more rounded, some fibers have internalized nuclei, and others are atrophic. One muscle fiber is necrotic and is undergoing phagocytosis. Connective tissue between fibers is increased (hematoxylin & eosin, ×295). C, Chronic neurogenic atrophy (juvenile spinal muscular atrophy). Notice grouping of fibers of same type and some atrophic angular fibers. Fat is increased between muscle fascicles (NADH-TR stain, ×125). D, Central core disease. Note pale areas of central cores in muscle fibers characteristic of this disease (NADH-TR stain ×200).