Severely Atrophic Muscle Fibers with Nuclear Clumps Survive many Years in Permanently Denervated Human Muscle

After complete lumbar-ischiatic spinal cord injury (SCI) the lower motor neuron (LMN) denervated human muscle fibers lose completely the myofibrillar apparatus and the coil distribution of myonuclei that are relocated in groups (nuclear clumps) in the center of these “severely atrophic” muscle fibers. In our cohort of patients, the “severely atrophic” myofibers are frequent in muscle biopsies harvested three to six years after SCI. Up to two years of LMN denervation the muscle fibers with nuclear clumps are 2 ± 5% (mean ± SD) of the total muscle fibers. The percentage increases to 27 ± 9% between three and six years of denervation (p< 0.001), and then abruptly decrease from the 6th year of LMN denervation onward, when fibrosis takes over to neurogenic muscle atrophy. Immunohistochemical analyses show that nuclear grouping occurs in both fast and slow muscle fibers. These results show that human muscle fibers survive permanent denervation much longer than generally accepted.