Denervation of primate extraocular muscle. A unique pattern of structural alterations

Extraocular muscles differ from most other skeletal muscles in terms of constituent fiber types and innervation pattern. The rules that govern fiber responses to various experimental interventions for most skeletal muscles, therefore, may not strictly apply to the extraocular muscles. In this study, denervation of the extraocular muscles of Cynomolgous monkeys, Macaca fascicularis, was accomplished by intracranial transection of the oculomotor nerve. Survival times of 3-167 days were allowed, and muscles were processed for analysis by light and electron microscopy. Short-term alterations involved all muscle fiber types and included retraction of neuromuscular junctions, mild myofibril disruption and inflammatory cell infiltration. Long-term morphopathological changes were most apparent in the orbital singly innervated fiber type and its global layer counterpart. These alterations consisted of dispersion of the mitochondrial aggregates which characterize this fiber type. Only occasional fibers (all types) exhibited severe vacuolar atrophy or myofilament breakdown despite the occurrence of only limited reinnervation. When extensive reinnervation did occur, the characteristic layered organization of the extraocular muscles was preserved and fiber type grouping was not apparent. Taken together, these findings indicate that the extraocular muscles exhibit a resilience to denervation beyond that observed for other skeletal musculature.