Structure and histochemistry of human extraocular muscle

Human extraocular muscles are arranged in overlapping, concentric zones of fascicles separated by a relatively thick layer of perimysial connective tissue. Three morphologically distinct fiber types are evident: coarse and granular fibers are similar to the type I and II fibers found in limb skeletal muscle, but fine fibers are unique to EOM. The histochemical staining pattern of the three fiber types is reflected in the fine structural features of the fibers, particularly for the size and distribution of mitochondria. EOM fibers have a rich endowment of mitochondria compared with limb skeletal fibers. Limb muscle biopsy combined with histochemistry has proven to be a useful tool in differentiating neurogenic and myopathic disorders and in diagnosing most neuromuscular disorders. In contrast, EOM biopsy and histochemistry have not yet proven to be useful in distinguishing myopathic from neurogenic disorders. Extraocular muscles normally have many of the features associated with myopathies of limb muscle including an increase in endomysial connective tissue, variation in fiber size, and the presence of both central nuclei and ragged-red fibers. Biopsies from patients with chronic progressive external ophthalmoplegia display an increased occurrence of these same features, but never the type grouping or group atrophy associated with neurogenic disorders of limb muscle. A further understanding of the pathogenesis of disorders of ocular motility may have to await the application of specific antibodies which readily identify proteins associated with specific myopathies or denervated fibers.