N-CAM is expressed in mature extraocular muscles in a pattern conserved among three species

Extraocular muscles express a number of characteristics not normally seen in mature skeletal muscle. The expression of neural cell adhesion molecule, N-CAM, was assessed immunohistochemically in the extraocular muscles of rabbits, monkeys, and humans to examine the hypothesis that there is greater complexity to extraocular muscle fiber types than the traditional description. The presence of N-CAM in normal mature extraocular muscles may play a role in the etiology and pathobiology of Graves' ophthalmopathy and/or other ocular myopathies.

Extraocular muscles from rabbits, monkeys, and humans were obtained and processed for immunohistochemical localization of the more mature, less sialylated form of N-CAM in histologic sections and in individual, intact myofibers prepared by in vitro digestion techniques. N-CAM localization was compared to the immunohistochemical localization of fast, neonatal, and developmental myosin heavy chain isoforms on serially sectioned myofibers. Additional sections were examined for the presence of the early appearing polysialylated form of N-CAM (polyNCAM).

N-CAM-positive myofibers were found in all six extraocular muscles of the three species examined. The rectus muscles showed an increased proportion of N-CAM-positive fibers compared to the oblique muscles. Individual myofibers in the rectus muscles were N-CAM positive extracellularly and intracellularly; in the oblique muscles, only extracellular staining was evident. Intact dissociated rectus muscle fibers were found that were N-CAM positive on their entire fiber surface. N-CAM was not regionally localized on these isolated fibers. Although polyNCAM was present in the extraocular muscles, only a few small diameter cells were positive for this form of N-CAM.

Populations of mature myofibers from all six extraocular muscles express N-CAM homogeneously on their cell surfaces. The orbital layer was almost completely N-CAM positive. When the staining pattern of the global layer of the rectus and oblique muscles is compared, not only did the rectus muscles have greater numbers of N-CAM-positive fibers, all rectus muscles showed both extracellular and intracellular N-CAM expression. This was in contrast to the oblique muscles, in which the number of N-CAM-positive fibers was reduced but N-CAM was only present extracellularly. Because of the exclusive involvement of the extraocular muscles in Graves' ophthalmopathy, the absence of N-CAM expression on other mature skeletal muscles, and the increased expression of N-CAM in those muscles (rectus) more affected in this disease, the authors postulate that N-CAM expression may at least be a marker for susceptibility to, if not play a clinically significant role in, the etiology and pathobiology of this disease. Complex patterns of association were demonstrated between N-CAM expression and myosin types, indicating that N-CAM-positive myofibers do not conform to a specific fiber type as traditionally defined.