Increased frequency of activated satellite cells in overacting inferior oblique muscles from humans.

PURPOSE Strabismus is an oculomotor disorder in which there is a misalignment of the visual axes of the eyes. Inferior oblique muscle (IOM) overaction is a common finding in comitant horizontal strabismus, but its origin is unclear. Recent studies have demonstrated that myogenic satellite cells (SCs) are still activated in adult extraocular muscles, with continuous myonuclear addition in normal uninjured muscles. The objective of this study was to determine whether there are differences in the processes of activation and proliferation of SCs in IOMs of patients with strabismus and IOM overaction and in patients with no history of strabismus. METHODS Cross sections of IOMs from strabismic and control groups were analyzed immunohistochemically for the presence of MyoD1 and myogenin, specific markers of activated SCs, and for c-Met, which is expressed in quiescent, activated, and proliferating SCs. RESULTS In overacting IOMs of 26 patients in the strabismic group and 10 patients in the control group, 28.8% and 3.0% of the myofibers, respectively, were associated with MyoD1-positive SC. The frequency of myogenin-positive SC was 30.8% in the strabismic group and 3.6% in the control group, and the frequency of presumptive SCs immunostained for c-Met was 33.6% in the strabismic group and 34.1% in the control group. CONCLUSIONS The presence of an increased number of activated SCs in overacting IOMs of the strabismic group in contrast to the frequency in the control group resembles the findings detected in developing, regenerating, or hypertrophic muscle tissue. High levels of MyoD1- and myogenin-positive SC in overacting IOMs support the hypothesis that these cells may be involved in alterations in IOM structure correlated with the overaction observed clinically.