Abstract: Objective To evaluate the quantity of muscle satellite cells in medial rectus of the patients with concomitant exotropia. Design Case-control study. Participants 18 pieces of medial rectus from the patients with concomitant exotropia and 14 pieces of healthy medial rectus collected from corneal transplant doners. Methods Histopathological observation was performed after the samples were sliced along axis of abscissa and stained by HE and M-cadherin immunohistochemistry. Positive results was analyzed quantitively according to double-blind comparision, then the relashionships between the number of muscle satellite cells and duration of diseases, operation age, or quantity of muscle cells. Main Outcome Measures The amount of muscle cells on transection of medial rectus in strabismus group and nomal group; the amount of muscle satellite cells. Results The amounts of muscle cells on transection of medial rectus in strabismus group（120.50±57.26） were significantly lower than in normal medial rectus (180.36±8.78) （P=0.000） and significantly correlated negatively with duration of disease （r=-0.919, P=0.000）. The amount of muscle statellite cells in medial rectus in strabismus group (8.25±5.85) were significantly lower than in normal group (42.50±5.12) （P=0.000） and significantly correlated with the amounts of muscle cells（r=0.849，P=0.000）. The amount of muscle satellite cells was significantly correlated inversely with duration of disease（r=-0.648, P=0.005） and their ages being operated（r=-0.600，P=0.01）. The amount of muscle statellite cells on medial rectus from intermittent exotropia patients was much more than that from stable exotropia ones(P=0.000). Conclusion Progressive atrophy of medial rectus in concomitant exotropia may be relevant to deficiency of the muscle satellite cells.

Key words: concomitant exotropia, medial rectus, M-cadherin, muscle satellite cell, immunohistochemistry