Activated satellite cells in medial rectus muscles of patients with strabismus

Abstract

Purpose: The goal of this study was to determine whether the medial rectus muscles of patients with a history of medial rectus underaction or overaction show alterations in the process of satellite cell activation when compared with normal age-matched control muscles.

Methods: Medial rectus muscles were obtained with consent from adult patients undergoing surgical resection due to medial rectus underaction or overaction and were prepared for histologic examination by fixation and paraffin embedding. Control muscles were obtained from cornea donor eyes of adults who had no history of strabismus or neuromuscular disease. Cross sections were obtained and stained immunohistochemically for the presence of activated satellite cells, as identified by MyoD immunoreactivity, and the presence of the total satellite cell population, as identified by Pax7 immunoreactivity. The percentages of MyoD- and Pax7-positive satellite cells per 100 myofibers in cross section were calculated.

Results: As predicted from results in the literature, MyoD-positive satellite cells, indicative of activation, were present in both the control and resected muscles. In the underacting medial rectus muscles, the percentages of MyoD- and Pax7-positive satellite cells, based on the number of myofibers, were approximately twofold higher than the percentages in the control muscles. In the overacting medial rectus muscles, the percentage of MyoD-positive satellite cells was twofold less than in the control muscles, whereas the percentage of Pax7-positive satellite cells significantly increased compared with that in the control specimens.

Conclusions: The presence of an increased number of activated satellite cells in the resected underacting medial rectus muscles and the decreased numbers of activated satellite cells in the overacting muscles was unexpected. The upregulation in the number of MyoD-positive satellite cells in underacting muscles suggests that there is potential for successful upregulation of size in these muscles, as the cellular machinery for muscle repair and regeneration, the satellite cells, is retained and active in patients with medial rectus underaction. The decreased number of activated satellite cells in overacting MR muscle suggests that factors as yet unknown in these overacting muscles are able to affect the number of satellite cells and/or their responsiveness compared with normal age-matched control muscles. These hypotheses are currently being tested.

FIGURE 1

Adult MR muscle from patients with underacting muscles immunostained for MyoD and dystrophin. (A) Arrows: MyoD-positive cells outside the dystrophin-positive sarcolemma. Note that the sarcoplasm of almost every fiber is positive for dystrophin. This is seen in regenerating muscle fibers, but not normal adult limb skeletal muscle myofibers. (B) A centrally located myonucleus (arrow). Bar, 20 µm.

FIGURE 2

Quantification of the percentage of myofibers with an associated cell positive for MyoD (A) and Pax7 (B) from normal adult and child MR muscles and from patients with MR underaction. *Significant difference from the control.

FIGURE 3

Adult MR muscle immunostained for Pax7. (A) Adult control MR muscle. Adult MR from patients with MR (B) overaction or (C) underaction. Bar, 10 µm.

FIGURE 4

Quantification of the percentage of myofibers with associated nuclei or satellite cells positive for MyoD from normal adult MR and MR of patients with MR underaction. *Significant difference from the control. O, no MyoD+ myonuclei in the sections.

FIGURE 5

Quantification of the percentage of myofibers with an associated cell positive for MyoD (A) and Pax7 (B) from normal adult MR and MR from patients with MR overaction. *Significant difference from the control.

FIGURE 6

Mean cross-sectional areas of myofibers from control, underacting, or overacting MR muscles. No significant difference was found in mean fiber areas between the two patient groups and the control muscle fibers.