Changes in muscle fiber size and in the composition of myosin heavy chain isoforms of rabbit extraocular rectus muscle following recession surgery

Purpose: To assess the changes in the size of muscle fibers and the composition of myosin heavy chain (MyHC) isoforms in the global layer (GL) and the orbital layer (OL) of rabbit rectus extraocular muscle (EOM) after recession. Methods: The right superior rectus muscles of two rabbits were harvested at 3 days or 1, 2, or 4 weeks after recession (eight rabbits in total). At each time point, one muscle was used for measuring the cross-sectional area of the muscle fibers and the other for identifying the composition of MyHC. The right superior rectus muscles of three additional naïve rabbits were used as controls. Results: The mean cross-sectional area of the OL fibers did not change significantly. However, that of the GL fibers significantly decreased at 3 days (P < 0.001) and 1 week (P = 0.024) postoperatively, and increased thereafter to reach the control levels at 2 and 4 weeks postoperatively. Three days after surgery, the total MyHC content and the proportion of type IIb MyHC (MyHCIIb) plus EOM-specific MyHC (MyHCeom) decreased and remained at its lower level for 4 weeks. Conclusions: Transient atrophy and regeneration were observed only in the GL, and the changes in the MyHCIIb plus MyHCeom appeared to be related to these changes.