Analyses of the mean cross-sectional areas of the myofibers that expressed the slow twitch MyHC were performed. The slow-twitch, MyHC-positive myofibers in the midregion of the superior rectus muscles 1 week after three sequential injections of BDNF showed a significant increase in mean cross-sectional area in the global layer myofibers only, with a mean of 464.7 ± 47.9 μm
2 in the control muscles, 616.5 ± 61.45 μm
2 in the superior rectus muscles contralateral to the treated muscles, and 949.9 ± 32.06 μm
2 in the BDNF-treated muscles (
Fig. 4A–C), a 104% increase in size compared with naïve controls and a 54.1% increase in size compared with the contralateral, untreated muscles. In the sections toward the tendon end, the mean cross-sectional areas of both the orbital and global myofibers positive for the slow-twitch MyHC isoform were significantly increased in the BDNF-treated superior rectus muscles compared with both the control and contralateral superior rectus muscles. For the orbital layer, mean cross-sectional areas of control myofibers positive for the slow-twitch MyHC isoform were 406.1 ± 103.7 μm
2 and the contralateral muscle fibers were 421.1 ± 69.49 μm
2 compared with a mean of 812.0 ± 67.67 μm
2 for the BDNF treated muscles, an increase of 99% and 92.8%, respectively. For the global layer, the mean cross-sectional areas of the control slow-twitch, MyHC-positive myofibers were 493.9 ± 76.96 μm
2 and 536.3 ± 60.27 μm
2 for the contralateral muscles compared with 883.1 ± 74.79 μm
2 in the BDNF-treated muscles, a 78.8% and a 64.7% increase, respectively. When the percentage of slow-twitch, MyHC-positive myofibers was examined in both the middle and tendon regions, only the orbital layer myofibers were significantly different. In the midregion, the percent of slow twitch myofibers in the control muscles was 33.1 ± 3.68% and 27.3 ± 2.9% positive in the contralateral superior rectus muscles, compared with 48.32 ± 4.1% in BDNF-treated muscles in the midregion, a 45.9% and 77% increase, respectively. In the tendon region, the orbital myofibers in control muscles were 46.69 ± 12.5% positive and 57.1% ± 7.8% positive in the contralateral muscles compared with 88.11 ± 3.8% in BDNF-treated muscles (
Figs. 4A,
4B,
4D), which represented an 88.7% and 54% increase after BDNF treatment, respectively.