For electron microscopy, spinal cords from five rats from each treatment group were processed into small 3 mm3 blocks that surrounded the injury epicenter and fixed for 1 hr in a mixture of glutaraldehyde (1.5%) and paraformaldehyde (3%), followed by washing three times in 0.1 M sodium cacodylate and 3 mM CaCl2. Samples were then postfixed in potassium ferrocyanide (0.8%) and osmium tetroxide (1%) for 1 hr followed by 3× washes in 0.1 M sodium cacodylate and 3 mM CaCl2 (All from Sigma). Ensuing a brief dH2O rinse, samples were embedded in Eponate 12 (Pelco, Clovis, CA) and cured at 60℃ for 2 days. Spinal cord sections (80 nm in thickness) corresponding to the site of the lesion were cut on a Reichert Ultracut E with a Diatome diamond knife, collected on formvar-coated 1 × 2 mm2 copper grids, and stained with uranyl acetate followed by lead citrate. Sections were examined on a Hitachi 7600 transmission electron microscope operating at 80 kV. The myelin index (MI) was measured by means of the ratio of axon diameter to axon diameter plus its myelin sheath (Wrathall et al., 1998; Pang et al., 2012).