Images of sagittal sections through the corpus callosum were acquired at 5,000× magnification for measurements of axon diameter and myelin thickness along with manual counting of degenerating axons, demyelinated axons, and excessive myelin figures. Analysis included 3 to 5 images per animal (at least 120 axons per image); 3 to 6 animals per condition were used. Degenerating axons were defined as axons with cytoskeletal changes (microtubule and neurofilament density extremely low or high with irregular spacing), axons filled with vesicles, or axons with swollen mitochondria (filling >50% of the axon cross-section). Demyelinated axons were manually counted based on a lack of detectable compact myelin and a diameter larger than 0.3 μm. Axons smaller than 0.3 μm were excluded from analysis as potentially unmyelinated fibers (27 (link)). An excessive myelin figure was identified as a myelin sheath that was not tightly enwrapping an axon but instead folded back on itself (i.e. redundant myelin) (13 (link), 22 (link)). These figures can be seen surrounding and extending from an intact axon or a degenerating axon or remaining without an axon. Within each counting frame, all apparently intact axons were measured to determine axon diameter and myelin thickness using Metamorph software (Molecular Devices), as previously detailed (28 (link), 29 (link)). These values were also used to calculate the g-ratio (axon diameter divided by myelinated fiber diameter) as an indicator of remyelination. Excessive myelin figures were measured along the lumen of the myelin sheaths, and that length was doubled to determine the length of each excessive myelin figure. Additional images were taken at 10,000× for illustration of pathology. Quantification included 26 mice (3 days: n = 3 sham, n = 4 TBI; 1 week: n = 5 TBI; 2 weeks: n = 5 TBI; 6 weeks: n = 4 sham, n = 5 TBI).