Histological assessment of nerve regeneration at four weeks after crush injury. At four weeks after crush injury, specimens of the crushed sciatic nerve were taken at 5 mm distal to the injury site from both sem (p38+/−) and wt (p38+/+) mice. As controls, tissue specimens were also taken from the contralateral uninjured sciatic nerve. (a) A 1 μm slice section was stained with toluidine blue and examined under a light microscope. Scale bar indicates 50 μm. (b) Axon diameter distribution of the sciatic nerve was evaluated using a set of three non-overlapping microscopic fields.