Axonal staining and measurement were performed as previously described [20 (link), 23 (link)]. Briefly, monoclonal antibodies against phosphorylated neurofilament heavy protein (SMI31, 1:1000, Covance, Battle Creek, MI, USA) or class III beta-tubulin (TUJ1, 1:1000, Biolegend, San Diego, CA, USA) were used. For postnatal DRG neurons, the lengths of the 15 longest axons in each chamber were measured using a microscopic computer imaging device (MCID) system. The axonal length was recorded for 3 days from DIV3 to DIV5. For adult DRG neurons, one time point axonal length was measured on DIV3 according to a published protocol [28 (link)]. Immunostaining for DRG tissues was performed according to a published protocol [24 (link)]. Briefly, L3–L6 DRGs were isolated, fixed in 4% paraformaldehyde, and embedded in paraffin. The slices were cut in 6 μm thickness. The following primary antibodies were used: rabbit anti-FOXP2 (1:50, Abcam) and goat anti-VAT1 (1:50, Santa Cruz). Cellular nuclei were stained with 4′,6-diamidino-2-phenylindole (DAPI) (1:10000, Thermo Fisher Scientific). A polyclonal antibody against protein gene product 9.5 (PGP 9.5, 1:1000; Millipore) was used to detect intraepidermal nerve fibers in plantar skin. The nerve fiber densities were calculated according to a published protocol [29 (link)].