Netrin-1 in stage 15–18 spinal cord. Netrin-1 IR is concentrated along the outer borders of the neuroepithelium before and during the growth of commissural fibers to the floor plate (ventral–midline region at the bottom). The floor plate also begins to display elevated netrin-1 IR after the crossing of the first few fibers. A–H, Transverse sections of stage 15 cord from caudal to rostral. I–P, Transverse sections of stage 18 cord from caudal to rostral. A,C, E, G, I,K, M, O, Processed with N1D. B, D, F,H, J, L, N,P, Adjacent sections processed with the β-tubulin antibody. Arrowheads designate early differentiating neurons before commissural fiber initiation (B) and commissural fibers growing ventrally to the floor plate (D, J) or crossing the floor plate (F, L). Scale bar: A,B, 30 μm; C–P, 60 μm.