Fig. 3. Development of central fibers in B/N mutant mice. Retrograde labeling of sensory neurons and motoneurons was performed using the lipid-soluble fluorescent tracer DiI in axial muscles (A-C,G) or dorsal root ganglia (D-F,H) at the lumbar level (see Materials and Methods). Micrographs of sections, through the lumbar spinal cord of E 15.5 embryos after DiI tracing show a normal complement of Ia fibers in control mice (arrowheads, A,D), which is absent in Nt3-/- mutant mice (B,E). Sections through the spinal cord of a B/N mouse reveals the presence of a some fibers projecting ventrally in the spinal cord (arrowheads, C,F). Labeling performed in the axial muscles (G) or in the DRG (H) of B/N mice at stage P0 shows the presence of only residual central fibers (arrows). Immunohistochemical analysis of P0 spinal cords from wild-type (I), Nt3-/- (J) or B/N (K) animals using an antibody against parvalbumin (PV), a marker specific for proprioception fibers, shows presence of PV-specific staining in only the wild-type spinal cord (arrows, I).
