Fig. 8.

Netrin-1 in tissues adjacent to the spinal cord. Elevated levels of netrin-1 IR were detected in tissues adjacent to the spinal cord before and during the period of pioneering commissural fiber growth to the floor plate. This was true of the notochord (located immediately ventral to the spinal cord) only before and during the early stages of commissural fiber growth, such as that in the very caudal cord at stage 15 (A) and stage 20 (Fig.4G). The early, temporally limited presence of netrin-1 IR in the notochord is consistent with the similarly restricted expression of netrin-1 mRNA in this structure (Kennedy et al., 1994). Elevated netrin-1 IR was also observed during the period of pioneering commissural fiber growth to the floor plate (e.g., stage 15, B and C, and stage 18,D) in the sclerotome located immediately lateral to the cord (with a potential contribution by migrating neural crest cells) and, to a lesser but more variable extent, in the more laterally located dermamyotome. Note that the paraxial tissues frequently became partially dissociated from the cord during harvesting, sectioning, mounting, or processing. A, Same section as in Figure3A. B, Section of Figure3E. D, Section of Figure3K. All of the sections were processed with N1D. Scale bar: A, 30 μm; B–D, 60 μm.