Development of the dorsal ramus of the spinal nerve in the mouse embryo: Involvement of semaphorin 3A in dorsal muscle innervation

The spinal nerve, which is composed of dorsal root ganglion (DRG) sensory axons and spinal motor axons, forms the dorsal ramus projecting to the dorsal musculature. By using the free-floating immunohistochemistry method, we closely examined the spatiotemporal pattern of the formation of the dorsal ramus and the relationship between its projection to the myotome/dorsal musculature and semaphorin 3A (Sema3A), which is an axonal guidance molecule. In embryonic day (E) 10.5-E11.5 wild-type mouse embryos, we clearly showed the existence of a waiting period for the dorsal ramus projection to the myotome. In contrast, in E10.5-E11.5 Sema3A-deficient embryos, the dorsal ramus fibers projected beyond the edge of the myotome without exhibiting the waiting period for projection. These results strongly suggest that the delayed innervation by dorsal ramus fibers may be caused by Sema3A-induced axon repulsion derived from the myotome. Next, by performing culture experiments, we confirmed that E12.5 mouse axons responded to Sema3A-induced repulsion. Together, our results imply that Sema3A may play a key role in the proper development of the dorsal ramus projection.