Development of the dorsal ramus of the spinal nerve in the chick embryo: A close relationship between development and expression of guidance cues

The spinal nerve, which is composed of dorsal root ganglion (DRG) axons and spinal motor axons, divides into ventral and dorsal rami. Although the development of the ventral ramus has been examined in considerable detail, that of the dorsal ramus has not. Therefore, we first examined the spatial-temporal pattern of the dorsal ramus formation in the chick embryo, with special reference to the projection to the dermamyotome and its derivatives. Next, we focused on two guidance molecules, chick semaphorin 3A (SEMA3A) and fibroblast growth factor 8 (FGF8), because these are the best candidates as molecules for controlling the dorsal ramus formation. Using in situ hybridization and immunohistochemistry methods, we clearly showed a close relationship between the spatial-temporal expression of SEMA3A/FGF8 and the projection of dorsal ramus fibers to the dorsal muscles. We further examined the axonal response of motor and DRG neurons to SEMA3A and FGF8. We showed that motor axons responded to both SEMA3A-induced repulsion and FGF8-induced attraction. On the other hand, DRG axons responded to SEMA3A-induced repulsion but not to FGF8-induced attraction. These findings suggest that FGF8-induced attraction may guide early motor axons beneath the myotome and that SEMA3A-induced repulsion may prevent these early motor axons from entering the myotome. Our results also imply that the loss of SEMA3A expression in the dorsal muscles may lead to the gross projection of the dorsal ramus fibers into the dorsal muscles. Together, SEMA3A and FGF8 may contribute to the proper formation of the dorsal ramus.