DEVELOPMENT OF THE SPINAL NERVES IN THE MOUSE WITH SPECIAL REFERENCE TO INNERVATION OF THE AXIAL MUSCULATURE

Development of the mouse spinal nerves was studied. On E11 (11th day o f gestation), the primitive spinal nerve fascicle extended ventrally i n the anterior half of the sclerotome. Spinal nerves in the forelimb r egion united with each other to form the primitive brachial plexus. Th eir terminal segment was covered by a peculiar cell mass. On E12, five primary branches developed along the primitive spinal nerve trunk. Th e ramus dorsalis was originally a cutaneous nerve, supplying two serie s of branches to the skin of the back. The medial series was derived f rom the dorsal ramus of C2-C8, and the lateral series from C8 and the more caudal dorsal rami. Nerves of the former series took the presegme ntal course through the intermyotomic space, while those of the latter the postsegmental course. The ramus cutaneus lateralis was a nerve th at took the presegmental course to become cutaneous. The ramus interco stalis externus was a muscle branch whose distribution was restricted within the segment. The ramus anterior was a muscle branch from the en d of the primitive spinal nerve trunk. The ramus visceralis connected a thoracic nerve with the para-aortic sympathetic cell cord. On E13-16 the ramus anterior secondarily gave off a cutaneous branch (ramus cut aneus anterior). The ramus intercostalis externus extended ventrally d eep to the intercostalis externus muscle, crossing just caudal to the ramus cutaneus lateralis that secondarily gave off branches to the obl iquus externus abdominis muscle.