The growth of segmental nerves from the brachial myotomes into the proximal muscles of the chick forelimb during development.

A study has been made of the growth of segmental nerves 13 to 16 (SN13 to SN16) into the chick limb bud, from the time when they have just reached the ends to the brachial myotomes (stage 21: Hamburger and Hamilton, '51), until they enter the newly formed ventral (stage 24) and dorsal (stage 25) pre-muscle cell masses in the limb bud. At stage 22 axon bundles of SN13 to SN16 have grown off the ends of their respective myotomes, and converge towards the most densely packed mesenchyme in the limb bud at segmental level 15. As a consequence, the first axon bundles of SN14 and SN16 have almost joined those of SN15, whereas the further removed SN13 axon bundles have not yet reached the level of SN15. By stage 23 the first axon bundles from SN14 to SN16 have joined at segmental level 15 to form a nerve which grows toward the ventral pre-muscle cell mass. At stage 24 axon bundles from SN13 have joined those from SN14 to SN16 to form the brachialis longus inferior nerve, which enters the densest region of the ventral pre-muscle. Other axons from SN13 to SN15 grow along the pathways provided by the early arriving axon bundles towards the ventral pre-muscle, but diverge from those at segmental level 14 to grow to the dorsal pre-muscle. By stage 25 axon bundles from SN13 to SN15 have joined to form the brachialis longus superior nerve which enters the densest region of the dorsal pre-muscle. At stage 26 a plexus has formed due to this pattern of growth of the segmental nerves between stages 22 and 25. It is suggested that pre-muscle cells synthesize a nerve growth factor which directs the growth of axons into the limb bud.