Proximal tissues and patterned neurite outgrowth at the lumbosacral level of the chick embryo: Deletion of the dermamyotome

The role of the dermamyotome (the dorsal portion of the somite which gives rise to muscles and dermis) in the development of patterned axon outgrowth was examined under conditions where limb development was substantially undisturbed. One or more chick dermamyotomes were removed before or during early neurite outgrowth and subsequent development was examined. Several developmental processes suspected to depend on the dermamyotome were not altered by its removal: (1) Neural crest cells that form sensory ganglia migrated and condensed in their normal segmental pattern. (2) The distal progression, dorsal-ventral organization, and segmentation of spinal nerves were unaltered. (3) Motoneuron pathway selection and projection patterns in the limb were normal in all respects. The most interesting finding was that the formation of the dorsal ramus is dependent on the nearby dermamyotome which provides the targets for this nerve. When a single or two adjacent dermamyotomes were removed, the metameric epaxial muscles derived from each dermamyotome were absent and the dorsal ramus extended into epaxial muscle in the closest adjacent segment. However, when dermamyotomes in both adjacent segments had also been removed or substantially reduced, the dorsal ramus did not form. These results strongly suggest that the target provides a chemotactic signal for proper outgrowth of dorsal ramus axons.