Abstract
Motoneurons grow into the chick hindlimb via consistent pathways, within which they make specific decisions leading to their correct targets. To determine which axonal guidance features dictate the position of the pathways and to examine the distribution of specific cues, we totally or partially ablated the early hindlimb bud and determined how the subsequent pattern of nerve outgrowth related to the distribution of tissue remnants. Our results suggest that local elements determine the gross anatomical pattern of outgrowth. First, determinants of individual pathways could be selectively removed without altering the pattern in other regions. Second, neurites were restricted to the plexus region at the base of the leg (within which, for unknown reasons, they proceeded posteriorly) unless distal permissive pathways or nearby target remnants were present. Finally, we found that the central region of the pelvic girdle, adjacent to the plexus region, determines the position where the major nerve trunks enter the leg. When gaps were introduced in this region of the girdle, nerves traversed the gaps and directly innervated adjacent muscle. The developing girdle is probably a nonpermissive environment for axon elongation, and axons enter the leg only where it is locally absent. Our results also support the concept that the specific cues that neurites use to reach their appropriate muscles are local. We find that neurites could make correct and specific decisions in the plexus region in the absence of all tissues distal to the pelvic girdle. This shows that the cues for these decisions are independent of the target and must reside in the local mesenchyme. In addition, when muscle remnants were present they were correctly innervated.