Development of the Pattern of Innervation of the Chick Limb

SYNOPSIS. There is a consistent repeatable pattern of innervation of the vertebrate limb by sensory and motor axons from the spinal cord. We have been looking at the factors which control the generation of this pattern in the developing chick limb. The gross pattern of nerves in the limb is controlled by the limb tissue and arises as a result of the axon's response to its local environment. Each segmental root innervates a characteristic territory in the limb. Each muscle is innervated by a characteristically located motoneurone pool in the cord. Three models explaining the generation of these patterns are presented: active selection, selective cell death, and passive deployment. A review of recent experiments leads us to conclude that there is evidence for both passive deployment and local, short range, active selection.