Motoneuron axon guidance: development of specific projections to two muscles in the embryonic chick limb.

In the embryonic chick limb, multiple mechanisms are believed to be responsible for a precise guidance of motoneuron axons to their appropriate targets from the earliest stages of axon outgrowth. In this report, I discuss one of these guidance mechanisms; a mechanism which appears to depend on a prelabeling of motoneurons prior to axon outgrowth. By focusing on the development of axonal projections to two specific muscles, I review the evidence that motoneurons destined for each muscle actively recognize environmental cues, and discuss recent experiments designed to identify the source of these cues.