The acquisition of motoneuron subtype identity and motor circuit formation.

Experiments in chick embryos using classical transplantation techniques introduced by Viktor Hamburger are reviewed; these demonstrated that chick-limb innervating motoneurons become specified by extrinsic signals prior to axon outgrowth and that they selectively grow to appropriate muscles by actively responding to guidance cues within the limb. More recent experiments reveal that fast/slow and flexor/extensor subclasses of motoneurons are distinct by E4-5 and that they exhibit patterned spontaneous activity while still growing to their targets. These observations are then related to the combinatorial code of LIM transcription factor expression, which has been hypothesized to specify motoneuron subtypes.