Rules of motor innervation in chick embryos with supernumerary limbs

The positions of motoneurons supplying individual muscles in chick embryos with grafted supernumerary limbs have been identified using retrograde transport of horseradish peroxidase. For a given muscle, motor pool location varied depending on the embryonic origin of the muscle, the position of the limb along the rostrocaudal axis of the body, and the limb's orientation with respect to the body wall. Limb muscles derived from the ventral part of the embryonic premuscle mass were always innervated by medially located motoneurons. Muscles derived from the dorsal portion were consistently innervated by motoneurons in either intermediate or far lateral positions. These relationships were invariant with changes in limb position and orientation. These findings indicate that motor axons can recognize and selectively innervate muscles derived from either dorsal or ventral muscle mass. In addition, the spinal nerves innervating each limb were identified. The type of limb plexus (e.g., crural, sciatic, or wing) and the peripheral branching patterns of the nerves within the limbs were also studied and found to be controlled by the limbs. The rostrocaudal variation in motor pool position similarly depended on the position and orientation of the limb. This rostrocaudal variation in motor pool position can be explained by the limb's ability to determine axonal outgrowth pathways and hence to constrain the possible target choices of outgrowing axons. The process of limb innervation involves interactions between motoneuron axons having intrinsic differences or specificities, and the character of the local environment of the limb into which they grow.