Studies of innervation patterns in grafted chick wings.

The pattern of innervation of 12 day embryonic chick wings was studied by morphological and electrophysiological techniques. Several types of chick wing grafts were performed to investigate the rules governing nerve pattern formation: whether the motor neurons actively seek out and connect with specific muscles or are passively guided and whether cell death plays any role in pattern formation. In each type of graft the outgrowing neurons were confronted with extra targets distal to the elbow. Two grafts replicated lower arm parts in series (proximodistal graft) or in parallel (forearm and hand replicated in the same pattern of asymmetry in a split limb graft); a third produced a mirror image limb. The flexor carpi ulnaris muscle (F.C.U.) was studied in these limbs. The innervation of the most distal f.c.u. muscle in proximodistal grafted limbs varied with the extent of host f.c.u. muscle present. The f.c.u. muscles in the split limb graft were always innervated. In both limb grafts the f.c.u. muscles were innervated by the appropriate neurons, as determined by retrograde HRP technique and electrophysiological recordings. This strongly suggests that the motor neurons are able to seek out and connect with their appropriate muscle. Results of the mirror image limb graft suggest that other factors may be involved. In addition, motor neuron cell counts were performed. From this one can infer that the normally occurring pattern of cell death was not affected by the grafts. Cell death is therefore excluded as a major factor in nerve pattern formation.