A morphological correlate of target recognition by regenerating motor axons in the cockroach

A specific cell recognition process during regeneration of severed axons of identified cockroach motor neurons eventually leads to the reformation of the original innervation pattern of target muscles in the leg. This occurs even though, at early times after nerve crush, the multiple branches of each regenerating axon grow into both appropriate and inappropriate muscles. In this study, we sought to examine whether there are any structural differences between regenerating axon branches in appropriate and inappropriate muscles that could lead to an understanding of why only those in inappropriate muscles are eliminated. A neuron subset-specific monoclonal antibody, NSS-2A, which labels the inhibitory motor neurons, was used to make their axon branches visible at various times after nerve crush. In inappropriate muscles, these axons grow primarily parallel to the muscle fibers and are later eliminated. In the appropriate muscles, these axon branches initially also grow parallel to the muscle fibers, but subsequently grow many interstitial collaterals. The formation of the collateral branches is a morphological correlate of the specific interaction of a neuron with its appropriate muscle. The simultaneous occurrence of axonal elimination and collateral sprouting supports the idea that the two processes are causally related, as suggested by the sibling neurite bias hypothesis.