Development of axon paths of motorneurons after removal of target muscles in a holometabolous insect.
The fate of leg motor neurons and the outgrowth of their axons was studied in fleshflies after removal of their target muscles before the onset of axogenesis. When a mesothoracic leg imaginal disc was extirpated in the prepupa the corresponding leg nerve was missing on the operated side of the five-legged adult fly. Horseradish peroxidase and cobalt filling revealed that the target-deprived motorneurons survive and their axons grow out through other nerve roots. The choice of exit of the axons is variable: adjacent pro- or metathoracic leg nerves on the ipsilateral side or nerves supplying thoracic muscles (e.g. indirect flight muscles). In order to see whether factors in the mesothoracic leg disc can attract mesothoracic motorneurons, implantation of supernumerary leg discs was made in prepupae. Motor innervation of the implanted legs was never seen. Our data thus indicate that under the present experimental conditions leg motor neurons are not dependent on their specific targets for survival. They can track along adjacent preexisting axonal paths to innervate other targets. Under normal conditions specific markers may be present in the appropriate targets and the appropriate preexisting pathways (that we disrupt in our experiments) may lead the axons to these markers.
D R
DR
Nässel
A
A
Helgee
P
P
Sivasubramanian
D001369:Axons / Q000502:physiology; D004175:Diptera / Q000254:growth & development*; D005121:Extremities / Q000294:innervation; D009046:Motor Neurons / Q000502:physiology*; D024510:Muscle Development; D009420:Nervous System / Q000254:growth & development*