The growth of neurites from explants of brachial spinal cord exposed to different components of wing bud mesenchyme

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Abstract
The extension of peripheral axons from the brachial spinal cord into the embryonic chick wing bud suggests that the target premuscle cell masses may act as the source of an adhesion gradient which establishes selective nerve pathways. Wing premuscle cell masses were explanted from different stage embryos and tested against age-matched brachial spinal cord for their ability to promote directional neurite outgrowth. It was found that target premuscle which does not contain myotubes could still elicit directional outgrowth in vitro. In contrast, skin and precartilage were unable to promote neuritic outgrowth significantly. Serum-free conditioned media were prepared from stages 20-38 premuscle and tested against age-matched spinal cord explants. There was an increase in the effects of conditioned media on neuritic outgrowth up to stage 35; conditioned media from older-stage pre- muscles had less effect than that of stage 35 premuscles. These results were shown to be dependent on the maturation of the premuscles and not on that of the spinal cord. When the premuscle conditioned media were preincubated over polylysine substrata, the ability to induce neuritic outgrowth was abolished from media derived from premuscle at stage 27 and older. Conditioned media derived from premuscle at stage 27 or older contain a polylysine-binding neurite-promoting factor which is present in greater amounts in more differentiated muscle. The time of first detection of neurite- promoting factors in stage 27 premuscle conditioned media correlates with the in vivo stages at which muscle-specific nerves branch from the main nerve trunks.