Axon guidance in muscleless chick wings: the role of muscle cells in motoneuronal pathway selection and muscle nerve formation.

The role of myogenic cells in accurate pathway selection and muscle nerve formation was studied in chick embryos. Myotubes were eliminated from the forelimb of the chick embryo by extirpating the somites, which give rise to myogenic cells. Other elements of the wing tissue, connective tissue, and cartilagenous elements derived from the somatopleure, were left intact. Injections of WGA-HRP were made into either dorsal or ventral nerve trunks in the wing and the positions of retrogradely labeled motoneurons determined. The positions of the motoneurons within the brachial lateral motor column were appropriate for the injection made. Thus, the accuracy of the motoneuronal projections was unaffected by the absence of muscle cells. The absence of myotubes was not correlated with the absence of muscle nerves. Muscle nerves were consistently observed in muscleless wings until stage 36, the oldest stage examined. Muscle nerves in muscleless wings differed from those in normal wings in that they were smooth and stubby, and lacked the normal pattern of intramuscular nerve branches. From these studies we conclude that muscle cells are not necessary for accurate motor axon guidance into the periphery along the routes of major nerve trunks, nor for the formation of muscle nerves. By inference, somatopleural derivatives provide sufficient cues for selection of specific axonal pathways and for patterning of muscle nerves within the chick limb.