Specificity of initial axonal projections to embryonic chick wing.
Anterograde transport of HRP was used to map initial segmental projection patterns of motor and sensory axons to the chick embryo wing. At the earliest stages of innervation, before the peripheral nerves had formed, motor axons were distributed as two loosely organized sheets, one directed towards the dorsal muscle mass and the other to the ventral. Labeled axons emerging from a given spinal cord segment were distributed within the sheets in a pattern that reflected the origin of the axons along the rostro-caudal axis of the lateral motor column. The segmental specificity of the motor projections was analyzed again after the axonal sheets had formed recognizable nerve trunks and muscle nerves. As was observed at the younger stages, axons were distributed in a pattern that was correlated with both the relative antero-posterior (A/P) and proximo-distal (P/D) position of the forming nerve, with cranial spinal nerves projecting to anterior and proximal limb regions. Whereas most axons emerging from a given ventral root projected in accord with what would be expected from the known adult motor pool maps, segmentally unexpected projections were frequently observed. The proportion of aberrantly projecting axons appears to be quite small, and in most embryos, it was impossible to determine whether the erroneous projections originated from unbranched axons or were collateral branches of others. The findings indicate that the initial projections of motor axons to the developing wing are patterned along the A/P axis of the wing and are largely accurate; however, the guidance processes are not sufficiently precise to exclude fibers entirely from inappropriate nerves. It is likely that later developmental processes act to fine-tune the initial projection pattern.