Muscle sensory innervation patterns in embryonic chick hindlimbs following dorsal root ganglion reversal

Previous studies suggest that sensory innervation of muscles is patterned by motor innervation. Muscle afferent projections mirror motor projections after various experimental manipulations and muscle afferents fail to project to muscle in the absence of motoneurons. It is not known, however, whether muscle afferents are specified with respect to the corresponding motoneurons or target muscles. To test this possibility we rotated three to four segments of neural crest in St. 15-17 chick embryos, leaving motoneurons intact, to reverse the rostrocaudal order of dorsal root ganglia (DRGs) T7/LS1-LS3. This caused sensory neurons derived from one segmental level to grow into the limb with motor axons from a different level. The resulting innervation patterns were assessed at St. 28-37 by injecting DiI and DiA into the sartorius and femorotibialis muscles or into the spinal cord and DRG. DiI labeling of crest prior to rotation showed that DRGs in the operated region were derived primarily from rotated cells. Muscle afferents from rotated DRGs grew to muscles in accord with their new rostrocaudal position, together with "inappropriate" motor axons from the same segmental level. The segmental distribution of sensory neurons innervating each muscle was more widespread in embryos operated at older than at younger stages. In contrast, sensory axons projected to the appropriate muscles in accord with their embryonic origin when segments of the whole neural tube, including motoneurons, were rotated, as reported previously. Thus, sensory neurons do not appear to be selectively matched with motoneurons or target muscles at stages when the corresponding motoneurons have clear identities.