Skin sensory innervation patterns in embryonic chick hindlimb following dorsal root ganglion reversals

During embryonic development skin sensory neurons in lumbosacral dorsal root ganglia (DRGs) establish their dermatomes and axonal projections in a precise, orderly fashion in the chick. To investigate mechanisms responsible for this specific outgrowth, the rostrocaudal order of DRGs T7-LS3 was reversed by rotating the corresponding segments of neural crest, either alone or together with the underlying neural tube in St.15-16 embryos. The resulting skin sensory innervation patterns, mapped physiologically or anatomically at St.29-40, differed between the two experimental groups. Following neural tube rotations DRGs tended to establish innervation patterns that were consonant with their original position in the embryo. Axons from these rotated DRGs generally projected into the appropriate pathways and innervated the appropriate region of skin. Neural crest rotations left the ventral neural tube (including the motor neuron precursors) largely intact. In this case rotated DRGs tended to establish innervation patterns in accordance with their new position in the embryo, almost as if no rotation had been made. These results cannot be explained solely by the inherent specificity of sensory neurons. Instead, the results are largely consistent with the suggestion (Honig et al., 1986; Landmesser and Honig, 1986) that motor axons can direct the outgrowth of sensory axons and thereby influence the establishment of sensory innervation patterns. Other mechanisms that may also affect the development of sensory innervation patterns are discussed.