Sensory afferents show appropriate somatotopy at the earliest stage of projection to dorsal horn

There is a controversy about the extent to which topographic projections in the CNS are determined by initial guidance of axons as opposed to remodeling during development. We have addressed this issue in the rat dorsal root ganglion (DRG) system because dorsal root axons have a readily demonstrable somatotopy in the dorsal horn of the spinal cord and because DRG neurons will ultimately be favorable for molecular analysis. By labeling sensory axons innervating ventral and dorsal skin with different lipid-soluble tracers, we have found that the somatotopic organization of the cutaneous afferent projection is present at the time dorsal root axons first penetrate the gray matter. Our results thus stand in contrast to those in the rat retinocollicular system where the mature retinotopically ordered projection emerges during the postnatal period through remodeling of an early, diffuse projection.