Interactions between dorsal root axons and their target motor neurons in developing mammalian spinal cord

We have utilized the lipid-soluble tracers Dil and DiA to investigate interactions between group la dorsal root afferent axons and their target motor neurons in developing rat spinal cord. We show here that la axons project toward motor pools in fascicles that exhibit a considerable degree of spatial order. A rough topography is present in that axons that innervate medially located axial motor neurons cross over others in the intermediate zone and follow a separate path along the midline toward their appropriate targets. Surprisingly, we have also found that motor neuron dendritic projections are well established in the transverse plane prior to the arrival of la afferents. Although dendrites from motor pools innervating limb muscles project directly in the path of incoming la afferents, they do not guide afferents to appropriate motor pools. The la afferents pass over the distal dendrites and grow all the way to the border between gray and developing white matter. A significant amount of terminal branching and bouton formation is in the vicinity of motor neuron somata and proximal regions of the dendritic arbors. Few boutons are found near dendrites that project dorsal to the motor pools, and virtually no boutons are found on dendrites in white matter. Our results show that la afferent axons are not guided to appropriate motor pools by random encounters with motor dendrites, and raise the possibility that mechanisms exist that promote an orderly projection of la afferents to particular regions of the ventral horn. The striking lack of innervation of white matter and dorsally directed dendrites by la afferents raises the question of whether descending and intersegmental systems have their initial interactions with these regions of the motor neuron dendritic arbor.