Reinnervation of Grafted Pacinian Corpuscles by Dorsal Root and Dorsal Column Axons

Abstract

Transplantation of various parts of the developing central nervous system (CNS) into the brain has proved to be a valuable tool for studying neuronal differentiation, connectivity and plasticity (Bjorklund and Stenevi, 1985; Raisman et al., 1987). In our experiments, we have used nonneuronal grafts from the periphery for testing the plasticity of dorsal root ganglion neurons. These pseudounipolar neurons are polarized: only a single process, crus commune, originates from the neuron, but it divides not far from the perikaryon into a peripheral and a central process. Both processes have the ultrastructure of axons, but differ in their polarity and form fundamentally different terminals. All central processes, i.e. dorsal root axons, terminate in the spinal cord in transmitter-releasing endings which contain a large number of synaptic vesicles, whereas peripheral axon terminals are designed to receive various kinds of stimuli either indirectly, through specialized receptor cells, or directly as transducing sensory endings which may be either free or connected with different types of corpuscular receptors; most of the peripheral sensory terminals are filled with mitochondria and contain vesicles only at their margins or extensions. We have attempted to answer the question whether the dorsal root ganglion neurons would respond to a change of the target tissue and alter the ultrastructure of their “central” endings, if their processes were connected to peripheral sensory corpuscles instead of to the spinal cord. To explore this possibility, we connected a graft of peripheral mechanoreceptors — Pacinian corpuscles — with either dorsal root axons or dorsal column axons and studied the process of axonal regeneration, the reinnervation of the grafted corpuscles and the ultrastructure of the regenerated endings.