Schwann cell-neuron relationships in spinal cord gray matter

Schwann cells develop within the ventral gray matter following exposure of lumbosacral spinal cords to x-rays in early postnatal rats. These ventral gray matter Schwann cell aggregates occurred in about 40% of the animals 8 or more weeks following irradiation. Light microscopically these cells appeared to be apposed to somata of large motor neurons, raising a question regarding the fate of axo-somatic synapses. This study focused on neuron-Schwann cell relationships and demonstrated ultrastructurally that the intraspinal Schwann cells established a variety of relationships with the neuronal somata and primary dendrites. These relationships ranged from direct contact without an intervening basal lamina to the presence of synaptic contacts intervening between neuron and Schwann cell basal lamina. Occasionally, the Schwann cells occupied an intermediate position between neurons and blood vessels, suggesting functions similar to those carried out by astrocytes. In these instances, as in all cases of Schwann cell-blood vessel contact, the vessels lacked their normal investiture by astrocytes. Light microscopic evaluation of synaptophysin-immunostained sections revealed decreased immunoreactivity in neuropil occupied by the Schwann cells but confirmed the presence of synapses on neuronal somata. Possible mechanisms underlying Schwann cell induction in the ventral gray matter are discussed. An understanding of the interactions between Schwann cells and the cellular constituents of the gray matter is important in light of attempts to enhance repair in the central nervous system by transplanting Schwann cells into that environment.