Morphological plasticity in the sympathetic chain

Abstract

The electron microscope has been used to study cellular changes that accompany the vigorous nerve sprouting which has been demonstrated previously in sympathetic chain by neurophysiological techniques. The ventral nerve roots C8-T3 were transected unilaterally in young cats. Fixation was by aldehyde perfusion after survival times of 3 days to 6 weeks. Following division of ventral roots C8-T3, about 90% of the preganglionic fibers were destroyed and digestion chambers containing organizing axonal debris were found in neurilemma cells and macrophages up to 3 weeks after operation. Completely eliminated were the large S1 axons of the pathway for pupillodilatation. Within 3 days after nerve section, numerous small groups of sheath cell “fingers,” oriented parallel to surviving axons, were found in the preganglionic trunk. These sheath cell assemblies are subsequently invaded by neurites (presumed collateral sprouts derived from surviving preganglionic axons), which possess less electron-dense cytoplasm than the neurilemma cells. Most collaterals arise from unmyelinated (S4) axons, and then ramify in parallel with the parent nerve fibers. Naked nerve sprouts were extremely rare, and axonal growth cones seldom encountered. Within 7 days after subtotal preganglionic section, presumed collateral sprouting of surviving axons was observed. Groups of neurilemma cell fingers accompanied by tiny neurites were observed very occasionally in normal tissue. It is possible that sprouting is a slow but continuing process in the normal animals and, if so, our experimental procedure greatly accelerates the process.