Observations on a transient phase of focal swelling in degenerating unmyelinated nerve fibers.

This study describes the nature and time-course of a swelling phase during the degeneration of unmyelinated nerve fibers, as observed in highly organized cultures of rodent sensory ganglia. Observations were made on nerve fascicles after they were cut and during nutritional deprivation. About 12 hr after nerve transection, large, clear vacuoles appear throughout fascicles distal to the cut. These vacuoles are most numerous at 24 hr and then gradually subside; after 48 hr, only small granules mark the severed fascicles. Electron microscopy shows that the vacuoles are, in fact, massive focal dilations of unmyelinated axons. Similar focal dilations in unmyelinated axons are observed if cultures are not refed for 5-7 days; under these conditions glucose concentrations fall below 20 mg/100 ml and degenerative changes begin to appear in neuronal somas. If the gas-tight assembly is opened and the culture refed, there is rapid disappearance of axonal dilations (usually within 1 hr) and recovery of many of the damaged neurons. Cooling (4 degrees C) prevents this reversal, suggesting that an active process is involved. It is postulated that the swellings result from the failure of active axolemmal ion-pumping mechanisms prior to loss of selective permeability in the axon membrane. The reasons for the focal nature of the swellings is unknown. A literature review indicates that a phase of focal swelling has frequently been observed during the degeneration of unmyelinated nerve fibers in vivo.