Studies on the Mechanisms Responsible for the Formation of Focal Swellings on Neuronal Processes Using a Novel In Vitro Model of Axonal Injury

Abstract

A novel in vitro model of axonal injury using PC12 cells was designed to introduce traumatic alterations on neuronal processes and to identify mechanisms responsible for the formation of focal swellings by observation with phase-contrast and transmission electron microscopes. The injury on the processes was produced by one-dimensional, horizontal oscillation. The fluid shear stress applied by the oscillation did not exceed 380 dyne/cm². The injured processes showed two forms. One involved an increase in the terminal diameter of the processes and the other entailed beading along the injured portions. Long-term observation of cellular responses to the mechanical insult disclosed that the terminal swelling coincided with the detachment of growth cones from the culture plate. The finding suggests that the detachment of the growth cone destroys the cytoskeletal network, which determines and maintains the cell shape, resulting in spherical deformation of the processes. When the cytoskeletal destruction occurred at non-terminal sites along the processes, spherical deformations developed slowly, and these appeared as beads. The beading also caused the detachment of the growth cones. As the most proximal bead grew, they absorbed the distal segment and their growth cones were pulled proximally with the spreading cytoskeletal destruction. The processes with terminal swellings as well as the bead segments showed regeneration with time evidence of and growth cone formation.