Abstract
Corticospinal axons show regressions during development but the manner of regressions of those axons after innervation into the spinal gray matter is largely unknown. We examined the regressive events observed in the corticospinal slice coculture by way of confocal time-lapse imaging of single axons labeled with EYFP. We made daily or short interval (30 to 60 min) image acquisitions from 5 to 13 DIV. From daily images, we obtained (1) gradual shortening, (2) amputation of the branches, and (3) disappearance of large part within a day. Acquisitions at shorter intervals allowed us to analyze retractions and degenerations in detail. Retractions occurred intermittently: it retracted at a certain constant rate for a period and stopped, and then resumed retraction at the same rate and stopped again. The rate of retraction was calculated to be 224 ± 42 μm/day (mean ± s.e.m.), which was consistent with the gradual shortening observed in daily images. Degeneration occurred on isolated axons and the part undergoing degeneration disappeared in 3-4 hours. Degenerating axons showed bright beaded spots arranged periodically. Those bright spots changed fluorescence intensity during degeneration but individual spot did not alter its position. The present observation and its detailed analysis on retraction and degeneration provide important clues to understand mechanisms of regressive events in CNS axons. [J Physiol Sci. 2008;58 Suppl:S120]
