STRUCTURE OF RETICULOSPINAL AXON GROWTH CONES AND THEIR CELLULAR ENVIRONMENT DURING REGENERATION IN THE LAMPREY SPINAL-CORD

The large larval sea lamprey is a primitive vertebrate that recovers c oordinated swimming following complete spinal transection. An ultrastr uctural study was performed in order to determine whether morphologic features of regenerating axons and their cellular environment would pr ovide clues to their successful regeneration compared to their mammali an counterparts. Three larval sea lampreys were studied at 3, 4 and 11 weeks following complete spinal transection and compared with an untr ansected control. Muller and Mauthner cells or their giant reticulospi nal axons (GRAs) were impaled and injected with horseradish peroxidase (HRP). Alternating thick and thin sections were collected for light a nd electron microscopy. A total of 9 neurites were examined. At all ti mes, growth cones of GRAs differed from those of cultured mammalian ne urons in being packed with neurofilaments and in lacking long filopodi a, suggesting possible differences in the mechanisms of axon outgrowth . Morphometric analysis suggested that GRA growth cones contact glial fibers disproportionately compared to the representation of glial surf ace membranes in the immediate environment of these growth cones. No d ifferences were found between glial cells in regenerating spinal cords and those of untransected control animals with regard to the size of the cell body and nucleus and the packing density of their intermediat e filaments. Glial fibers in control animals and glial fibers located far from a transection were oriented transversely. Glial cells adjacen t to the transection site sent thickened, longitudinally oriented proc esses into the blood clot at the transection site. These longitudinal glial processes preceded the regenerating axons. Desmosomes were obser ved on glia adjacent to the lesion but were scarce in the lesion durin g the first four weeks post-transection. These findings suggest that l ongitudinally oriented glial fibers may serve as a bridge along which axons can regenerate across the lesion. The presence of desmosomes mig ht prevent migration of astrocytes near the transection, thus stabiliz ing the glial bridge. (C) Wiley-Liss, Inc.