Watery and dark axons in Wallerian degeneration of the opossum's optic nerve: different patterns of cytoskeletal breakdown?

In this paper we report a qualitative morphological analysis of Wallerian d egeneration in a marsupial. Right optic nerves of opossums Didelphis marsup ialis were crushed with a fine Forceps and after 24, 48, 72, 96 and 168 hou rs the animals were anaesthetized and perfused with fixative. The optic ner ves were immersed in fixative and processed for routine transmission electr on microscopy. Among the early alterations typical of axonal degeneration, we observed nerve fibers with focal degeneration of the axoplasmic cytoskel eton, watery degeneration and dark degeneration, the latter being prevalent at 168 hours after crush. Our results point to a gradual disintegration of the axoplasmic cytoskeleton, opposed to the previous view of an "all-or-no thing" process (Griffin et al 1995). We also report that, due to an unknown mechanism, fibers show tither a dark or watery pattern of axonal degenerat ion, as observed in axon profiles. We also observed fibers undergoing early myelin breakdown in the absence of axonal alterations.