Myelin debris clearance during Wallerian degeneration in the goldfish visual system.

We have examined the clearance of myelin debris from the visual pathways of the goldfish during Wallerian degeneration. Both the rate and pattern of myelin disappearance from the optic nerve and tract were determined using immunohistochemistry on frozen sections, as well as plastic sections and electron microscopy. Animals with and without regenerating optic axons were examined in order to determine whether the axons play a role in myelin clearance. We found that myelin is cleared at different rates along the visual paths. Thus, virtually all myelin debris is gone in the optic tract and distal optic nerve stump by 4 weeks after surgery, while in the cranial nerve segment, myelin clearance is still incomplete at 6 weeks postoperative. These temporal and spatial patterns of myelin clearance are the same in animals with and without regenerating axons, thus indicating that growing axons do not influence this process. Finally, ultrastructural observations revealed that both astrocytes and microglia participate in phagocytosing myelin debris in the optic nerve, while in the tract, the vast majority of debris is removed by microglia alone. These data are discussed with regard to possible mechanisms controlling the differential expression of myelin clearance.