Temporary colonization of the site of lesion by macrophages is a prelude to the arrival of regenerated axons in injured goldfish optic nerve

In crushed goldfish optic nerve, regenerating axons cross the site of lesio n within 10 days following injury. Some 30 days later, Schwann cells accumu late at the lesion, where they myelinate the new axons. In this study, we h ave used immunohistochemistry and electron microscopy to examine the cellul ar environment of the crush site prior to the establishment of Schwann cell s in order to learn more about the early events that contribute to axonal r egeneration. During the first week following injury, macrophages enter the site of lesion and efficiently phagocytose the debris. The infiltration of macrophages precedes the arrival of regenerating axons that abut and surrou nd these phagocytes. Based on EM morphology and phagocytic capacity, macrop hages of the type observed at the site of lesion are not present in the deg enerating distal nerve segment, where debris clearance is shared between co nventional microglia and astrocytes over a period of several weeks. During this period, axon bundles emerging distally from the injury zone become enw rapped by astrocyte processes, thereby re-establishing the characteristic f ascicular cytoarchitecture of the optic nerve. The process of fasciculation also leads to the displacement of myelin debris to the margins of the fibe r bundles, where it is trapped by the astrocytes. Our results suggest that the early robust appearance of macrophages at the lesion, and their effecti veness as phagocytes compared with the microglia distally, may contribute t o the vigorous axonal regeneration across the crush, beyond which axons-exc epting the pioneers-extend through newly formed debris-free channels deline ated by astrocyte processes.