Astrocytes and regenerating axons at the proximal stump of the severed frog optic nerve.

We have studied the growing tip of the severed frog optic nerve, a central nervous system pathway that successfully regenerates. Since reconnection with the distal stump was prevented, guidance of the growing axons along anterogradely degenerating axons and their debris was precluded. One week after nerve section, there was vigorous mononuclear macrophage activity at the cut end, which quickly subsided. Phagocytosis of the remaining debris of retrograde degeneration in the proximal stump was carried out by astrocytes. Regenerating axons appeared at the tip of the stump 3 weeks after the cut. They were preferentially located near the periphery of the stump, in close proximity to astrocytes of the glia limitans. Eight weeks after the cut, regenerating axons formed a region of outgrowth protruding from the tip of the proximal stump. They were always accompanied by astrocytes, and no myelin-producing oligodendrocytes were seen in the outgrowth.