Observations on Wallerian degeneration in explant cultures of cat sciatic nerve.

Wallerian degeneration was studied in vitro using explant cultures of cat sciatic nerve. As these cultures contain no macrophages they highlight the responses of Schwann cells to myelin sheath breakdown. Although there were regional variations in the changes observed in these cultures with respect to time, the sequence of events which lead to Schwann cell proliferation and to fragmentation and liberation of myelin debris into the endoneurial space was established. The initial event was rejection of myelin sheaths by Schwann cells. Liberated Schwann cells then proliferated within the basal lamina tube. In nerve fibres containing proliferating Schwann cells, myelin debris passed through breaks in the basal lamina tube into the endoneurial space. Schwann cells also escaped from the basal lamina tube with the myelin debris. Following the loss of the luminal contents the basal lamina tube collapsed and the intratubular Schwann cells formed bands of Büngner. The Schwann cells which migrated into the endoneurial space and subsequently onto the culture dish retained contact with each other. These studies indicate that rejection of myelin internodes by their supporting Schwann cells set in train a series of events in which Schwann cells and degenerating myelin behaved as separate components. Schwann cells were not involved in phagocytosis or degeneration of myelin. We conclude that Schwann cell proliferation in Wallerian degeneration is directed towards re-establishing cellular continuity within the basal lamina tube which is lost when Schwann cells reject their myelin sheaths.