Studies of the Impregnation of Nervous Tissue Elements I. Impregnation of Axons and Myelin

All metallic impregnation methods depend on the presence of reducing groups in tissues. By testing the effect of various reduction indicators on sections of nervous tissue it was found that some reduction systems do not have a high enough oxidizing potential to serve for the visualization of either axons or myelin. Others can visualize myelin sheaths. The systems having the highest potential visualize also axons. Although myelin can be oxidized more easily than axons, axons are probably relatively richer in reducing groups. Impregnation by AgNO3 or Ag-diammine results in the formation of silver nuclei in both axons and myelin. Subsequent exposure of such sections to the action of reducing agents increases the visible impregnation of the axons much more than that of the myelin. This effect can be ascribed to the low dielectric constant of myelin. Gold toning serves a dual purpose: (a) coating with gold the more heavily impreg-nated structures, and (b) clearing the background through the displacement of the silver from the less intensely impregnated areas. The mechanism of metallic impregnations and the rationale of the different steps in Bielschowsky's procedure for axons are discussed