A Method for Impregnating Myelinated Axons in Adult Central Nervous System1

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Abstract
A method has been developed for impregnating myelinated axons in blocks of brain tissue of adult animals. Tissue pieces were fixed in osmium tetroxide-potassium di-chromate for 30 to 250 days, followed by immersion in 0.75% AgNO3 solution for 5 to 7 days. The pieces were treated with 10% formalin for 2 to 3 days, followed by a second impregnation in 0.75% AgNO3 solution for 1 day and finally treated with 10% formalin for 4 to 5 days. The tissue was rapidly dehydrated and imbedded in 62d`C paraffin under vacuum. Sections were cut at 50 μ, mounted on slides, cleared with cedarwood oil, and covered with Canada balsam without a coverslip. Preparations have been stable for 1 1/2 years. The advantage of the present technique is that the course of a single myelinated axon can be followed for many millimeters in a section. Such fibers appear to retain their normal diameters so that populations of different anatomical or functional categories of axons can be estimated.