THE BEADED FORM OF MYELINATED NERVE-FIBERS

When the nerves are lightly stretched and fixed by freeze-substitution , their fibers show the form-change termed ''beading'' which consists of a series of undulating constrictions and swellings in the internode s. This form change has not ordinarily been seen in chemically fixed n erves, or when it has, it has been ascribed to a pathological change o r an artifact. We now report that beading is also retained in normal n erves when, following a light maintained stretch, they are fixed with aldehydes at a temperature close to 0 degrees C. The degree of beading in single fibers teased from the aldehyde fixed nerves was graded and found to be maximal at 0 degrees C, falling off with increased temper ature until, at temperatures above 16 degrees C, most fibers showed no beading or a very mild beading. The fibers of nerves cold-fixed at 0 degrees C displayed the characteristics as freeze-substituted fibers, but with a somewhat smaller number of maximally beaded fibers and an 1 8% reduction in microtubule numbers in the axons. Desheathing or slitt ing the sheaths of the nerves before cold-fixation increased the proba bility of retaining beading. Exposure of stretched nerves to the aldeh yde fixative at room temperatures for times as short as 3-5 min before they were cold-fixed showed a diminished degree of beading, indicatin g that aldehydes can have a deleterious effect on the beading mechanis m which we hypothesize to be present in the fiber. This action is dist inct from the general cross-linking action of aldehydes.