Changes in Intermediate-Sized Filaments During Myogenesis and Neurogenesis

Abstract

Between 1960 and 1970 several studies included electron micrographs that revealed a class of filaments morphologically identical in a variety of normal and abnormal cells. They were described as free actin filaments, unstriated myofibrils and skeletal elements. In neurites, where they were known as “neuro-filaments”, they were postulated to mediate the centrifugal flow of axoplasm. Finding that these filaments could not be decorted with HMM rendered some of these interpretations dubious (Ishikawa et al. 1969). Studies of myogenic, fibrogenic, and chondrogenic cells demonstrated that these filaments averaged 10 nm in diameter, were of indeterminate length, and most intriguingly, reacted to colcemid and vinblastine by aggregating into immense “lakes” or cables (Ishikawa et al. 1968; Holtzer et al. 1970). This reaction (Figs. 1 a, b, 2, and 3) is as diagnostic for these 10 nm filaments as is the formation of arrowhead complexes following HMM decoration for the actins. We concluded that the 10 nm filaments constitute a distinct type of organelle and termed them “intermediate-filaments” (IF), being thicker than actin, but thinner than myosin, filaments (Ishikawa et al. 1968).