Preliminary Biochemical Investigations of the Intermediate Filaments 1

Preliminary biochemical investigations of differentiating cardiac muscle first revealed the occurrence of a third class of filaments, differing in appearance and reactivities from the thick and thin filaments of the sarcomere. These inter-mediate filaments were more resistant to extraction with neutral salts, fl-mer-captoethanol, and urea than were the thick filaments, thin filaments, or Z material. Combined with the data from the accompanying report, sufficient evidence is presented to discount virtually all previous models of myofibril-logenesis. In 1965, Heuson-Stiennon (11) first reported the general occurrence of a hetero-geneous class of filaments in presarcomere myoblasts of developing striated muscle and indicated that the observed filaments were not identical to the sarcomeric thin filaments, as was (and still is) commonly assumed. In the accompanying report, visual identification of at least three classes of filaments is presented and is compared with the data from striated muscle presented by Heuson-Stiennon (11), Kelly (19), and Ishikawa et al. (16, 17). The present investigation, initiated in 1967 and complete