The Role of Intermediate (10-nm) Filaments in the Development and Integration of the Myofibrillar Contractile Apparatus in the Embryonic Mammalian Heart

Most eukaryotic cells possess three major distinct classes of fibrous organelles that are independently organized and function as elements of the cytoskeleton. These elements of the cytoskeleton include microtubules (25 nm), which form elaborate cytoplasmic networks; actin or thin filaments (6 nm), which form cytoplasmic stress fibers; and the intermediate filaments (10 nm). The term “intermediate” filaments has been applied to this third class of cytoplasmic fibrous proteins because their mean diameter at the ultrastructural level lies between the mean diameter of actin and microtubules. Investigations have shown that both actin (Clarke and Spudeck, 1977; Pollard and Werhing, 1974; Stossel, 1978) and microtubules (Stephens and Edds, 1976) are involved in various aspects of cell motility and also in the movement of cellular organelles. The role of the intermediate filaments in cell function is unresolved at present. The intermediate filaments were initially regarded as a disaggregation, or degradation product, or myosin and/or microtubules and thus until recently attracted little attention. Current biochemical and immunofluorescent methods have established the intermediate filaments as a distinct class of cytoplasmic proteins that differ with respect to the physical properties of their subunits. In contrast to the proteins actin and tubulin, which are the major structural protein subunits of microfilaments and microtubules, respectively, the intermediate-filament proteins do not appear to be highly conserved (Bennett et al., 1979; Lazarides and Balzer, 1978; Shelanski and Liem, 1979) and exhibit a relatively high degree of tissue specificity. The intermediate filaments have been divided into several subclasses on the basis of biochemical and immunochemical data, and their constituent proteins have been named accordingly. These subclasses at present include: (1) prekeratin tonofilaments found in epithelial cells (Franke et al.,1978a,b, 1979b; Sun et al., 1979) and cells of epithelial origin; (2) vimentin or decamin filaments (Bennett et al., 1978b, 1979; Franke et al.,1978a, 1979a) found in fibroblasts and other cells of mesenchymal orgin; (3) desmin filaments (Izant and Lazarides, 1974; Lazarides and Hubbard, 1976; Lazarides, 1978a; Lazarides and Balzer, 1978) or skeletin (Campbell et al.,1979) of smooth muscle, which have also been identified in the cytoplasm and Z lines of skeletal and cardiac muscle; (4) neurofilaments of neurons; and (5) glial filaments, which are present in astrocytes (Shelanski and Liem, 1979) but not in all types of glial cells (Liem et al., 1978; Schlaepfer, 1977; Schlaepfer and Lynch, 1977). Current studies have shown that it is not uncommon to find two of these classes of intermediate filaments coexisting in the same cell type. It is also quite possible that more than two classes of intermediate filaments can be present in a single cell type (Lazarides, 1980).