Cytoskeleton organization in differentiating mouse teratocarcinoma cells.
In addition to the well-known cytoskeleton actin and tubulin, new intermediate-sized filaments have been isolated that ensure cellular connections. Their proteins, referred to here as IFP (intermediate filament proteins), comprise keratin desmin, vimentin, glialin and neurofilin. We review some of these new findings, with emphasis on the IFP expressions in the differentiation of embryonic cells and the deviations encountered in tumor cells. The first measurements on embryonic and differentiated mouse teratoma carcinoma cells established that all cell lines contain cytoplasmic microtubules. Although actin is also present in all cells, its organization differs markedly in differentiated derivatives. Since then, extensive studies have confirmed these findings and refined our understanding of the adaptation of cytoskeletons during differentiation to fulfill thier function. Fibrillar IFP structures have diameters ranging from 80 to 150 A, by electromicroscopic measurements. The specificity established in various structural functions have emerged from protein assays by two-dimensional electrophoresis and by immunofluorescence. With mouse teratoma cells, the incipient formation of the three structural components is followed by applying antibodies of high specificity for keratin, desmin and vimentin. During in vitro differentiation of teratoma carcinoma cells, as in animal tissue, keratin is expressed always in the endodermal cells and desmin in the muscle cells. Vimentin, which is known to be restricted to mesenchymal tissue in animals, occurs in all cells which have acquired the potential for unlimited growth in culture. In embryonic developments, all cells of mouse blastocyst express microtubules which is consistent with the function attributed to tubulin. The structure of actin, by contrast, changes during the first morphological step of differentiation. The outer trophectodermal cells contain actin cables, whereas the inner cell mass contains actin in a diffuse state. It has been established that keratin fibers appear in trophoblastic cells, i.e., in the earliest embryonic differentiation of epithelial character. Research on the development of IFP is rapidly progressing. Recent results of several groups are discussed.