FITC-labelled antibody staining of tropomyosin-containing fibrils in smooth, cardiac and skeletal muscle cells, prefusion myoblasts, fibroblasts, endothelial cells and 3T3 cells in culture.

FITC-labelled antibodies to purified chicken gizzard smooth muscle tropomyosin were prepared and used to stain muscle and non-muscle cells in culture. Skeletal muscle myoblasts stained both diffusely throughout the cytoplasm and in fine filamentous structures. Once myotubes developed the staining was localized exclusively in the I-band region of the myofibrils. Similarly, cardiac muscle cells stained in the I-band alone. Primary and subcultured smooth muscle cells, irrespective of their state of differentiation, stained exclusively in long, straight fibrils. The staining of the fibrils was interrupted with stained regions 1-2 micrometer long and unstained spacings 0.5 micrometer. Interrupted fibrils were also observed in fibroblasts and endothelial cells, however their staining reaction was very weak (almost indistinguishable from that with pre-immune serum) and they were few in number. 3T3 cells demonstrated moderate staining in interrupted fibrils. Sheaths of very fine fibrils staining with a similar intensity were also found throught the cytoplasm. Interruptions in these fine fibrils were often aligned to give the whole cell a striated appearance. Sheaths of fibrils were not found in the other cell types studied.