Tonofilaments in normal human bronchial epithelium and in squamous cell carcinoma

Tonofilaments in the cells of squamous cell carcinoma of human bronchial epithelium were observed and compared to those in normal bronchial epithelium in 2 patients. In the normal tissue, only small numbers of tonofilaments were observed, and these were found only in the basal cells of the epithelium. In the tumor cells, the quantity of filaments increased considerably. They appeared in the cytoplasm both as branching and anastomosing bundles (tonofibrils) and also as single filaments about 40 A in thickness. These bundles were particularly well developed in the core of narrow cytoplasmic processes of tumor cells. The thickness and appearance of single tonofilaments were similar to those of the cytoplasmic microfilaments also observed in this study. It is suggested that the development of tonofilament bundles in the cytoplasmic processes of tumor cells may provide direct motive force for the deformation of the shape of the cell and potential invasiveness of these tumor cells.