A Fibrillar Structure in Rat Fibroblasts as Seen by Electron Microscopy

During the course of comparative tissue culture studies of a strain of normal rat fibroblasts and its malignant cell derivative, 1 we obtained preparations of both of these cell types which under the electron microscope† showed an unmistakable fibrillar structure in the thinly spread cytoplasm. The fibrils, of an estimated thickness of 10 to 100 mμ, may converge and diverge in fan-like formations of great regularity (Fig. 1) but are more often seen gathered together in long bands of varying widths (Fig. 2). The composite bands are seen clearly in phase microscope movies of the cells, 3 but it is only by the increased resolution available with the electron microscope that the nature of the bands becomes clear. They are apparently composed of long thin converging fibrils which in general radiate from the dense central area. That the fibrils are not produced by a wrinkling of the formvar membrane on which the cell is stretched is indicated by their uniformity, by their sharp change in direction in certain areas of the periphery, and by their fine converging and diverging structure. Besides this, no wrinkles were seen in the membrane outside of the cellular area where clear visualization is possible. Of interest are the relations of these bands to the general structure. The mitochondria, often long and thin, seem frequently to be associated with individual fat droplets. Some of the elongated mitochondria are found in areas which show many microsomes, with the whole area bounded by parallel fibrous bands. This appears to be a static representation of the streams of material which can be seen in the movies of these cells. In other areas, the great masses of mitochondria may be seen crossing those fibrous bands.