Shape of large bound polysomes in cultured fibroblasts and thyroid epithelial cells.

Large bound polysomes were observed by conventional electron microscopy in surface or en face views of the rough endoplasmic reticulum (RER) in two cultured cell types. Cultured thyroid follicular epithelial cells and dermal fibroblasts, both from rats, were prepared for electron microscopy. Ultrathin sections were cut in the plane of the flattened cells to maximize the incidence of RER surface views. Some observations were also made on tissue sections of rat thyroid. Most of the large, RER-bound polysomes in both cell types appeared as two parallel rows of ribosome, thus resembling the shape of long hairpins, although probably closed at both ends. The two parallel rows of ribosomes were about 14 nm apart, and the center-to-center distance between ribosomes in the strands averaged 25 nm. Most of the large bound polysomes in thyroid epithelial cells were presumably making thyroglobulin subunits (330 kDa), while a majority of those in the fibroblasts were probably making prepro-alpha chains of collagen I (150 kDa). It was not possible in this material to see complete large polysomes, because their size usually caused them to extend out of the plane of section. In addition to the hairpin polysomes, there were smaller numbers of other forms. A characteristic large spiral polysome was seen occasionally in both cell types and contained as many as 31 ribosomes. One or two dense particles were sometimes seen in the center of spiral or circular polysomes. The consistent hairpin shape of most large bound polysomes observed in this study suggests that their shape is quite stable.