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.

Circular polysomes predominate on the rough endoplasmic reticulum of somatotropes and mammotropes in the rat anterior pituitary.

We have studied the shape and size distribution of membrane-bound polysomes in somatotropes and mammotropes, which are the sources, respectively, of growth hormone and of prolactin in the rat pituitary. The observations were made in conventional electron micrographs of these cells in situ, where occasional surface or en face views of the rough endoplasmic reticulum allow the polysomes to be seen as rows of ribosomes arranged in distinctive patterns on the membranes. It is possible by this means to characterize the shape and number of ribosomes for the total population of bound polysomes in the respective cell types. The great majority of membrane-bound polysomes in these two cell types (81% in somatotropes, 78% in mammotropes) have an approximately circular shape and contain an average of 6.8 (somatotropes) or 6.5 (mammotropes) ribosomes, which is an appropriate size for translation of the polypeptide hormones produced by these cells. About 17% of the membrane-bound polysomes in somatotropes and 20% in mammotropes have a spiral shape, resembling somewhat the letter "G," and contain about eight to nine ribosomes in both cell types. The preponderance of circular polysomes on the rough endoplasmic reticulum of somatotropes and mammotropes suggests the possibility that ribosomes (or the 40S ribosomal subunit) may recycle on the polysome after the translation of growth hormone or of prolactin.