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 somato- tropes and mammotropes, which are the sources, re- spectively, of growth hormone and of prolactin in the rat pituitary. The observations were made in conven- tional electron micrographs of these cells in situ, where occasional surface or en face views of the rough endo- plasmic 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 charac- terize 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 mam- motropes) have an approximately circular shape and contain an average of 6.8 (somatotropes) or 6.5 (mam- motropes) ribosomes, which is an appropriate size for translation of the polypeptide hormones produced by these cells. About 17% of the membrane-bound poly- somes 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 mammo- tropes suggests the possibility that ribosomes (or the 40s ribosomal subunit) may recycle on the polysome after the translation of growth hormone or of prolactin.