An Unusual Arrangement of Ribosomes in Mesenchymal Cells

It has been established for some time that cytoplasmic ribosomes may be arranged in characteristic linear or spiral configurations (1, 2). Recently it has been shown that a multiple ribosomal structure is the active unit in protein synthesis (3-6) and hypothesized that individual ribosomes of a unit are held together by a strand of messenger R N A since the collection of ribosomes dissociates rapidly when exposed to low concentrations of ribonuclease (7-9). In studies of the rabbit reticulocyte, which synthesizes a single protein, hemoglobin, Warner et al. (3) and Slayter et al. (9) have described, in fractionated material, extended arrays of ribosomes connected by a strand approximately 10 A in width. The most frequent structure they encountered was a pentaribosomal unit, although some larger units contained six or even seven ribosomes. These structures, termed polyribosomes or polysomes, were in linear arrays when seen in positive contrast whereas in negative contrast clusters predominated. They considered that the linear array is the actual configuration in vivo and that the clusters seen in negative contrast were the result of drying. The thin thread, 10 A wide, which connects ribosomes, was thought to be a single strand of RNA. This work recently has been reviewed by Huh in (10). In our current studies of the rat umbilical cord, we have observed, at certain stages of gestation, an unusual arrangement of ribosomes in relation to the endoplasmic retieulum of mesenchymal cells. The array of ribosomes, in sectioned material, appears to bear a resemblance to the polyribosomes described, in fractionated material of the rabbit reticulocytes, by Slayter et al. M A T E R I A L S A N D M E T H O D S