Sites of Nucleolus Production in Cultured Chinese Hamster Cells

Chinese hamster cell strains in the early passages in culture display wide variation in number of nucleolus-like bodies per cell, though such strains are characteristically euploid. A variety of criteria indicate that the nucleolus-like bodies are true nucleoli. Their Azure Band fast green-staining properties indicate the presence of RNA and protein; they have typical nucleolar fine structure, including both fibrous and granular components; radioautography reveals that their patterns of uptake of uridine-3H into RNA are similar to those reported for nucleoli of other cell types; actinomycin D, at a level which selectively inhibits ribosomal RNA synthesis, greatly reduces their RNA synthesis and also causes segregation of fibrous and granular nucleolar components. Colchicine was used to experimentally fragment the nuclei of these cells into a number of separate karyomeres, each presumably containing some, or only one, of the chromosomes of the complement. Almost all the karyomeres contain nucleolus-like bodies which, by the same criteria applied to the multiple nucleolus-like bodies of uninuclear cells, appear to be true nucleoli. The nucleoli of individual karyomeres of the same cell often differ from each other in fine structure while the multiple nucleoli of a uninuclear cell generally resemble each other. The evidence presented in this study indicates that Chinese hamster cells contain many nucleolus-producing sites scattered through the genome. Diploid, nontransformed Chinese hamster cells in tissue culture were found to possess widely varying numbers of nuclear bodies which appeared cytologically to be nucleoli. This finding seemed inconsistent with numerous studies demonstrating that somatic cells are normally characterized by a set number of nucleoli which consistently arise in association with specific chromosomal loci. The number of nucleoli produced is found to be one per haploid chromosome complement, i.e., two per diploid cell, in a great many organisms, for instance, Hyacinthus (de Mol, 1926, 1928), Crepis (Nawaschin, 1927), Vicia (Heitz, 1931 a), Zea (McClintock, 1934), Allium (Heitz, 1931 b), Drosophila (Kaufman, 1934), certain species of Chironomus (Beermann, 1960), Xenopus (Fischberg and Wallace, 1960), frog (Parmenter, 1940), axolotl (Dearing, 1934) and other amphibians (Fankhauser and Humphrey, 1943). However, two nucleoli per genome are characteristic of a few species of grasshopper (Carothers, 1913), ginkgo (Lee, 1954) and Chironomus tentans (Balbiani, 1881). These data are consistent with biochemical evidence from Xenopus and Drosophila that cistrons coding for ribosomal RNA, which is synthesized in the nucleolus (Perry, 1962; Brown 248 on A ril 4, 2017 D ow nladed fom Published January 1, 1969