The nucleolus, chromosomes, and visualization of genetic activity

By the time ofMontgomery's classic paper on the nucleolus in 1898 (1), there were already 700 or so articles with observations on this nuclear organelle, beginning with a study by Fontana in 1781 entitled "Venom of Vipers." The early cytological emphasis on the nucleolus undoubtedly was due to the high visibility of the organelle in interphase nuclei ofmost cell types ; however, the fact that nucleoli are directly involved with chromosomal activity was not demonstrated until Heitz (2) and McClintock (3) showed that nucleoli form during telophase at specific chromosome regions called "nucleolar organizers" (NOS) by McClintock. In 1940, Caspersson and Schultz (4), using ultraviolet (UV) absorption spectra, concluded that both nucleoli and cytoplasm ofcells are generally rich in ribonucleic acid (RNA) . Brachet (5) came to the same conclusion independently after discovering that RNase treatment ofamphibian oocytes removed the basophilic components ofboth cytoplasm and nucleoli . Following these early observations, Caspersson and co-workers produced convincing evidence that a positive relationship exists between nucleolar size and levels of RNA and protein synthesis in cells (6) . In the early 1950s, Estable and Sotelo (7) used a silverstaining technique on a variety of cells, and suggested that during interphase, nucleoli contain a threadlike structure, termed the "nucleolonema," which associates with all of the chromosomes during mitosis and regroups at the NO during telophase, after which the nucleolus acts as a collecting site to accumulate additional material . Although the assignment of a hereditary continuity to the nucleolonema independent of chromosomes turned out to be incorrect, this provocative concept spurred interest by many investigators regarding the function of the nucleolus in cell metabolism . A comprehensive 1955 review by Vincent (8) provides a nice overview of earlier research on nucleoli .