Active nucleolus organizers are precisely positioned in adult central nervous system cells but not in neuroectodermal tumor cells.

Active nucleolus organizing regions (NOR) were stained with silver in isolated nuclei and chromosomes, monolayer cultures, and tissues for electron microscopy. In nuclei isolated in 0.8-1.3 M urea, only the NOR stained with silver, and round or fiber-like regions with a diameter of 200-250 nm with a substructure were delineated at appropriate nucleolar or acrocentric chromosomal sites. In whole cells, additional (non NOR) silver binding regions were noted. Electrophoresis studies on nuclear isolates indicated at least two different nuclear protein subsets were responsible for the observed silver binding; the NOR protein(s) was most tightly bound with respect to urea. Interphase cells displayed a more extensive NOR network than mitotic cells, suggesting an increase in rDNA transcription during interphase. Mature sperm cells showed no active NOR regions. Interphase neuroectodermal tumor cells generally contained large multiple NOR indicating extensive activity; the position of NOR in the nucleus was variable from cell to cell, even in cloned lines. In contrast to tumor lines, central nervous system (CNS) cells from various species all showed highly reproducible or non-random NOR locations within the nucleus of each cell type. The NOR of large neurons were always central and single, whereas small granule cell neurons displayed a few small NOR that were positioned more peripherally. These findings suggest that in highly differentiated cells, the NOR region is precisely positioned in the nucleus, regardless of non-identical chromosome locations of the NOR in different species. The variability of NOR in tumor cells indicates profound changes in nuclear structure that may be part of the neoplastic transformation.