The relationships between ribosomal genes and fibrillar centers in thyroid cells cultivated in vitro

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Abstract
Despite the fact that the fibrillar centers of the nucleolus and the chromosomal nucleolar organizers (NOR) are similarly stained with the NOR-Ag technique, there remain some questions about the identification of fibrillar centers as NOR. The distinct delineation of the fibrillar centers in porcine thyroid cells allowed us to determine whether there was a numerical equivalence or correlation between fibrillar centers and NOR. Hybridization in situ and Ag staining performed on pig chromosomes showed that pairs 8 and 10 contained [ribosomal] rDNA sites. Ag staining of thyroid cells in EM microscopy showed that the fibrillar centers and their surrounding layer of dense fibrils were the sites of Ag deposit. Chromatin fibers were demonstrated within the fibrimlar centers through the aid of the osmium-ammine reaction and with the oxidized diaminobenzidine technique. It was observed that in cultured thyroid cells the fibrillar centers could be identified in the light microscope as argyrophilic spherules, and easily counted. The number of fibrillar centers was variable according to culture conditions. In cells cultured for 5 h, the mean number of fibrillar centers was 1.7. After 5 days of cultured, the number of fibrillar centers increased, reaching a mean value of 5.93. When thyroid cells were stimulated with thyrotropin, the number of fibrillar centers again increased to a mean value of 7.54. Apparently, the relationship between fibrillar centers and NOR is not a simple proportionality: the number of fibrillar centers increases with increased cellular activity. In active cells each NOR may pass through several fibrillar centers.