The quantitative relationship of the fibrillar centres and other nucleolar components to changes in growth conditions, serum deprivation and low doses of actinomycin D in cultured diploid human fibroblasts (strain mrc-5)

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Abstract
The proportions of the 4 components of nucleoli: namely, granular component, fibrillar component, vacuolar space and the fibrillar centre material, were calculated by electron microscopical stereological procedures for human diploid fibroblasts (strain MRC-5) under 6 different culture conditions. Using nucleolar volumes obtained by light microscopy of unsectioned cells, estimates of the volume of each constituent per nucleolus were obtained. From the size and number of fibrillar centres encountered it was possible to estimate approximately their numbers per nucleolus. This ranged from an average of 42 ±4 to 234 ±25, depending on the level of cell activity, the number rising with increasing cell activity. Their volumes were inversely proportional to their number per nucleolus, indicating a possible fusion with cell inactivation. The number of fibrillar centres exceeds the number of nucleolus organizers in man (which is ten) and is nearer the number of ribosomal genes, which has been quoted at between 100 and 400 for diploid cells. The volumes of granular and fibrillar components also reflect changes in cell activity. A different response follows drug-induced inactivity when compared with the less artificial inactivation resulting from confluence or serum starvation. There was less fibrillar component in the actinomycin D-inactivated nucleoli. It is suggested that the nature of the fibrillar component may not be the same in cells in different states and that the simple interpretation that this is the transcriptional component may need to be revised. The change to fewer larger fibrillar centres upon nucleolar inactivation may be a consequence of 3 simultaneous processes. First, that the organizers increase in size by the condensation of previously active organizer chromatin, which is withdrawing from its transcriptional configuration. Secondly, this process may be accompanied by the fusion of the resultant larger nucleolar organizer regions. And finally, the increase in sizes of fibrillar centres may be further affected by an accretion of some non-chromatin material, possibly matrix or skeletal protein material, onto the organizers.