Cytologic demonstration of differential activity of rRNA gene clusters in different human tissues

rRNA gene activity was evaluated by cytologic methods in cultured human cells from two different tissues grown under controlled experimental conditions. The modal and average numbers of silver positive nucleolus organizers (NOs) per cell as well as the distribution of cells with different numbers of silver positive NOs and different combinations of D- plus G-group silver stained chromosomes, were evaluated. Statistically significant differences in the average number of silver positive NOs per cell between leukocytes and fibroblasts grown under standard experimental conditions have been demonstrated. The observed differences became sharper in cells cultured under more restrictive conditions. Also, differences in the frequency of silver positivity of specific chromosomal NOs located on individually identified chromosomes were observed in cells from the same tissue. Furthermore, differences in the frequency of activation of rDNA clusters located on the same chromosome were also observed between cells from the two tissues. The possible biologic meanings of these findings are discussed

The transcriptional activity of individual ribosomal DNA gene clusters is modulated by serum concentration

The activity of ribosomal gene clusters has been studied by cytological methods in human cultured cells grown in different amounts of serum under controlled experimental conditions. It has been shown that increasing amounts of serum induce an increase in ribosomal RNA synthesis at the single cell level. Furthermore, the concomitant identification of individual rRNA gene clusters by fluorescent techniques allowed us to demonstrate: (1) that individual gene clusters have differential transcriptional activity and differential frequency of activation; (2) that ribosomal gene activity is closely associated with the amount of silver-positive gene product and; (3) that environmental variations modulate rRNA synthesis by repressing or derepressing specific gene clusters

Silver positivity of the NOs during embryonic development of "Xenopus laevis".

Transcriptional activity of ribosomal RNA (rRNA) genes is detectable around blastula-gastrula transition during the embryonic development of amphibians and other non-mammalian systems. The silver staining reaction, known to selectively stain transcriptionally active nucleolus organizer regions (NORs) both in interphase and metaphase chromosomes allowed us to follow the activation of the NORs during the embryonic development of Xenopus laevis

Hormone-modulated rRNA gene activity is visualized by selective staining of the NOs.

The role of Growth Hormone and Dexamethasone in the regulation of rRNA gene activity was evaluated on cultured human fibroblasts by the cyto-chemical method of selective silver staining. By this method the transcriptionally active r-gene clusters can be specifically visualized in individual cells. Statistically significant increases in the rate of rRNA transcriptional activity were demonstrated after hormone administration