Nucleolar organizer regions (Ag-NORs) in multinuclear osteoclasts

Osteoclasts are multinuclear bone-resorbing cells. Little is known about regulation of gene expression in these cells and how it changes during the aging of an osteoclast, which is indicated by an increasing number of nuclei. Silver staining of nucleolar organizer regions, so-called Ag-NORs, allows us to assess the activity of rRNA expression. Using this technique, we show that individual nuclei of osteoclasts can be stained specifically and that the majority of individual nuclei contained Ag-NORs. The number and size of Ag-NORs varied, indicative of difference in the progression of the cell cycle through the G(1) phase. The relative activity of Ag-NORs was not synchronized on the level of individual nuclei, regardless of the number of nuclei per osteoclast. Surprisingly, the total activity of Ag-NOR appeared to be correlated to the total nuclear area of an osteoclast. We conclude that regulation of gene expression in multinuclear osteoclasts most likely occurs on the level of individual nuclei.