Expression of silver-stained nucleolar organizer regions is coupled to cell cycle in rat thymic cells

To directly analyze the relationship between the expression of silver-stained nucleolar organizer regions (AgNORs) and cell proliferation, thymic cells from newborn rats were separately sorted at the G0-G1-phase and early-mid and late-mid S-phase of the cell cycle according to their DNA content. Different AgNOR-derived parameters (mean area and numbers of AgNORs per cell and mean AgNOR-particle area) were evaluated after silver staining of cytospins. A linear correlation was observed between the mean area and numbers of AgNORs per cell, both parameters increasing progressively from G0-G1-phase to early-mid and late-mid S-phase. An increase of the mean AgNOR-particle area was also seen between G0-G1 and S-phase but this was not significant along the S-phase. A bias on the selection of S-phase cells linked to intrathymic maturation can be ruled out as S-phase cells labeled with bromodeoxyuridine were found throughout the thymus, and, moreover, the analysis of the frequency distribution of nuclear area did not show a bimodal pattern. It is concluded that the expression of AgNORs--evaluated as AgNOR area/cell or AgNOR numbers/cell--is causally or indirectly coupled to DNA synthesis and, thus, AgNORs can be considered as a cell proliferation marker.