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

AbstractTo 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 iate‐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. © 1993 Wiley‐Liss, Inc.