Figure 6.

p21 upregulation and cell cycle exit. A, To determine whether p21 upregulation correlated with cell cycle exit, a model of p21 colocalization with BrdU and Ki-67 at t_0pi, t_18pi, t_24pi, and t_30pi is proposed. This model was built on the idea that p21 is upregulated in G₁ and persists after cell cycle exit. At a time before one full cycle (t_18pi), more p21⁺/BrdU⁺ should be in the cell cycle (Ki-67⁺) than out of the cell cycle (Ki-67^–). As the BrdU⁺ cells progress through G₁, cells that upregulate p21 should exit the cell cycle and downregulate Ki-67 expression. B, C, Triple-labeled nuclei (white arrows) were readily distinguishable from cells labeled with p21 and BrdU only (yellow arrows; B). The number of p21⁺, BrdU⁺, Ki-67⁺, and p21⁺, BrdU⁺, and Ki-67^– per field was counted in untreated slices. As predicted, a decrease in the number of triple-labeled cells over time was countered by an increase in p21⁺, BrdU⁺, and Ki-67^– cells (D). Scale bar, 20 μm.