Figure 4. DNA damage in β-cells in vivo. In order to measure DNA synthesis and damage in β-cells in vivo, mice were administered BrdU in drinking water for 3 d. Pancreata were harvested and immunostained for BrdU (green), insulin (blue), and γH2AX (red). Under normal conditions (A–C), approximately 7% of BrdU positive β-cells also exhibited γH2AX (red) expression. In the course of studying small molecules identified in high throughput screens,¹³ however, we found that intraparenchymal injection of DMSO in the pancreas resulted in expression of γH2AX (red) in over 25% of BrdU positive β-cells (D–F), quantified in (L). Thus, BrdU incorporation can represent DNA damage in a significant percentage of β-cells in vivo as well as in vitro (Fig. 3). Arrows in (D–F) mark region magnified in G–I respectively. *p < 0.01, **p < 0.001. Blue nuclear counterstain is DAPI. Scale bars (A–F) = 50 μm, (G–I) = 10μm.