Figure 5. Progenitor cells may be induced and converted into glucagon and subsequently into insulin-expressing cells.

Next to a pulse of BrdU at three weeks of age and examination a week later, a quantitative analysis established a 2.35 increase in the number of proliferating cells in POE::Glucre pancreas compared to POE controls (A-B). Importantly, most BrdU-labeled cells are not detected within the islets, but rather near or within the duct epithelium (B). It is worth noticing that this location corresponds to the cluster of glucagon-producing cells consistently observed in this genotype. Within the duct epithelium, scattered endocrine cells could be detected, some co-expressing the glucagon and sometimes somatostatin hormones (arrowheads in C-D). Along the same line, islet cells positive for the duct marker genes CK19 and SPP1 are observed adjacent to the duct epithelium (arrowheads in E-F). Notably, the duct lining also appears to contain numerous cells positive for the proendocrine marker gene Ngn3 (G-H), but negative for Pdx1 (I). (J-L) Infection of controls (K) and double-transgenic (L-O) pancreata using a construct containing a CMV promoter driving the constitutive expression of a c-Myc tag (the latter being not expressed in wild-type tissues - J). Due to the method used, two weeks post-infection, most exocrine cells are labeled by the virus, whereas only very few islet cells are (islet underlined in K). Importantly, a majority of double-transgenic endocrine cells appear marked by the virus, suggesting their ductal or acinar origin (L). These cells express the insulin hormone (M) and are β-galactosidase-positive (N-O), indicating that they once expressed the glucagon hormone. (H-I, M, O: Green fluorescence corresponding to residual GFP expression after bleaching of the sections).