Conversion of adult δ cells into insulin-producing cells upon Pax4 ectopic expression. (A–C) Representative photographs of lineage tracing experiments performed on 2- and 5-mo-old pancreata (n = 4). Although β-galactosidase⁺ cells did not label insulin⁺ cells in Sst-Cre::ROSA26-β-gal control islets (A), immunofluorescence experiments showed several insulin⁺ cells positive for the β-galactosidase tracer in Sst-Cre::Pax4-OE pancreata (B and C, insets and arrows), indicative of a conversion of δ cells into insulin⁺ cells. (D) Further examinations revealed that 3.1% of insulin⁺ cells in 2-mo-old transgenics expressed β-galactosidase, and this proportion was maintained in 5-mo-old Sst-Cre::Pax4-OE animals. (E and F) Immunofluorescence analyses assessing Hhex expression in somatostatin-expressing cells in controls (E) and Sst-Cre::Pax4-OE (F) pancreata. Note the detection of somatostatin⁺/Hhex⁻ cells solely within the islets of Sst-Cre::Pax4-OE transgenics (F, arrows in inset). For the purpose of clarity, in selected photographs, islets are outlined with dashed white lines. Statistics were determined using a Mann–Whitney test (***, P < 0.001; *, P < 0.05). Bars: 50 µm; (insets) 20 µm. INS, insulin; SST, somatostatin.