Muc1 expression during pancreas development. Double-immunofluorescence was performed to detect Muc1 (green) and the pan-epithelial marker E-cadherin (red) in sections of embryonic and neonatal pancreata. Muc1 is already strongly expressed in the E11.5 pancreatic epithelium (A-C), when most cells are still undifferentiated progenitors. From E15.5 (D-F) through birth (G-I) Muc1 expression remains strong and becomes restricted to cells lining ductal (du) and acinar (ac) lumens. Islet cells (is) do not express Muc1 at any stage. Scale bar: 100 μm. Abbreviations: ac, acinus; du, duct; is, islet.