Comparison of Muc1 and CK19 staining. Double-immunofluorescence was performed to detect Muc1 (green) and cytokeratin-19 (CK19, red) in sections of embryonic and adult pancreata. (A-C) At E15.5, CK19 is undetectable while Muc1 is strongly expressed throughout the pancreatic ductal epithelium. (D-F) CK19 expression is first detected in larger ducts at E17.5, but remains weak or undetectable in smaller ones, while ducts of all sizes express Muc1. (G-I) In the adult pancreas, Muc1 protein is detected in all ducts, albeit at slightly lower levels in intralobular (closed arrowhead) than intercalated ducts (open arrowhead). (Insert in G demonstrates that Muc1 is expressed by all duct cells.) CK19 exhibits the opposite staining pattern, highest in intralobular and lowest in intercalated ducts. Scale bars: 100 μm.