Neurog3+ cells arise from Muc1+ ductal cells during embryogenesis. To detect the short-term differentiation potential of Muc1+ cells, Muc1IC2/IC2; Rosa26EYFP/+ embryos were labeled by TM administration to pregnant dams at E13.75 (A, C) or E14.75 (B, D), and harvested ~12 hours later for immunofluorescence analysis (n = 2 for each timepoint). (A-B) Short-term labeling of Muc1+ cells (green) reveals their presence within the E-cadherin+ ductal network (blue, open arrowheads), as well as their contribution to Neurog3+ cells (red, closed arrowheads). (C-D) Similar analysis of islet hormone expression indicates that Muc1IC2 labeling (green, open arrowheads) is excluded from insulin+ (red) and glucagon+ cells (white), confirming that these cells derive from Muc1+ progenitors but do not themselves express Muc1. Scale bars: 50 μm in A-B and 100 μm in C-D.