Fig. 9. Two models for the lineage of exocrine, endocrine and duct cells. In both models, the pancreatic duct progenitors only express Pdx1 from E9.5 to E12.5. In Model 1, the duct lineage diverged from that of the endocrine/exocrine cells before E8.5. The endocrine and exocrine cells are distinguished by the presence of Ngn3 expression. In Model 2, the duct and endocrine/exocrine lineage diverged between E9.5 and E12.5. In the later model, suppressive or inductive signals are needed to allow for the development of duct fate.