The importance of mesenchymal-epithelial interactions for normal development of the pancreas was recognized in the early 1960s, and mesenchymal signals have been shown to control the proliferation of early pancreatic progenitor cells. The mechanisms by which the mesenchyme coordinates cell proliferation and differentiation to produce the normal number of differentiated pancreatic cells are not fully understood. Here, we demonstrate that the mesenchyme positively controls the final number of ␤-cells that develop from early pancreatic progenitor cells. In vitro, the number of ␤-cells that developed from rat embryonic pancreatic epithelia was larger in cultures with mesenchyme than without mesenchyme. The effect of mesenchyme was not due to an increase in ␤-cell proliferation but was due to increased proliferation of early pancreatic duodenal homeobox-1 (PDX1)-positive progenitor cells, as confirmed by bromodeoxyuridine incorporation. Consequently, the window during which early PDX1؉ pancreatic progenitor cells differentiated into endocrine progenitor cells expressing Ngn3 was extended. Fibroblast growth factor 10 mimicked mesenchyme effects on proliferation of early PDX1؉ progenitor cells and induction of Ngn3 expression. Taken together, our results indicate that expansion of early PDX1؉ pancreatic progenitor cells represents a way to increase the final number of ␤-cells developing from early embryonic pancreas. Diabetes 56:1248-1258, 2007 E pithelium-mesenchyme interactions play a crucial role during organogenesis. They are mediated at least in part by soluble factors produced by the mesenchyme and acting on the epithelium (1). Evidence points to a crucial role for epithelialmesenchymal interactions in cell proliferation and differentiation during pancreatic development (2). However, the mechanisms by which the mesenchyme coordinates cell proliferation and differentiation to produce a normal number of differentiated pancreatic cells are not fully understood.The mature pancreas contains endocrine islets composed of cells producing hormones, such as insulin (␤-cells) and glucagon (␣-cells), and exocrine tissue composed of acinar cells producing enzymes (e.g., carboxypeptidase-A) secreted into the intestine. The pancreas originates from the dorsal and ventral regions of the foregut endoderm. Recently, important findings have shed light on the processes controlling pancreatic endocrine cell development. Studies of genetically engineered mice identified a hierarchy of transcription factors regulating pancreas organogenesis and islet-cell differentiation (3-5). The endodermal region committed to a pancreatic fate first expresses transcription factor pancreatic duodenal homeobox-1 (Pdx1). Pdx1 is detected in mouse embryos on embryonic day 8.5 (E8.5) (E9 in rats) in early pancreatic progenitors. During adulthood, Pdx1 expression becomes largely confined to ␤-cells, where it activates insulin gene transcription (6). Disruption of the Pdx1 gene in mice or human leads to pancreatic agenesis (7,8). These data indicate that P...

The importance of mesenchymal-epithelial interactions in the proliferation of pancreatic progenitor cells is well established. Here, we provide evidence that the mesenchyme also controls the timing of ␤-cell differentiation. When rat embryonic pancreatic epithelium was cultured without mesenchyme, we found first rapid induction in epithelial progenitor cells of the transcription factor neurogenin3 (Ngn3), a master gene controlling endocrine cell-fate decisions in progenitor cells; then ␤-cell differentiation occurred. In the presence of mesenchyme, Ngn3 induction was delayed, and few ␤-cells developed. This effect of the mesenchyme on Ngn3 induction was mediated by cell-cell contacts and required a functional Notch pathway. We then showed that associating Ngn3-expressing epithelial cells with mesenchyme resulted in poor ␤-cell development via a mechanism mediated by soluble factors. Thus, in addition to its effect upstream of Ngn3, the mesenchyme regulated ␤-cell differentiation downstream of Ngn3. In conclusion, these data indicate that the mesenchyme controls the timing of ␤-cell differentiation both upstream and downstream of Ngn3. Diabetes 55:582-589, 2006