Fig. 3. Dynamics of endocrine specification and differentiation. (A-F) Brightfield photomicrographs of Pdx1 immunostaining at various stages of mouse dorsal pancreas (dp) development. From E11.5-E15.5, Pdx1 is expressed throughout the pancreatic epithelium (as well as in the posterior stomach, st), and is subsequently downregulated in acini (ac) and ducts (du) while being maintained in islet ß-cells (is). (G-L) Confocal immunofluorescence photomicrographs at equivalent stages, for the pan-epithelial marker E-cadherin (green) and the islet precursor marker Ngn3 (red). Ngn3 expression is rare at E11.5, dramatically peaks during the secondary transition (E13.5-E15.5) and declines again at E17.5, becoming undetectable in neonatal and adult pancreas. Arrowheads indicate proto-acinar clusters at the periphery of the branched epithelium, from which Ngn3 expression is consistently excluded. (M-R) Confocal detection of glucagon (green) and insulin (red). Glucagon⁺ -cells are relatively common at E11.5 and E13.5, wheras large numbers of insulin⁺ ß-cells are not detected until after E13.5. From E17.5 onwards, endocrine cells aggregate into recognizable islets, with ß-cells at their cores and -cells distributed peripherally. Scale bar in all images, 50 µm.