Fig. 1. Dorsal Pdx1-positive endoderm cells interact with endothelial cells more extensively than ventral Pdx1-positive endoderm cells. Double cell labeling for Pdx1 (DAB immunostaining, brown) and ß-galactosidase (X-gal, blue) on transverse sections of Flk1^lacZ/+ embryos at E8.5-9.5 (somite stages are indicated on the top of each panel). nc, notochord. The data shown are representative of multiple embryos assayed and sections throughout midgut regions. The boxed regions in D,F,H,J,L,N are magnified in E,G,I,K,M,O, respectively. (A, F) Before the emergence of Pdx1-positive cells, the aorta only has limited contact with the dorsal endoderm, laterally. (B,C) By 12-15S, the aorta moves medially and interacts extensively with the endoderm in a portion of the Pdx1-positive domain, first evident at 15S. No mesenchyme cells were detected between portions of the endoderm and the aorta at this stage (green arrow). (D) As Pdx1-positive cells form the dorsal pancreatic bud, dorsal mesenchyme cells (long black arrow) interpose between them and the fused aorta. At this stage, small capillaries can be found in the dorsal mesenchyme (E, arrows). (F,G) At 8S, the prospective ventral foregut endoderm is in the proximity of the sinus venosus. (H,I) At 10S, during embryo turning, the vitelline vein on the embryo's right side is distal to the initial few Pdx1-positive cells in the ventral endoderm. (J-M) A thin line of mesenchyme cells continuously separates the few, nascent Pdx1-positive endoderm cells from the vitelline veins (K,M; cells at end of green arrow). (N) By 26S, when the ventral pancreatic domain is clearly surrounded by mesenchyme cells, small capillaries can be found in the mesenchyme (O, arrow), as seen in the dorsal region. Original magnifications are indicated at the bottom of each panel.