Fig. 1. Advent of the spleno-pancreatic mesenchyme and the relationship to the SMP. (A) Reconstruction of an E10.5 embryo following analysis by optical projection tomography (OPT). Yellow and red boxes indicate the planes of the sections shown in B and C. (B,C) Cross-sections through an E10.5 embryo in the region of the stomach (B) and dorsal pancreas (C). Boxed region in C indicates the embryonic position of the images shown in D-G, and of comparable images presented throughout the manuscript. (D-G) Expression of early spleen marker genes in transverse sections through the dorsal pancreatic bud in E10.5 embryos. In situ hybridisation showing expression of Hox11 (D) and Nkx2.5 (E) (seen as red signal) in combination with pancreatic marker anti-PDX1 antibody (green). The double label highlights the relationship of spleen and pancreas location in the early mesenchyme. Expression of Wt1 (F) and capsulin/Pod1 (G) (single label shown as white signal) overlaps Hox11 expression domain and confirms that the dorsal mesenchyme is the region of the spleen rudiment. There is close association with the thickened mesodermal layer that does not express the spleen markers at a high level. The approximate position of the embryonic midline is indicated by the broken line in D. The thickened epithelium that surrounds the mesenchyme is outline by the broken lines in D and E. br, branchial arches; dp, dorsal pancreatic bud; e, eye; fb, forebrain; fl, forelimb; h, heart; hl, hindlimb; st, stomach. Scale bars: 150 µm.