Fig. 2. SMP formation in the developing mouse embryo. (A-C) Timecourse showing the development of the SMP during mid-gestation. (A) Transverse section through an E9.5 wild-type embryo stained with antibodies for phalloidin (green) and PDX1 (red). The position of the dorsal pancreatic bud is shown with the PDX1 antibody. The white box indicates the region that is magnified (4x) in B, which highlights the organised cellular structure of the SMP that is revealed following staining for f-actin. The cells are elongated and situated perpendicular to the dorsoventral embryonic axis. (C) At E10, the SMP (white arrowheads) and spleno-pancreatic mesenchyme have grown laterally to the left. The SMP that was situated on the right side is now absent and is replaced by a thin mesothelial layer (yellow arrowheads). (D) Dh/Dh mutant embryos at E9.5 differ from wild type in that an unorganised mesenchymal layer of cells surround the midline-positioned dorsal pancreatic bud. The white box indicates the region magnified (4x) in E; the cells are round and densely packed. (F-N) At E9.5, the initially bilateral SMP is characterised by elongated nuclei (F-H). These are lost on the right-hand side in a ventral-to-dorsal direction over the course of 24 hours, and in their place nuclei resembling those in the underlying mesenchyme and adjacent mesothelia are observed (I-N). The yellow boxes in F,I and L indicate the regions magnified (4x) in G,H,J,K,M,N. Dorsal and ventral (in C only) pancreatic buds are shown in red (PDX1 antibody stain). dp, dorsal pancreatic bud. Scale bars: 60 µm in A,C-E.