Fig. 3. The SMP is under the genetic control of LR signalling. Asymmetric expression of Pitx2 (A) and Barx1 (B) on the left-hand side of the mesenchyme and associated SMP (arrowheads) is detected at E9.5. At E10.5, Pitx2 is not detected, while Barx1 (arrowheads in C) is detected in the extreme lateral domain of the spleno-pancreatic mesenchyme. Bapx1 expression is bilateral at E9.5 (D), but is higher on the leftward side (arrowheads) and is expressed in the left lateral domain at E10.5 (E). The pancreatic bud is outlined by a broken red line. (F,G) Immunohistochemical analysis of the inv mutant mouse using phalloidin (green) and PDX1 (red) antibody at E10.5. SMP formation is associated with the leftward-oriented spleno-pancreatic mesenchyme in wild type (F) and inv heterozygous embryos (data not shown), but develops in the opposite orientation in the homozygous mutant embryos (G).