Fig. 9. Model for asymmetry in the spleno-pancreatic region. The model describes the major events that occur in lateral organ morphogenesis. The left SMP is under the influence of the LR genetic cascade and is maintained whereas the right SMP is lost. Cell proliferation in the SMP appears to be the motive force in the lateral growth that occurs in the SMP between E9.5 and E10.5 of development. During this period, the population of mesenchymal cells underlying the SMP expands considerably and the dorsal region is induced (perhaps by the associated SMP) to become spleen. Outgrowth of the spleno-pancreatic region is compromised by loss of Bapx1, as is Fgf10 expression in the ventral SMP. The high expression of Fgf10 from the SMP may be the chemotactic factor to which the dorsal pancreas responds.