Fig. 9. Model of Tbx1 function in pharyngeal development (E9.5-E11.5). (Top) In wild type, pharyngeal arches develop as bulges on either side of the pharyngeal endoderm (bold lines, coronal section; rostral, top; caudal, bottom). The arrows indicate approximate time in development; left, earliest, right, latest times. Tbx1, shown as red boxes on the endodermal lining, marks the position of the future pharyngeal pouches, which will extend outward to meet the surface ectodermal clefts. Tbx1 is expressed in the PE during pouch outgrowth but becomes downregulated once the pouches form. The gene is also expressed in the core mesenchyme of the developing arches, indicated by red boxes and circles. It is likely that Tbx1 in the endoderm is involved in epithelial-mesenchymal interactions with the core mesoderm (red arrows). (Bottom) Lack of distal pouch development in Tbx1^-/- and conditional null embryos prevents the segmentation of the distal apparatus, altering NCC migration, growth and differentiation.