Figure 9. Model for the role of Tbx4 and Tbx5 in lung and trachea development.

(A) Lung and trachea specification begins at E9.0 in the ventral foregut and at this time Tbx5 expression (light purple) is adjacent to the presumptive endoderm. Later, Tbx4 and Tbx5 expression (dark purple) is in mesenchyme associated with the lung and trachea. Tbx5 but not Tbx4 is important for specification of bilateral lung buds and the trachea. (B) Magnification of box shown in (A) representing the events in the growing tip during branching morphogenesis. Grey denotes epithelium and purple denotes mesenchyme. Tbx4 and Tbx5 interact with each other and act upstream of the Fgf10 signaling pathway. Decrease in Tbx4 and Tbx5 affects mesenchymal Fgf10 expression and expression of its targets in the epithelium – Bmp4, Spry2 and Etv5 – but not the expression of the epithelial Fgf10 receptor Fgfr2. In addition to Fgf10 expression in the mesenchyme, Tbx4 and Tbx5 also control the expression of an unknown factor(s) (X) that is essential for activation of the Fgf10 signaling pathway. Furthermore, Tbx4 and Tbx5 act upstream of Wnt2 in the mesenchyme. (C) In the trachea and the main stem bronchi Tbx4 and Tbx5 either control Sox9 expression, which in turn regulates cartilage condensation, or Tbx4 and Tbx5 regulate another factor (X) essential for chondrogenesis secondarily affecting Sox9 expression.