Fig. 2

Developmental stages and the WNT/β-catenin signaling. In the embryonic stage (<E9.5 in mouse, <4 weeks in human), Wnt2/2b is expressed within surrounding mesenchyme for the anterior foregut specification and the onset of lung specification. In the pseudoglandular stage (E9.5 to 16.5 in mouse, 4 to 16 weeks in human), several Wnt ligands, receptors, and extracellular modulators are expressed within proximal or distal airways and surrounding mesenchyme and coordinate proximal-distal patterning of branching morphogenesis. In the canalicular stage (E16.5 to 17.5 in mouse, 16 to 24 weeks in human), WNT/β-catenin signaling regulates the balance of proliferation and differentiation of the distal epithelium. In the saccular to alveolar stage (E17.5 to postnatal day 5, postnatal day 5 to day 30 in mouse, 24 weeks to 36 weeks, 36 weeks and after in human), WNT/β-catenin signaling may organize the alveolarization and vascularization and the formation of secondary septa