Figure 1.

WNT/β-catenin pathway on the embryological pulmonary development in humans. Five stages classically follow one another: embryonic, pseudo-glandular, canalicular, saccular, and alveolar. WNT/β-catenin signaling reaches a maximum activation at the 17th week (end of the pseudo glandular stage or beginning of the canalicular stage). In the middle of the canalicular stage (around the 21st week), canonical WNT pathway activity decreases dramatically, a necessary prerequisite for the correct realization of the alveolar stage. In the event of premature birth, the thick alveolar walls do not allow sufficient gas exchanges, which can lead to pulmonary hypoxia. Hypoxia activates the canonical WNT/β-catenin, impairs the alveolar stage, and promotes the synthesis of myofibroblasts and subsequent fibrosis, with abnormalities in the respiratory function.