Lung development: AT1 and AT2 property

The human respiratory system consists of the upper and the lower respiratory tracts. Anatomically, the lower respiratory tract consists of the trachea, bronchi, bronchioles (terminal bronchioles and respiratory bronchioles), alveolar duct, alveolar duct sacs, and alveoli. Alveoli are composed of two epithelial cell types, cuboidal alveolar type 2 (AT2) cells that secrete surfactant to prevent alveolar collapse and function as stem cells to regenerate alveolar type 1 (AT1) cells during damage repair, and squamous AT1 cells that cover most of the surface area of the alveoli and mediate gas exchange. Previous studies mainly focused on AT2 cells; this review summarizes the current studies on lung development and property of AT1 cells.