Fig. 1

Diagram of the alveolar-capillary membrane illustrating the physical stimuli experienced by cells at physiological conditions of normal breathing. In both alveolar epithelial cells and capillary endothelial cells, cyclic stretch caused by inspiration/expiration changes cell size and modifies the forces (represented by red arrows) acting on the cytoskeleton through cell-cell junctions and focal adhesions to the extracellular matrix (ECM) scaffold. Capillary endothelial cells are also subjected to shear stress forces caused by blood circulation. The stiffness of the ECM is sensed by the cytoskeleton through physical contacts in focal adhesions. Oxygen concentration experienced by cells changes along the airways tree and the capillary system. Alveolar epithelial cells are subjected to an air-liquid interface environment. See text for detailed explanation of the effects of these physical stimuli on stem cells