Fig. 2.

Cell-cell junctions: Tight junctions, adherens junctions, and desmosomes are the three main junctional complexes connecting adjacent epithelial cells. Tight junctions are the most apical protein complexes and regulate epithelial barrier paracellular permeability. Adherens junctions and desmosomes stabilize cell-to-cell adhesion and maintain lung homeostasis. They also regulate the actin-cytoskeletal organization and confer mechanical strength to the alveolar epithelial barrier. Gap junctions are essential for intercellular communication and secretion of surfactant. Hemidesmosomes bind basal epithelial cells to the basement membrane. Hereditary mucoepithelial dysplasia (HMD) affects desmosomal structures. ZO, zona occludens; IFs, intermediate filaments.