ABSTRACT
The alveolar epithelium is a thin continuous liquid lining layer primarily composed of two types of epithelial cells, i.e., alveolar type I (ATI) and alveolar type II (ATII) cells. ATI cells have a broad flattened morphology and cover about 95% of the gas exchange surface of the lung. Whilst, ATIIs are small cuboidal cells with characteristic lamellar inclusions and apical microvilli that line the remainder of the alveolus (about 5%). ATII cells make and secrete a pulmonary surfactant, which reduces the surface tension in the alveoli preventing alveolar collapse during respiration and reducing the energy required to inflate the lungs, thereby increasing pulmonary compliance. These cells also transport ions from the apical to the basolateral surface so as to keep the alveoli relatively fluid free and have roles in the innate immune response. Importantly, they are the progenitor cells for the alveolar epithelium in the adult lung. Having a very thin cytoplasm, a limited number of mitochondria and covering a higher surface area, ATI cells are easily damaged during lung injury, after which they are replaced by ATII cells through the alveolar epithelium regeneration process. Despite ATII cells are considered an essential part of this process, numerous interstitial lung diseases are characterized by hyperplastic ATIIs: in fact, they may also contribute to the fibroproliferative reaction by secreting a number of growth factors and proinflammatory molecules.