Ultrastructure of the Alveolar Cell Complex of Human Lung and its Functional Correlation: A Review

The human lung is a dynamic organ of constant biotransformation. Out of 40 different cell types of lungs, Type‑1 and Type‑2 pneumocytes, pulmonary capillary endothelium, and cells of lung interstitium play a vital role in functioning of the lung. Exploring the structure of these cells would shed light on the pathogenesis of various diseases affecting the lungs. A systematic literature search was conducted using three research databases – Medline, Google Scholar, and Cochrane. Out of ninety articles published between the year 1981 and 2020, 41 articles were included in the study based on the inclusion and exclusion criteria. Among the cellular components of the alveolar cell complex of the lung, the Type‑2 pneumocytes show versatility in its function in the form of surfactant synthesis, immune reaction, and regenerative capacity. The pathogenesis of the lung injury is due to the loss of functions of the Type‑2 pneumocytes and the intercellular junction functional discrepancy. Irrespective of the cause of lung disease, there is an increase in permeability in the alveolocapillary membrane leading to interstitial edema. This, in turn, leads to hyperventilation, low oxygen saturation, and a decrease in lung capacity. The pericytes present in the interstitium have a preventive role to play in lung injury and potential stem cell function. Keywords: alveolar epithelial cells, capillary endothelium, lung interstitium, type-1 pneumocytes, type-2 pneumocytes