Secretion and fluid transport by alveolar type II epithelial cells

Alveolar type II cells are cuboidal epithelial cells that line parts of the alveolar wall and are readily recognized by their characteristic lammelar inclusions. These inclusions are the predominant intracellular storage form of surface active material. In adult rat lungs, type II cells comprise 14 percent of total parenchymal lung cells and over 3 percent of the alveolar surface. Physiologic functions of type II cells include: synthesis, storage, and secretion of surface active material; differentiation into alveolar type I cells; hyperplasia and adaptation after lung injury; secretion of non-surfactant proteins; and transport of fluid and electrolytes from the alveolar subphase into the interstitium. In this review we discuss two of these functions, namely: secretion of surface active material and transepithelial fluid transport.