[Development of connective tissue and surfactant production in fetal lung]

The important processes in fetal pulmonary development are devoted to gain large surfaces for gas exchange and surfactant production. Basement membrane components, collagen fibers and elastic fibers are formed immediately around the epithelium in the early stage of lung development. Primordia of alveolar septa composed of elastic fibers and smooth muscle cells, develop in the glandular stage and form alveolar septa by protruding into the glandular lumina in later stages. The differentiation of alveolar walls, including the appearance of surfactant-producing cells, begins earlier in the proximal portion than in the distal portion of primitive alveoli. The process of fibrosis and regeneration in lung fibrosis defectively mimics the embryogenesis of the lung. Pulmonary structural remodeling occurs when the epithelial basement membrane, a scaffold for regenerating epithelial cells, is lost because of intra-alveolar fibrosis. In panacinar emphysema, simple and dilated air spaces are formed by the degradation of elastic fibers in alveolar septa, and this process may be reverse to the formation of alveolar septa in developing lung.