pneumocytes

Pneumocytes are specialized epithelial cells lining the alveoli of the lungs. There are two major types: alveolar type I (AT1) and alveolar type II (AT2) cells.

AT1 cells are extremely thin, squamous cells that cover about 95% of the alveolar surface and form the primary barrier for gas exchange between air in the alveoli and the blood in surrounding capillaries. They are highly flattened, have extensive cytoplasmic processes, and have limited capacity to divide, relying on AT2 cells for regeneration after injury.

AT2 cells are more cuboidal and are fewer in number but crucial for alveolar homeostasis. They produce and secrete pulmonary surfactant, a phospholipid-protein mixture that reduces surface tension and prevents alveolar collapse at end expiration. Surfactant is stored in lamellar bodies and contains specific proteins SP-A, SP-B, SP-C, and SP-D associated with innate immunity and surfactant function. AT2 cells also serve as progenitor cells that can proliferate and differentiate into AT1 cells during repair of the alveolar epithelium.

Clinical relevance: surfactant deficiency or dysfunction in newborns leads to respiratory distress syndrome. Injury to alveolar epithelium and surfactant failure contribute to acute respiratory distress syndrome in adults. Normal lung maintenance requires turnover and coordinated activity of both cell types during development and regeneration.