pneumocytter

Pneumocytter, commonly called pneumocytes in English, are the epithelial cells that line the alveoli in the lungs. They are categorized into two main types: type I and type II.

Type I pneumocytes are extremely thin, squamous cells that cover about 95 percent of the alveolar surface. They form the principal gas-exchange barrier between air and blood. Their thin cytoplasm and tight junctions minimize diffusion distance and prevent fluid leakage into the airspaces. They have limited capacity to proliferate.

Type II pneumocytes are cuboidal cells scattered among the type I cells. They secrete pulmonary surfactant, a lipid-protein complex that reduces surface tension and stabilizes the alveoli during breathing. Type II cells also function as progenitor cells for the alveolar epithelium; after injury, they proliferate and differentiate into type I cells to restore the alveolar lining. They contain lamellar bodies that store surfactant.

Surfactant comprises phospholipids such as dipalmitoylphosphatidylcholine and surfactant proteins SP-A, SP-B, SP-C, and SP-D. Beyond reducing surface tension, these components contribute to innate immunity and maintenance of the alveolar surface film.

Clinical relevance: injury or dysfunction of pneumocytes contributes to conditions such as acute respiratory distress syndrome (ARDS) and other forms of alveolar damage. Infections, inhalational injuries, or chronic lung disease can impair pneumocyte function and alveolar repair processes.

Development: during fetal lung development, type II pneumocytes appear early and begin surfactant production toward the end of the second trimester, enabling postnatal respiration. Alveolar maturation continues after birth.