Which cells act as progenitors to replace Type I cells and produce surfactant?

• A. Type I cells
• B. Type II cells
• C. Type III cells
• D. Type IV cells

Answer: (B)

Alveolar Type II pneumocytes are the cells that act as progenitors to replace Type I cells and they are the primary source of surfactant. Type II cells are cuboidal and sit alongside the thin, expansive Type I cells; they proliferate in response to injury and can differentiate into Type I cells to reestablish the gas-exchange surface. They also produce surfactant, a lipid-rich mixture that reduces surface tension and prevents alveolar collapse. Type I cells form the thin barrier for gas exchange but do not serve as progenitors, and the other listed cell types are not the alveolar epithelial progenitors or surfactant-producing cells.