Which type of pneumocytes are specialized for gas exchange and do not regenerate?

• A. Type 1
• B. Type 2
• C. Both
• D. None

Answer: (A)

Type I pneumocytes are the thin, squamous cells that line most of the alveolar surface. Their extremely flat, minimal cytoplasm creates a very thin diffusion barrier between the air in the alveolus and the blood in the capillaries, which is ideal for efficient gas exchange of oxygen and carbon dioxide. They cover the majority of the alveolar surface, but they have little or no capacity to divide and renew themselves. When injury occurs, regeneration mainly comes from Type II pneumocytes, which proliferate and can differentiate into Type I cells to repair the lining. Type II cells are cuboidal, produce surfactant to reduce surface tension, and act as the progenitor population for the alveolar epithelium. Because Type I cells are specialized for gas exchange and do not regenerate, the correct choice emphasizes Type I pneumocytes.