type II pneumocyte
Summary
Type II pneumocytes are cuboidal epithelial cells that produce pulmonary surfactant and serve as progenitor cells for type I pneumocytes. They are crucial for reducing surface tension in alveoli and maintaining alveolar stability, preventing collapse during expiration.
Detail
Type II pneumocytes (also called granular pneumocytes) are specialized epithelial cells located in the alveolar epithelium, comprising about 5% of the alveolar surface but 60% of alveolar epithelial cells by number. Their primary function is synthesizing, storing, and secreting pulmonary surfactant via lamellar bodies - organelles containing phospholipids (primarily dipalmitoylphosphatidylcholine/DPPC) and surfactant proteins (SP-A, SP-B, SP-C, SP-D). Surfactant reduces surface tension according to the Law of Laplace, preventing alveolar collapse and reducing the work of breathing. Type II pneumocytes also serve as progenitor cells, differentiating into type I pneumocytes during lung repair. They contain characteristic lamellar bodies visible on electron microscopy and express surfactant protein markers. Dysfunction leads to conditions like neonatal respiratory distress syndrome (surfactant deficiency in premature infants), acute respiratory distress syndrome (ARDS), and idiopathic pulmonary fibrosis. These cells are also targets for various toxins and can undergo malignant transformation in adenocarcinoma.
Sources
- Robbins and Cotran Pathologic Basis of Disease
- West's Respiratory Physiology
- First Aid for the USMLE Step 1
- Murray & Nadel's Textbook of Respiratory Medicine
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