Production and Degradation
Surfactant production in humans begins in Type II cells during the terminal sac stage of lung development. Lamellar bodies appear in the cytoplasm at about 20 weeks gestation. These lamellar bodies are secreted by exocytosis into the surface water layer lining the alveolar airspace, where the surfactant forms a meshwork of tubular myelin. Term infants are estimated to have an alveolar storage pool of approximately 100 mg/kg of surfactant, while preterm infants have an estimated 4–5 mg/kg at birth.
Clara cells also produce a component of lung surfactant.
Alveolar surfactant has a half life of 5 to 10 hours once secreted. It can be both broken down by macrophages and / or reabsorbed into the lamellar structures of type II pneumocytes. Up to 90% of surfactant DPPC (dipalmitoyl phosphatidylcholine) is recycled from the alveolar space back into the type II pneumocyte. This process is believed to occur through SP-A stimulating receptor mediated, clathrin dependant endocytosis. The other 10% is taken up by alveolar macrophages and digested.
Read more about this topic: Pulmonary Surfactant
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