text: Surfactant production in humans begins in type II cells during the alveolar 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 alveolar lining fluid, where the surfactant forms a meshwork of tubular myelin Full 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.
  Club 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 (dipalmitoylphosphatidylcholine) 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 dependent endocytosis. The other 10% is taken up by alveolar macrophages and digested.