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Surfactant phospholipids are synthesized in the endoplasmic reticulum by multiple enzymatic processes and then transported through the Golgi apparatus to their storage site in lamellar bodies. PC is synthesized mainly from CDP choline and diacylglycerol through the cholinephosphotransferase pathway, phosphatidylglycerol and phosphatidylinositol from phosphatidic acid through the phosphatidate cytidylyltransferase pathway.
All surfactant proteins are synthesized by the type II pneumocytes, SP-A also by Clara cells in the bronchiolar epithelium.
In type II pneumocytes, SP-A is synthesized and stored mainly in multivesicular bodies of the cytosol and secreted into the alveolar spaces through pathways different from those involved in the secretion of surfactant phospholipids and hydrophobic proteins. Uptake and recycling of SP-A by type II cells has recently been verified.
Unlike SP-A, SP-B and SP-C are synthesized in endoplasmatic reticulum, stored in lamellar bodies, and co-secreted with phospholipids into the alveolar spaces. This explains why the relative content of hydrophobic proteins is higher in lamellar bodies than in extracellular surfactant. Both hydrophobic proteins can be taken up by type II cells for recycling.