Lamellar body of a type II alveolar cell in the lung (dog)

Electron microscopy. Extrusion of surfactant material out of a type II alveolar cell (pneumocyte II, great alveolar cell). After fusion of a multilamellar body (1) (cytosome) with the apical cell membrane (note microvilli 2) the electron-dense lamellar content or surfactant (consisting of phosphatidylcholine, phosphoglycerol, cholesterol and SP-proteins) will be discharged. After release the secretion spreads over the alveolar surface and forms an extracellular coating. This detergent reduces the surface tension at the air-fluid interface, it also contributes to stabilize alveolar diameters. At (*) remnants of surfactant the so-called tubular myelin (after adequate fixation). Several packed lamellae resemble stacks of the crystalline phase of a polar lipid-water system.