Detail of lamellar body (surfactant) and type I alveolar cell in lung (rat)

Electron microscopy. After fixation the extracellular lining of surfactant (phosphatidylcholine, phosphoglycerol, cholesterol and proteins) will often be present as free packed lamellae in the alveolar space (4). This so-called tubular myelin is observed as stacks (5) of lipid crystals and aqueous layers of proteins and proteoglycans (crystalline phase of a polar lipid-water system). (1) point to type I alveolar cell, (2) indicates common basal lamina of alveolar cell and endothelium (3) of capillary (C). Note the transport vesicles (*) in both (1) and (3). (↔) is the air-blood barrier is the so-called thin-walled area. Thin-walled areas are most favourable to gas exchange and alternate with thick-walled areas (not shown here) consisting of supporting fibers, extracellular matrix and cells of the alveolar framework that separate the alveolar epithelium from the capillaries.