Figure 2

Scheme showing the route for de novo surfactant (DPPC) synthesis, secretion as lamellar bodies, transposition to tubular myelin and reutilization of components in lung type II cells. Phospholipids are represented by filled circles (polar head group) with fatty acid chains projecting into the air away from the polar hypophase. Potential examples of single sites of surfactant protein function (SP-A, SP-B, SP-C and SP-D) are shown; multiple functions and sites are likely. New concepts concerning the surface reservoir are indicated by the multiple phospholipid layers at the air-liquid interface [45, 105]. Note the extensive extracellular processing of the surfactant as it is secreted at the apical surface of the type II cell, the air-hypophase interface and reutilization.