Phagocytic potential of pulmonary alveolar epithelium with particular reference to surfactant metabolism.

Finely divided thorium and carbon were injected into the rat lung via the trachea and their uptake was studied electron microscopically. Small amounts of thorium were found in the lamellar vacuoles of the granular pneumocytes. Their presence was maximal at 24 hours and they later disappeared from these cells. This phenomenon is thought to represent a passive inflow as the vacuoles open on to the alveolar surface to discharge their content of surfactant, rather than evidence that the granular pneumocyte is concerned in surfactant disposal. An explanation reconciling the secretory nature of the vacuoles with their previously demonstrated content of lys,osomal enzymes is advanced. Small quantities of thorium dioxide were ingested by the