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 lysosomal enzymes is advanced. Small quantities of thorium dioxide were ingested by the squamous pneumocytes and a little transferred to interstitial macrophages, but this is a minor route of disposal, the bulk of the material being ingested by alveolar macrophages. The limited phagocytic potential of the alveolar epithelium contrasts with that of the alveolar macrophages and is not taken to support the concept of an epithelial derivation of alveolar macrophages.
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