Pathways of clearance in mouse lungs exposed to iron oxide aerosols

The history of particle clearance was studied in lungs of mice serially sacrificed at intervals up to 14 months following single exposures to an aerosol of submicronic, particulate, iron oxide used as a similitude for atmospheric dust. Clearance was followed by light microscopy in unstained and Prussian blue stained frozen and plastic embedded sections, as well as by electron microscopy, where iron oxide can be recognized by its form. Related problems were investigated through histochemical demonstration of acid phosphatase activity in pulmonary lysosomes and Prussian blue staining of various tissues after administration of iron compounds by gastrointestinal and vascular routes. The iron particles settle extensively but not uniformly on pulmonary alveolar surfaces. Clearance is centripetal and involves two mechanisms, an extracellular mechanism fed by fluid currents sweeping across the surfcace, and cellular mechanism principally involving alveolar macrophages. In the early post exposure period both actively remove deposited particles predominantly through the pulmonary airways. By 24 hours uncleared residues have become ingested and clearance thereafter results mainly from cellular action. Macrophages enter bronchial passages where they sometimes continue to pursue normal activities. A chronic phase of clearance begins when deposited particles become sequestered in macrophages of pulmonary connective tissues. These cells are reached by several routes, not least by crossing the bronchial epithelium. Particle clearance from these macrophages is very slow, and residue-containg cells eventually congregate in lymphoid tissues surrounding major bronchi. These findings are discussed as they help to develop an overall picture of clearance from the lungs and as they bear on related topics, such as functional roles of alveolar and pulmonary connective tissue macrophages and the pathogenesis of chronic bronchial disease.