The localisation of particles retained in the trachea of the rat.

Particles of 133BaSO4 were deposited on the surface of rat trachea by intra-tracheal injection. Aggregates of particles were located in the trachea by autoradiography and electron microscopy. Following deposition, particles not rapidly removed by muco-ciliary clearance remained on the epithelium for some time. After 2 hr most had been ingested by macrophages on the surface, though some were still free in the mucus. By 24 hr, 74 per cent. of the aggregates remaining were beneath the epithelium in the lamina propria, and after 7 days almost all of them were in or beneath the epithelium. All the buried particles identified by EM were within macrophages. After 24 hr the particles in the tracheal wall were beneath epithelium which was not ciliated columnar, but cuboidal or flatter, with fewer or no ciliated cells and infiltrated with lymphocytes. It is suggested that particle retention in airways is accomplished by ingestion by macrophages which then migrate through this type of epithelium.