The manner in which dust particles are transported from alveoli into the pulmonary parenchyma and lymph nodes has been the subject of controversy for over 8o years. During the past 50 years opinions in regard to this mechanism have become largely set, with only a few dissenting. The prevailing concept is that macrophages emigrating to the free surface of the respiratory membrane engulf dust particles and then immigrate into the parenchyma. This opinion is recorded by most authors of standard textbooks. The contrary opinion, that dust particles per se penetrate the pulmonary parenchyma and are carried by tissue fluid and lymph to their destination, finds support in but a few texts."' Several authors adopt a "middle of the road" opinion and state that both mechanisms are operative.
Some of the earlier writers who argued for direct penetration of dust particles into lung tissue were Slavjansky' (I869), Ruppert" (1878), and Arnold" (1885). In more recent times Drinker' (922) maintained that certain silicates may penetrate directly into the pulmonary parenchyma. Cameron and Lang' (1933) denied that emigrating macrophages immigrated back into the lung tissue, while Robson, Irwin, and King (1934) held that silica particles penetrated the lung substance without the aid of phagocytic cells.
The theory of the "emigrating-immigrating" macrophage was supported by v. Ins' (I878), Schottelius (I878), and Shingu (1910). In I9I4 Klotz26 stated, "It would appear, however, with the more recent studies that the migration of the dust particles from the air sacs occurs only through the agency of certain wandering cells." Permar mn 1920, gave this concept the much needed and widely quoted experimental support. Other supporters of this vew include Haynes (1931), Gardner' (1932), and Belt (I934).
A number of writers espoused both concepts. They are Knauff (1867), Slavjansky (i869), Ruppert (I878), Lemon and Higgins (1932), Robertson (1941), and Drinker (1952).
During microscopic studies of the lungs of animals subjected to experimental pneumoconioses by inhalation as well as by intratracheal injection, large numbers of extracellular particles were noted in interstitial positions. The existence of the extracellular particles in such sites was difficult to explain on the basis of the prevailing concept. Furthermore, it was recalled that in many sections of lungs from human necropsy material the arrangement of interstitial anthracotic pigment deposits suggested that the material was extracellular. It was noted frequently that assumptions of an intracellular position of the pigment were arbitrary and rested on very tenuous grounds.
The theory that particles per se may penetrate the lung substance seemed intriguing and appeared to have support in past experimental observations. The following experiments were designed to test this possibility.
