Morphology of pulmonary intravascular macrophages (PIMs) in ruminants: ultrastructural and cytochemical behavior of dense surface coat.

Recent studies have indicated that pulmonary intravascular macrophages (PIMs) are a resident cell population which in structure and function resemble mature macrophages of the mononuclear phagocyte system (MPS) in various domestic species, particularly the ruminants. The ultrastructural features of PIMs of the goat and calf lungs were studied by using vascular perfusion and direct airway instillation of fixatives. Staining with tannic acid as a component of paraformaldehyde-glutaraldehyde-based fixative revealed the presence of an electron-dense coat on the surface of the cell membrane of the PIMs. The surface coat disappeared after heparin infusion and after enzymatic digestion with lipolytic lipase, suggesting that the surface coat was predominantly lipoprotein in nature. The lipoprotein coat was organized in the form of a linear chain of spherical globules with a consistent periodicity created by the intervening translucent space between individual globules. The surface coat was separated from the outer-leaflet of the cell membrane by an empty space measuring 35-39 nm in width. PIMs possessed a significant number of coated pits and coated vesicles, the cell organelles of receptor-mediated endocytosis of lipoproteins. In concurrence with the coated pits and vesicles, microtubules, multivesicular bodies, and lipoprotein-positive vesicles were also observed. It is conceivable that PIMs are involved in lipid metabolism and are the major source of vasoactive substances, which significantly influence both the dynamics of pulmonary circulation and the surfactant turnover of the ruminant lung.