Ultrastructural analysis of asteroid bodies: evidence for membrane lipid bilayer nature of components.

Electron microscopic examination of multiple asteroid bodies (ABs) from pulmonary tissue revealed that the core and spokes of these structures are composed of phospholipid bilayers organized in lamellar and tubular forms. These morphologic patterns followed the ones described previously for the lipid-water bulk interface systems. On light microscopy the ABs appeared surrounded by a halo of vacuoles corresponding ultrastructurally to loosely arranged myelin membranes. Transitional forms, between the tightly arranged phospholipid membranes forming the spokes and the loose myelin membranes surrounding them, were seen. Myelin membranes were seen in the cytoplasm of multinucleated giant cells, independent of ABs, in close association with intracellular spaces (lumina) and lined by abundant microvilli identical to the surface microvilli. These spaces were present in central areas of the cell and probably represent surface membrane remnants of partially fused cells. No centrioles, true microtubules, or collagen were seen in the multiple ABs examined. Immunoperoxidase stains for vimentin and tubulin were also negative in ABs. No calcium or phosphorus was seen by X-ray microanalysis. Thus the morphologic evidence is supportive of the hypothesis that, during or after the process of fusion of activated macrophages, the resulting excess of membranes (surface and lysosomal) contributes to the formation of ABs. No evidence supporting the notion that ABs are formed by the aggregation of preexisting cytoskeletal components was found.