Ruthenium red staining of HIV-1 infected macrophages. Monocyte-derived-macrophages infected with HIV-1 (NLAD8) for 14 days were fixed on ice in the presence of ruthenium red (RR) dye and embedded in Epon for transmission electron microscopy as described . (A) Viral assembly compartments negative for the RR dye were observed such as the one which is framed. Electron-dense deposits of ruthenium red-positive material were seen in lipids droplets, which lied deep within macrophages and were especially numerous near HIV-1 virus-containing vacuoles (see white asterisks). However, a majority of virus-containing compartments remained RR negative (see black asterisks). (A') Enlargement of the framed area in A. (B) Viral assembly compartments containing viral particles positive for the RR dye were also observed. Note the presence of a microchannel emanating from the central compartment (black arrowhead). (C) A "sponge-like structure" is shown in the center of the panel exhibiting highly interconnected membranes. Such structures were positive for the RR dye and very frequently were found in the vicinity of viral compartments (see above the structure). Below the structure, note the presence of numerous secondary lysosomes containing small osmiophilic particles (a few examples are pointed by black arrowheads). Bars, 400 nm.