Nature and significance of the electron-dense bodies of the endospores of rhinosporidium seeberi: their reactions with MTT (3-[4,5-dimethyl-2-thiazolyl]-2,5-diphenyl-2H-tetrazolium bromide) and TMRE (tetramethyl-rhodamine ethyl ester).

The most conspicuous internal structures of the endospore of Rhinosporidium seeberi are the 10-16 spherical, 1.0-1.5-microm bodies that have been termed electron-dense bodies (EDB) or lipid bodies (LB); some authors have regarded them as nutritive stores of lipid or protein while others have regarded them as DNA-containing, ultimate generative units of R. seeberi. The literature is reviewed as supporting either view. We report, for the first time, (i) reactions of the endospores with the salt MTT (3-[4,5-dimethyl-2-thiazolyl]-2,5-diphenyl-2H-tetrazolium bromide) and (ii) ultrastructural appearances; and suggest that both views on the nature of spherical bodies are valid (i.e. the endospore contains both EDB, and lipid or protein bodies). Well-marked reduction of the MTT with the production of deep-purple staining was seen in a proportion of the spherical bodies, probably the EDB, suggesting that they are actively metabolizing, viable elements with dehydrogenase activity, and that these bodies are the thick-walled electron-dense bodies described as EDB and visualized in the transmission electron microphotograph illustrated in this paper. The spherical bodies showed fluorescent labeling with acridine orange and with ethidium bromide supporting the idea that they contained nucleic acids. TMRE (tetramethyl rhodamine ethyl ester), a mitochondrion-specific dye, also labeled the intra-endosporial spherical bodies. Other bodies (LB) of a similar size that were MTT-non-reducing, electron lucent, and have no organized structure, are probably the lipid or protein containing, inert, nutritive storage bodies suggested by previous authors.