Electron cytochemical evidence for lysosomal-like equivalents in Histoplasma capsulatum.

Electron cytochemical localizations of acid phosphatase, aryl sulfatase, deoxyribonuclease, adenylate cyclase, and c-AMP phosphodiesterase activity sites in thin sections of cells of the two growth phases of the zoopathogenic Histoplasma capsulatum are described and illustrated by transmission electron micrographs. Various activity sites of these enzymes included the cytomembranes of the nucleus, mitochondria, and endoplasmic reticulum. At the same time, electron opaque reaction products were sequestered within membrane-bound, vacuolar regions of the cytosol. These vacuoles may be ontogenically related to membranous or vesicular inclusions commonly seen in thin sections of glutaraldehyde osmium tetroxide-fixed cells. These enzymatically-active vacuoles are believed consistent with previous descriptions of fungal lysosomal-like structures found in certain other fungi. Lysosomal-like vacuoles of H. capsulatum may provide a means of compartmentalization of various hydrolytic enzymes involved in catabolism and mobilization of storage reserves, and perhaps to function as well in other aspects of the life cycle of this important pathogenic dimorphic fungus.