The appearance of fungi in tissues

Abstract

The fungi most commonly encountered in tissue sections in addition to the dimorphic fungi include the hyaline molds belonging to the genera Aspergillus, Scedosporium, and Fusarium, the zygomycetes, and the yeasts, C. albicans and C neoformans. The hyphal forms of Aspergillus species cannot usually be distinguished from those of the other hyaline molds. The pseudohyphae of Candida species can usually be distinguished by observing the regular points of constriction at the sites of blastoconidia production, simulating the shape and arrangement of a link sausage. The co-presence of budding yeast forms also supports the presumptive identification of Candida species. True hyphae may also be seen. Except for rare occasions, the yeast forms of C. neoformans do not produce pseudohyphae. The yeast cells are spherical, vary in size, and are commonly widely separated by capsular material. The yeast forms of C. glabrata are smaller, do not vary in size, and sometimes aggregate in tight clusters. The yeast forms of the dimorphic molds are less frequently encountered, except in endemic areas. Blastomyces dermatitidis and P. brasiliensis produce large yeast cells measuring between 10 to 15 μm in diameter, and are spherical and thick-walled. The yeast cells of B. dermatitidis bud singly, and are attached by a broad base. The yeast cells of P. brasiliensis produce multiple buds, each attached by a narrow connection, simulating a "mariner's wheel." The yeast cells of H. capsulatum are small, ranging from 2 to 5 μm in diameter, and tend to be regular in size. They cluster within the cytoplasm of epithelioid macrophages. When observed in H&E-stained sections, they appear to be surrounded by a pseudocapsular halo. The yeast forms of S. schenckii are often difficult to see in tissue sections. When present, they lie singly and have a distinctive elongated cigar shape, and some cells may be oval to spherical. The endospore-containing spherules of C. immitis are usually easy to identify in tissue sections. Empty, immature spherules are in the size range of B. dermatitidis. If they are not accompanied by the larger endospore-containing forms in other fields of view, a diagnosis may be difficult to establish. The 2 to 5 μm cyst forms of P carinii may have an appearance similar to immature spherules of C. immitis and must be included in the differential diagnosis if cyst-like structures devoid of endospore formation are observed. Recently Hayden and colleagues developed an in situ hybridization assay using specific probes to identify the fungi mentioned within this manuscript, in addition to several additional species. The clinical utility of these probes will be determined as experience is gained with their use.