Angioinvasion by mucoraceous zygomycetes. Angioinvasion by mucoraceous zygomycetes. (A) H&E-stained section of a vessel filled with hyphae and inflammatory cells. Tissues infected with the mucoraceous zygomycetes often demonstrate extensive angioinvasion by fungal elements. Neutrophils are the predominant inflammatory cells responding to an infection with these agents. The inflammatory cells stain basophilic (dark), while the hyphae demonstrate an inconspicuous hyaline staining that can often be overlooked. Pale-staining hyphae (arrowheads) often look like hole or bubbles in the tissue. The hyphal elements are nearly obscured on this H&E section due to the inflammatory process seen. For these reasons, it is recommended that a fungus-specific stain such as GMS or PAS be performed to better characterize fungal elements in tissues having thrombosed vessels, extensive tissue necrosis, or hemorrhage. (B) H&E-stained section of a vessel with typical zygomycete hyphal elements in a patient with neutropenia. In contrast to panel A, the hyphal elements are much more prominent in this H&E preparation despite their hyaline staining, due largely to the absence of the inflammatory cells. (C) GMS-stained section of a vessel containing coenocytic fungal hyphae typical of the zygomycetes. Wide ribbon-like hyphae with broad-angle branching is seen (arrowhead). GMS provides excellent contrast to help distinguish hyphae, which stain black or dark gray, from the tissue elements or inflammatory cells, which stain with the pale counterstain. (D) GMS-stained section of a vessel with fungal hyphae penetrating the vessel wall. Infections originate in mucocutaneous sites, from which they may spread hematogenously. Fungal elements invade through the vessel walls from their tissue sites. Mycotic emboli disseminate and may thrombose small vessels in which they are lodged. Fungal elements may invade normal or devitalized tissues at these remote sites by directly penetrating the vessel wall (arrow). All four panels are the same magnification. Bar, 50 μm. Julie A. Ribes et al. Clin. Microbiol. Rev. 2000; doi:10.1128/CMR.13.2.236