Ultrastructural Studies on the Yeastlike and Mycelial Phases of Sporotrichum schenckii

Abstract

Fine details of the internal and external morphology of the yeastlike and mycelial phases of the dimorphic fungal pathogen Sporotrichum schenckii as seen in ultrathin sections are described and illustrated by electronphotomicrography. Comparisons of yeastlike phase ultrastructure were made using two different methods of fixation and embedding. The internal morphology of the two forms of yeastlike S. schenckii was in many ways similar to that of similarly dimorphic fungi and yeasts studied by other authors. However, the use of the glutaraldehyde-osmium in agar fixation technique suggests the presence of an electron transparent capsular or slime layer with associated electron dense microfibrils to be present external to the cell wall of the yeastlike phase but not the mycelial phase. Mycelial phase S. schenckii was found to contain many of the internal microstructures reported for other filamentous dimorphic fungi. Conidia production in agitated liquid culture was found to be restricted since only rare sessile conidia were observed.

Sporothrix schenckii and related species of Ceratocystis.

“…The cell wall thickness in mycelial-phase cells was 80 to 140 nm. Each sessile conidium had degenerative changes in its internal organization, and a thickened plate-like structure was present at the point of hyphal attachment (85). The cell walls of pigmented conidia were 330 nm thick and had two layers; these were 210 nm thick (inner layer) and 120 nm thick (outer layer).…”

Ultrastructural studies of the mycelium-to yeast transformation of Sporothrix schenckii

“…Scattered throughout the cytoplasm are organelles described previously for the ovoid YP cell ofS. schenckii (15). Conspicuous material binding dialyzed iron is present along the outer limits of the cell wall, and appears to consist of lighter inner and a more electron-dense outer layer measuring about 60 and 20 nm, respectively.…”

“…The hyphal wall appeared finely roughened with what may be a similar substance, but its relative thickness was always much less conspicuous than that observed for the yeastlike cell. Although this microfibrillar material was demonstrable by the usual heavy-metal staining used in electron microscopy (14,15), staining with dialyzed iron enhanced markedly the electron density of the microfibrillar material. This selective ultrastructural staining characteristic with dialyzed iron suggests that this microfibrillar material associated typically with the yeast cell wall of S. schenckii is composed in part of an acid mucosubstance similar to that described by Mahvi et al (19) for the mucoid capsule of Cryptococcus neoformans.…”

“…The YP of S. schenckii is characterized by ovoid to fusiform cells which may show either single or multiple budding (6). The use ofglutaraldehyde-osmium in agar fixation techniques suggests the presence of an electron-transparent capsular or slime layer with associated electron-dense microfibrils to be present external to the cell wall of the YP cell but not the MP cell (15). Figure 1 shows an ultrathin section through a budding ovoid YP cell from a control YP culture after fixation in glutaraldehyde and staining with dialyzed iron.…”

“…In recent years, several studies on the fine structure of the yeastlike phases (YP) and mycelial phases (MP) of S. schenckii have appeared in the literature (14,15,18), whereas aspects of the ultrastructural reorganization during in vitro YP-to-MP conversion have been studied by Lane and Garrison (16). To date, no comparable information is available on sequential ultrastructural events of the reverse MP-to-YP conversion of this dimorphic fungus.…”