Sporotrichosis, which is caused by the dimorphic fungus Sporothrix schenckii, is currently distributed throughout the world, especially in tropical and subtropical zones. Infection generally occurs by traumatic inoculation of soil, plants, and organic matter contaminated with the fungus. Certain leisure and occupational activities, such as floriculture, agriculture, mining, and wood exploitation, are traditionally associated with the mycosis. Zoonotic transmission has been described in isolated cases or in small outbreaks. Since the end of the 1990s there has been an epidemic of sporotrichosis associated with transmission by cats in Rio de Janeiro, Brazil. More than 2,000 human cases and 3,000 animal cases have been reported. In humans, the lesions are usually restricted to the skin, subcutaneous cellular tissue, and adjacent lymphatic vessels. In cats, the disease can evolve with severe clinical manifestations and frequent systemic involvement. The gold standard for sporotrichosis diagnosis is culture. However, serological, histopathological, and molecular approaches have been recently adopted as auxiliary tools for the diagnosis of this mycotic infection. The first-choice treatment for both humans and cats is itraconazole.

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Sporothrix schenckii and Sporotrichosis

Sporulation of the baker's yeast Saccharomyces cerevisiae is a response to nutrient depletion that allows a single diploid cell to give rise to four stress-resistant haploid spores. The formation of these spores requires a coordinated reorganization of cellular architecture. The construction of the spores can be broadly divided into two phases. The first is the generation of new membrane compartments within the cell cytoplasm that ultimately give rise to the spore plasma membranes. Proper assembly and growth of these membranes require modification of aspects of the constitutive secretory pathway and cytoskeleton by sporulation-specific functions. In the second phase, each immature spore becomes surrounded by a multilaminar spore wall that provides resistance to environmental stresses. This review focuses on our current understanding of the cellular rearrangements and the genes required in each of these phases to give rise to a wild-type spore.

Ascospore formation in the yeast Saccharomyces cerevisiae

Dimorphism in Histoplasma capsulatum: a model for the study of cell differentiation in pathogenic fungi.

The polyphyletic origins of ophiostomatoid fungi

Sporothrix schenckii and related species of Ceratocystis.

Fine details of the internal and external morphology of the in vitro mycelial phase (MP) to yeastlike phase (YP) transition of the dimorphic fungal pathogen Sporothrix schenckii are shown in electron micrographs of ultrathin sections. Morphological transformation at the ultrastructural level was observed to occur by direct formation of budlike structures at the tips and along the hyphae and by oidial cell formation. Direct budding of yeast from conidiospores was not observed. Early transitional forms arising by direct blastic action from the MP possessed conspicuous electron-dense microfibrillar material at the outer limits of the cell wall. The electron density of this microfibrillar material was enhanced by staining with acidified dialyzed iron. It is believed that this extracellular material may be composed in part of an acid mucosubstance. No acid phosphatase activity was associated with this microfibrillar material. This substance was found to be a characteristic of the outer limits of the cell wall of the YP of S. schenckii. Oidial YP cell formation occurred later during the transition. The cell wall of the developing oidial YP transitional form arose from an inner layer of the converting hyphae. No consupicuous alterations of the cytoplasmic content of the parent MP cell was observed during MP-to-YP transition. It is suggested that the MP-to-YP transition of S. schenckii may be regulated by at least two mechanisms involving alterations of the biochemical and/or biophysical nature of the cell wall of the MP cell in response to the conversional stimuli.

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

Electron microscopy of 8 strains of Sporothrix schenckii and 1 strain each of Ceratocystis stenoceras, C. pluriannulata, C. ulmi, and C. minor revealed the presence of unusual osmiophilic structures (EOB) which appeared as normal organellar components of young cells of these fungi. In S. schenckii and C. stenoceras, these structures were markedly osmiophilic, reacted strongly with thiocarbohydrazide, could be partially solubilized with the lipid solvent sodium methoxide, and appeared to possess lipase activity. On subsequent cellular ageing, lipid bodies were commonly seen in intimate association with the EOB. Eventually, the EOB underwent degeneration and extensive vacuolization. It is suggested that these structures are composed in part of lipoidal material in possible association with a protein matrix, and may be in some manner involved with lipid metabolism of mechanisms of lipid storage.

Ultrastructural observations of an unusual osmiophilic body in the hyphae of Sporothrix schenckii and Ceratocystis stenoceras.

Aspects of the fine structure as well as electron cytochemical localization studies of certain hydrolytic enzymes were examined by electron microscopy of ultrathin sections of the vegatative hyphae and conidia of the phycomycetous fungus, Entomophthora coronata. This entomogenous fungal organism is of interest since it has been increasingly implicated as the etiologic agent of phycomycosis of man and animals. On thin section, hyphal cells were frequently observed with septa while the cytoplasm was multinucleate. The conidium was bound by a multilayered cell wall. The cytoplasm of ungerminated conidia characteristically contained large numbers of a class of cytoplasmic organelle found in loose aggregates with lipid storage bodies. Similar organelles were observed in the cytoplasm of hyphal cells from 7-day old cultures. This round to oval to slightly reniform structure was bound by a single limiting membrane and composed of an electron dense, slightly granular matrix without evidence of crystalloid formation. The limiting membrane of the lipid storage bodies was observed to be intimately associated with that of one or more of these microbody-like organelles. This intimate association of the two cytoplasmic organelles suggests that the microbody-like organelle may be involved in some manner with lipid metabolism during the life cycle of the fungus. Cautious interpretations of electron cytochemical localization studies suggested that lipase, nonspecific esterase, and possibly aryl sulfatase were associated with the microbody-like organelles. Neither peroxidatic nor acid phosphatase activity could be demonstrated with these organelles of the conidial cytoplasm.

Ultrastructural and electron cytochemical studies of Entomophthora coronata.

Fine details of perithecial morphology and aspects of ascospore formation of Ceratocystis stenoceras (Robak) C. Moreau are shown in electron micrographs of ultrathin sections. The envelope of the perithecial body proper consisted of two zones of cells which differed morphologically one from another. Loose aggregates of small electron opaque particles were present at the outer wall surface which may be responsible for the characteristic pigmentation of the body and neck. Cells comprising the ostiolate neck may arise as modifications of spindle-shaped cells of the inner zone of the perithecial envelope. Cell walls of the neck and of zones 1 and 2 may be composed in part of periodic acid-reactive polysaccharide. The perithecial envelope and ascogenous cells were separated by a band of several parallel, double-layered membranes which may function in some manner with biosynthetic activities of ascospore production. In general, the mechanics of ascospore formation by C. stenoceras were in most ways in agreement with recent reports of ascospore outogeny in other ascomycetous fungi. Mature ascospores were somewhat lenticular in shape and the outer space wall was finely sculptured.

Garrison Rg

Papers

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

Ultrastructural observations of an unusual osmiophilic body in the hyphae of Sporothrix schenckii and Ceratocystis stenoceras.

Ultrastructural and electron cytochemical studies of Entomophthora coronata.

Perithecial ultrastructure and formation of ascospores of Ceratocystis stenoceras (Robak) C. Moreau.

Electron microscopic analysis of yeastlike cell formation from the conidia of Sporothrix schenkii.