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Electron microscopy of yeastlike cell development from the microconidium of Histoplasma capsulatum
Abstract:
Fine details of the sequential morphological events occurring during transition of microconidia (spores less than 5 micrometer in diameter) to the yeastlike phase of Histoplasma capsulatum as seen in ultrathin section are described and illustrated by electron micrographs. Masses of microconidia were obtained when the fungas was grown on a garden soil extract medium. Spores were incubated under in vitro environmental conditions conducive for phase transition (an enriched medium at 37 degrees C). Within 48 h of incubation, the microconidia either germinated to give rise to a short mycelium or the germ tube process became a yeast mother cell without further extension. The wall of the yeast mother cell was thin and smooth, and its cytoplasmic content was ultrastructurally complex, consisting of numerous lipid bodies, vacuoles, glycogen-like deposits, and membrane systems. Within 96 h, the mother cell underwent multipolar budding to form simultaneously linear hyphal and/or ovate yeastlike daughter cells. During the transition, new cell wall materials of the germ tube, the mother cell, and yeastlike daughter cells arose by blastic action from the innermost layer(s) of the wall of the precursor form. Lomasome-like vesicles were often seen in association with areas of new cell wall formation. After organellar migration into and septation of the daughter cells, the yeast mother cell's cytoplasmic content underwent marked degenerative changes.
Insights
Histoplasma capsulatum microconidia transition into yeast cells through germ tube development. This process involves cell wall remodeling and eventual degeneration of the mother cell.
Area of Science:
- Mycology
- Cell Biology
- Microbiology
Background:
- Histoplasma capsulatum is a dimorphic fungus that exists as mold (microconidia) at ambient temperatures and yeast at body temperatures.
- The transition from microconidia to yeast is crucial for the pathogenesis of histoplasmosis.
Purpose of the Study:
- To elucidate the sequential morphological events during the in vitro transition of Histoplasma capsulatum microconidia to the yeastlike phase.
- To detail the ultrastructural changes in cell wall formation and cytoplasmic content during this dimorphic transition.
Main Methods:
- Cultivation of Histoplasma capsulatum microconidia on a garden soil extract medium.
- Incubation of microconidia under conditions favoring yeast phase transition (enriched medium at 37°C).
- Observation and illustration of morphological changes using electron microscopy of ultrathin sections.
Main Results:
- Microconidia germinated into mycelium or directly formed yeast mother cells within 48 hours.
- Yeast mother cells exhibited complex ultrastructure with lipid bodies, vacuoles, and glycogen.
- Multipolar budding produced hyphal and yeastlike daughter cells within 96 hours.
- New cell wall formation occurred via blastic action, with lomasome-like vesicles associated with synthesis.
- Yeast mother cells showed degenerative changes after daughter cell formation and septation.
Conclusions:
- The study provides detailed ultrastructural insights into the morphological transformation of Histoplasma capsulatum from microconidia to yeast.
- The findings highlight the dynamic cell wall remodeling and cytoplasmic reorganization during dimorphism.
- Understanding these transition mechanisms is vital for comprehending fungal development and host-pathogen interactions.