Three-dimensional reconstruction of a pathogenic yeastExophiala dermatitidiscell by freeze-substitution and serial sectioning electron microscopy

The structure of a budding cell of the pathogenic yeast Exophiala dermatitidis was observed in three dimensions after freeze-substitution, serial ultrathin sectioning and computer reconstruction. The nucleus occupied about 10% of the cell volume. The spindle pole body was composed of two disk elements connected by an intervening midpiece, and occupied about 0.01% of the cell volume. The cell wall consisted of an inner transparent layer, a middle electron-opaque layer, and an outer fibrous layer. The mitochondria occupied about 10% of the cell volume. There were numerous mitochondria in the mother cell and the bud, but no 'giant mitochondrion' was seen. The ratio of mitochondrial volume within the bud to the mitochondrial volume of the cell was close to the ratio of bud:cell cytoplasmic volume. The results emphasize the importance of good cryofixation for 'perfect' preservation of yeast cell structure.