The ultrastructure of yeast: cell wall structure and formation.

Yeasts are unicellular eukaryotes, and are used widely as a model system in basic and applied fields of life science, medicine, and biotechnology. The ultrastructure of yeast cells was first studied in 1957 and the techniques used have advanced greatly in the 40 years since then; an overview of these methods is first presented in this review. The ultrastructure of budding and dimorphic yeast cells observed with a scanning electron microscope (SEM) and a transmission electron microscope (TEM) after thin sectioning and freeze-etching are then described, followed by discussion of the regeneration of the cell wall of Candida albicans protoplasts detected by cryosectioning. C. albicans protoplasts are regenerated to synthesize microfibrils on their surface. They are aggregated into thicker bundles which are intermeshed, forming a wide-meshed network of long fibrils. These microfibrillar structures are chains of beta-1,3-glucan which are broken down after treatment with beta-1,3-glucanase. Morphologically identical microfibrils are synthesized in vitro by a cell-free system in which the active cell membrane fraction as a source of beta-1,3-glucan synthetase and UDP glucose as the sole substrate are used. The diameter of an elemental fibril of beta-glucan is estimated to be 2.8 nm from the pattern of autocorrelation of the image obtained by computer processing. In contrast, in the presence of aculeacin A the formation of normal fibrillar nets or bundles is significantly inhibited, resulting in the occurrence of short fibrils. These electron microscopic data suggest that aculeacin A inhibits not only the synthesis of beta-1,3-glucan but the aggregation of microfibrils of this polysaccharide, allowing formation of the crystalline structure. On the basis of the cumulative data obtained from the electron microscopic studies, we are led to the assumption that de novo synthesized beta-glucan chains might initially form fine particles which are then transformed into thin fibrils with single to multiple strands which appear to be oriented parallel to each other so that they develop into fibrillar structures. This process of assembly of beta-glucan molecules leads to the development of a fibrous network within the regenerating Candida cell wall. Third, the mechanism of cell wall formation is shown by low-voltage (LV) SEM and TEM, using various techniques and computer graphics, of the regeneration system of Schizosaccharomyces pombe protoplasts: after 10 min of regeneration, the protoplasts begin to grow fibrillar substances of a beta-glucan nature, and a fibrillar network covers the surface of all protoplasts. The network is originally formed as fine particles on the protoplast surface and these are subsequently lengthened to microfibrils 2 nm thick. The microfibrils twist around each other and develop into 8 nm thick fibrils forming flat bundles 16 nm thick. Interfibrillar spaces are gradually filled with amorphous particles of an alpha-galactomannan nature and, finally, the complete cell wall is formed after 12 h. Treatment of reverting protoplasts with RuO4 provided clear TEM images of glucan fibrils with high electron density. The relationship between cell wall regeneration and intracellular organelles was examined by using serial thin sections stained with PATAg and computer-aided three-dimensional reconstruction. The secretory vesicles in a protoplast had increased markedly by 1.4, 3.4, and 5.8 times at 1.5, 3.0, and 5 h, respectively. Three-dimensional analysis indicates that Golgi apparatuses are located close together in the nucleus of the protoplast and are dispersed into the cytoplasm during the progress of cell wall formation.