Under usual culture conditions, Saccharomyces is ellipsoidal/ovoid in shape and approximately 5-10 ¡j,m long by 3-7 //m wide. This is referred to as the yeast form. Figure 3.1 shows a scanning electron micrograph (SEM) of a cell in the yeast form. Cell division is by budding; that is, a smaller ovoid daughter cell forms as a projection from the surface of the mother cell. Haploid cells are generally about one-half the volume of diploid cells. The characteristic shape is maintained by a rigid cell wall that completely surrounds the plasma membrane of Saccharomyces. Changes in this shape involve remodeling of the cell wall and occur during budding, mating, and pseudohyphal differentiation. Under nutrient stressed conditions certain strains undergo a shape change, forming filaments consisting of short chains of individual cells called pseudohyphe. In the pseudohyphal form the cells are elongated rather than ovoid in shape, and form chains of cells because the daughters do not detach from the mother even though cytokinesis is complete (Figure 3.2). Pseudohyphae penetrate into the solid substrate growth medium making them difficult to wash off, so-called invasive growth.
The plasma membrane surrounds a cytoplasm that is organized structurally by a cytoskeleton and is divided into membrane-bound compartments including a nucleus, mitochondria, pexoisomes, a vacuole, a Golgi complex, vesicles of various types, and the endoplasmic reticulum (ER). In many respects the ER, Golgi, vacuole, secretory vesicles and endocytic vesicles, and plasma membrane should be considered to be part of an interconnected system. Newly synthesized membrane and membrane proteins move from the ER to the Golgi and then to either the plasma membrane or the vacuole via secretory vesicles. Additionally, plasma membrane is internalized to form endocytic vesicles that fuse eventually with the vacuole. These combined processes of secretion (or exocytosis) and endocytosis are often referred to as membrane trafficking processes. Various particulate structures are found in the cytosol including ribosomes and proteasomes (essential protease complexes involved in the degradation of soluble proteins).
Figure 3.1 SEM of S. cerevisiae. A dividing cell exhibiting the yeast form is shown using SEM. Arrows point to the birth scar (BirS) and bud scar (BS), indicating that this cell has formed one bud prior to the one that is currently forming. Taken from Walker (1998). Reproduced with permission of John Wiley & Sons Limited
Figure 3.2 Pseudohyphal growth pattern. In starvation conditions, certain Saccharomyces strains alter their shape and budding pattern. This altered growth pattern, shown in this figure, is referred to as the pseudohyphal morphology. An elongated cell shape and a unipolar pattern of budding characterize the pseudohyphal growth. In the unipolar budding pattern, new buds form only at the end of the cell opposite to the birth end (the distal pole) in both the mother and the daughter cell. Adapted from Taheri et al. (2000) by permission of Oxford University Press
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