Anatomical analysis of Saccharomyces cerevisiae stalk-like structures reveals spatial organization and cell specialization

Recently we reported an unusual multicellular organization in yeast that we termed stalk-like structures. These structures are tall (0.5 to 3 cm long) and narrow (1 to 3 mm in diameter). They are formed in response to UV radi ation of cultures spread on high agar concentrations. Here we present an an atomical analysis of the stalks. Microscopic inspection of cross sections t aken from stalks revealed that stalks are composed of an inner core in whic h cells are dense and vital and a layer of cells (four to six rows) that su rrounds the core. This outer layer is physically separated from the core an d contains many dead cells. The outer layer may form a protective shell for the core cells. Through electron microscopy analysis we observed three typ es of cells within the stalk population: (i) cells containing many unusual vesicles, which might be undergoing some kind of cell death; (ii) cells con taining spores (usually one or two spores only); and (iii) familiar rounded cells. We suggest that stalk cells are not only spatially organized but ma y undergo processes that induce a certain degree of cell specialization. We also show that high agar concentration alone, although not sufficient to i nduce stalk formation, induces dramatic changes in a colony's morphology. M ost striking among the agar effects is the induction of growth into the aga r, forming peg-like structures. Colonies grown on 4% agar or higher are rem iniscent of stalks in some aspects. The agar concentration effects are medi ated in part by the Ras pathway and are related to the invasive-growth phen omenon.