RÉSUMÉ
Ultrastructural changes in the cell envelope of the yeast Yarrowia lipolytica as a stress response were examined using electron microscopy. The formation of new cellular surface structures, including membrane vesicles, pore channels, and wall surface globules, were shown for the first time under conditions of oxidative (endogenous and exogenous) or thermal stress. This demonstrates once again that under stress conditions the microorganisms reveal properties previously unknown for them. Particularly noteworthy is the accumulation of silicon in the surface globules, which was revealed by X-ray microanalysis of the elemental composition of thin sections of cells. A multilayered plasmalemma instead of a 3-layered one is also characteristic for stressed cells. The envelope modifications above were observed only as a stress response and were not detected in stationary-growth-phase yeast cells that assume different physiological states. A decrease in the intracellular level of cAMP allows us to assume that a common factor activates defensive mechanisms thus explaining the similarity of the response under different stress conditions. The data presented not only enable visualization of the yeast stress response and add to our awareness of the diversity of adaptive reactions, but they also raise questions about the interrelations of the stress phenomena and their functional necessity in the cell.