Thermodynamic and fluid properties of cells, tissues and membranes

This dissertation studies cellular rearrangements in tissues and attempts to establish the role of physical properties of cells, tissues and membranes in several biological phenomena. Using experiments and statistical mechanical modeling, we study cell sorting, tissue engulfment, single cell motion and membrane fluctuations.; When cells of two different types are mixed together, they sort out, with the less cohesive tissue surrounding the more cohesive one. This sorting out resembles the phase separation of a mixture of immiscible liquids. We have measured the rate of sorting in tissues and compared it with a cellular automaton based model of cell aggregates. We have also established that cell sorting agrees well with the theory for phase separating fluids.; Engulfment is the spreading of one type of tissue over the surface of another tissue placed adjacent to it. Differences in adhesion cause an imbalance of surface tension forces which drives tissue spreading. We have quantitatively studied engulfment between different tissue types and compared the experimental rate with results from computer simulations and a liquid model. Our results suggest that simple physical principles can model tissue motion.; Studying the motion of single cells in aggregates is important to understanding the overall pattern formation in tissues. We characterized cell motion in different types of adhesive aggregates to elucidate the role of adhesion in cell motion. We also observed that the cells exhibited a novel type of statistics including correlations and collective motion. Membrane deformations of cells played a negligible role in large scale cell motion. Our results indicate the importance of correlated motion for cells to move long distances in tissues.; At the single cell level, tension of the cell membrane and intracellular membrane can play an important role in cell shape changes, regulation of cell motility and membrane dynamics. We used optical tweezers to measure the membrane tension of tubulo-vesicular networks obtained from Golgi and Endoplasmic Reticulum (ER) membranes within cells. As expected on the basis of some previous experiments, the ER has a higher membrane tension than the Golgi.