Mammalian cells can adopt a large variety of shapes often associated with a specific internal organisation called cell polarity, which is instrumental for cell migration and tissue morphogenesis. The capacity of cells to control their shape and polarity relies in part on a thin layer of actin filaments associated to the plasma membrane, the actin cortex. We will use novel tools that we developed to measure locally and simultaneously, in live cells, the parameters defining the physical state of the actin cortex (its thickness, stiffness, viscosity and molecular tension), while controlling its polarity using optogenetics. This study will be first performed on single cells in a controlled micro-environment, then in the context of a polarised epithelial monolayer. With this project, we propose to understand how, in addition to molecular components, mechanical properties of the cell cortex can also be polarised, and how this polarity contributes to single cell and tissue morphogenesis.