Cell migration is a fundamental property of all animal cells which is involved in processes such as embryogenesis, immune response, tissue regeneration and cancer metastasis. Directional migration requires cell polarization which could happen in response to external signals but also spontaneously. Polarization involves the change in the cell edge activities from random distribution of protrusion and retraction to their separation into two large regions corresponding to the leading and trailing edges of the cell. Many studies investigated cytoskeletal mechanisms of protrusions and retractions but it is still not well understood how the cell orchestrates protrusion and retraction along its edge and what triggers transitions between these two types of activity. These questions are essential to understand both cell polarization and cycles of protrusion and retraction which characterize exploratory edge dynamics before polarization. This thesis focuses on the analysis of cell edge dynamics during the initiation of motion and on the mechanisms of transition (switches) between protrusion and retraction. We considered the cell polarization and protrusion-retraction cycles as two related phenomena and aimed to identify common mechanisms. Live cell imaging, cytoskeletal inhibitors, traction force microscopy, patterned substrates, micromanipulation, and computational analysis and modeling were used to examine cell polarization in the experimental model of fish epithelial keratocytes that are characterized by simple and regular shape and robust polarization and motion. We found that protrusion-retraction switches happened at maximal distance from the cell center both during cell edge fluctuation and directional motion and did not depend on the edge orientation with respect to the cell motion direction. Computational model demonstrated that switches at a threshold distance were sufficient for self-organization of edge activity leading to spontaneous polarization and stable cell shape and motion suggesting that distance-sensing is a fundamental mechanism of cell symmetry breaking. Next we investigated the mechanisms of distance-sensing focusing on two hypotheses: traction forces and tridimensional cell shape. Traction force may increase with the distance from the cell center (e.g., due to a build-up of actomyosin network) leading to detachment of the edge and initiation of retraction. We discovered that traction forces indeed increased with the distance and that protrusion-retraction switches occurred near maximal forces. Local external force also induced protrusion-retraction switch. However, inhibition of contractility reduced traction forces and abolished the dependence of force on the distance, but did not prevent cell polarization. Taken together, these results suggest that traction forces are sufficient, but not necessary mediators of distance-dependent switch and cell polarization. In an alternative mechanism, tridimensional shape of the cell edge may depend on the distance from the cell center and affect the balance of forces at the edge, inducing switches. We have locally modified this tridimensional force balance by using substrates with topographic features and showed that switches preferentially happen near these features. These results provide a novel framework to understand cell edge dynamics and symmetry breaking in terms of protrusion-retraction switches and physical force balance at the cell edge.