Cell migration is a fundamental process during mammalian brain development since all neurons migrate to their final location. Cajal-Retzius (CR) cells are transient neurons that control the laminar organization of the cerebral cortex and form a monolayer at its the surface. It has been shown that CR cells are generated by focal sources surrounding the cerebral cortex and disperse on its surface. However, how the monolayer distribution of CR cells is achieved during development and what cell contacts are involved remains largely unknown. By candidate approach, we have recently shown that the inactivation of the transcription factor Ebf3 leads to a normal spreading of CR cells, followed by an abnormal local agglutination. The goal of this project is to unravel the mechanisms controlling CR cells distribution. To this aim, we will perform a detailed study of Ebf3 function in migration and adhesion by combining: i) phenotypic analyses of mutant mice; ii) cortical time-lapse imaging studies and ex-vivo contact inhibition assays comparing the behaviour of wild-type, mutant, and Ebf3-overexpressing CR cells; iii) cell-sorting and comparative transcriptome profiling. In parallel, to determine how CR cells interact with their environment we will set up a new approach to visualize cell contacts. We will take advantage of a strategy developed in worms and drosophila that relies on the reconstitution in trans of two membrane-bound fragments of the green fluorescent protein (GFP). These two fragments are not fluorescent, but upon physical interaction, they form a fluorescent GFP, thereby providing an on/off system to visualize physical interactions of CR cells with other CR cells or with surrounding cortical structures. Overall, this project will provide new insights on the mechanisms controlling CR cells distribution and on Ebf3 function in brain development. Furthermore, it will generate novel and essential tools to analyze cell/cell physical interactions in mammals.