Morphology and functions of the leading process. (A) The elaborate leading processes of tangentially migrating neurons are continuously remodeled as migration progresses, with individual branches growing and collapsing. (i) A chemoattractant (concentration gradient indicated by orange shading) stimulates signaling at growth cones (red dashes). A growth cone in a higher concentration of chemoattractant generates a stronger signal and becomes dominant. Presumably, the cell has a mechanism to compare the signals from different growth cones and determine which is stronger, but the mechanism is unknown (black arrow). In this illustration, the right-hand process has more signal and becomes dominant. (ii) A dilation in front of the nucleus, containing the centrosome and Golgi apparatus (red dot), translocates to the branch point. (iii) The nucleus moves to the branch point and the centrosome relocates into the dominant process. The cycle continues in a new direction. (B) Multipolar neurons extend and retract processes in various directions from the cell soma. At any given time one process is dominant, recruits the centrosome, and directs movement. Other processes can take over, presumably based on relative stabilization of growth cones by short-range and long-range signals. It is unclear how the signal strengths at different growth cones are compared (black arrow). (C, i) Radially migrating neurons undergoing locomotion along radial glia have a simple, relatively unbranched leading process. Branching may be suppressed by long-range signaling from receptors at the tip of the leading process (black arrow). (ii) Mutations that interfere with adhesion also destabilize the leading process and induce branching. (D) The leading process in neurons undergoing somal translocation is anchored to the cells or extracellular matrix at the pia. Branching is also suppressed, perhaps by unidentified long-range inhibitory signals (black arrow). Question marks represent unknown mechanisms.