Sonic hedgehog control of size and shape in midbrain pattern formation

Little is known about how patterns of cell types are organized to form brai n structures of appropriate size and shape. To study this process, we emplo yed in vivo electroporation during midbrain development to create ectopic s ources of Sonic: Hedgehog, a signaling molecule previously shown to specify different neuronal cell types in a concentration-dependent manner in vitro . We provide direct evidence that a Sonic Hedgehog source can control patte rn at a distance in brain development and demonstrate that the sire, shape, and orientation of the cell populations produced depend on the geometry of the morphogen source. Thus, a single regulatory molecule can coordinate ti ssue size and shape with cell-type identity in brain development.