In vitro assays are important tools for investigating how the intricate network of neuronal connections is established during brain development. We have been using different in vitro systems to study the cellular strategies involved in the formation of neuronal projections between the cerebral cortex and subcortical areas, and to examine mechanisms that control the assembly of local cortical circuits (for review,seeBolz et al., 1993 ; Bolz, 1996 ; Bolz and Castellani, 1997 ). Results from these studies indicated that diffusible and membrane-associated molecules provide crucial information for the establishment of layer-specific extrinsic and intrinsic cortical connections.
