Formation of target-specific neuronal projections in organotypic slice cultures from rat visual cortex.

A characteristic feature of the mammalian cortex is that projection neurons located in distinct cortical layers send their axons to different targets. In visual cortex, cells in layers 2 and 3 project to other cortical areas, whereas cells in layers 5 and 6 project to subcortical targets such as the lateral geniculate nucleus. The proper development of these projections is crucial for correct functioning of the visual system. Here we show that specific connections are established in an organotypic culture system in which rat visual cortex slices are co-cultured with another slice of the visual cortex or with a thalamic slice. The laminar origin and cellular morphology in vitro of cortical projections to other cortical regions or to subcortical targets are remarkably similar to those seen in vivo. In addition, axons of projecting cells are not restricted to particular pathways, but appear instead to grow directly towards their appropriate target. These observations raise the possibility that chemotropic attraction from the target areas may play an important part in the development of the cortical projection pattern.