In vitro experiments reconstituting topographic map formation

Topographic axonal projections are a prevalent feature of brain connectivity. The retinotectal mapping of the chick is the best-studied model system of this type of neuronal connectivity. Its formation is commonly explained by interactions between graded markers of the ephrin-A/EphA family expressed on both retinal ganglion cell growth cones and on the tectal target area. Surprisingly, most insights into retinotectal development have been gathered through in vitro rather than in vivo experiments. In vitro assays not only enabled the biochemical identification of the postulated molecular markers but also helped to understand the signals conveyed by them. Thus, it was established in vitro that forward (ephrin-A->EphA) as well as reverse signalling (EphA->ephrin-A) are simultaneously needed for topographically appropriate guidance of retinal axons. However, no in vitro assay yet exists that fully reproduces topography formation. New in vitro techniques such as micro-contact printing or micro-fluidic networks may help to improve existent assays and to identify a sufficient set of functional components that reconstitutes topography formation.