In vitro experiments reconstituting topographic map formation

Abstract

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.