How axons see their way--axonal guidance in the visual system

In humans up to 80% of the information received from the outside world is processed by the visual pathway. Therefore, understanding the molecular and cellular bases of the formation of the retinofugal projection has been in the focus of research during the last decades. Besides our interest in the development of the visual pathway per se this circuit is also an excellent model system to study axon guidance, midline crossing, and formation of topographic neuronal maps in general. The generation of genetic animal models as well as the design of in vitro loss- and gain-of-function paradigms have provided insight into transcriptional networks, identified signalling molecules, extracellular matrix components, morphogens, and activity patterns which are involved in the establishment of the visual pathway. To provide a picture as complete as possible, we will summarize molecular mechanisms involved in axon guidance and retinotopic mapping as well as neuronal activity shaping retinal and thalamocortical projections focusing on the mouse as a model system and highlight discoveries made in other organisms that contribute to our understanding.