Mechanisms and molecules controlling the development of retinal maps

All mature vertebrates exhibit precise topographic mapping from the retina to the tectum, or its mammalian homologue, the superior colliculus (SC). In frogs and fish the development of this projection is precise from the outset; in avians retinal axon targeting is more diffuse but respects a coarse topographic matching; and in rodents early projections show no topographic specificity. Topography in avians and rodents emerges from a process of branch extension, arborization, and elimination of aberrant axonal projections. Despite these differences, the basic mechanisms controlling the development of this retinotopy are conserved. It has been hypothesized that molecules distributed in a position-dependent manner in the retina and the tectum or SC control the development of these maps. A number of candidate molecules have been identified on the basis of their distribution, or their ability to influence axonal growth in vitro. In addition, transcription factors and signaling molecules are expressed in a position-dependent manner and may regulate the expression of molecules involved in retinotopic map formation.