MECHANISMS AND MOLECULES CONTROLLING THE DEVELOPMENT OF RETINAL MAPS

All mature vertebrates exhibit precise topographic mapping from the re tina to the tectum, or its mammalian homologue, the superior colliculu s (SC). In frogs and fish the development of this projection is precis e from the outset; in avians retinal axon targeting is more diffuse bu t respects a coarse topographic matching; and in rodents early project ions show no topographic specificity. Topography in avians and rodents emerges from a process of branch extension, arborization, and elimina tion of aberrant axonal projections. Despite these differences, the ba sic mechanisms controlling the development of this retinotopy are cons erved. It has been hypothesized that molecules distributed in a positi on-dependent manner in the retina and the tectum or SC control the dev elopment of these maps. A number of candidate molecules have been iden tified on the basis of their distribution, or their ability to influen ce axonal growth in vitro. In addition, transcription factors and sign aling molecules are expressed in a position-dependent manner and may r egulate the expression of molecules involved in retinotopic map format ion.