Molecular Development of Sensory Maps Representing Sights and Smells in the Brain

retinotectal and olfactory systems has clear distinctions, Introduction which relate in part to differences in their functional Defining the molecules and mechanisms that control requirements. In the retinotectal system, the main objecthe establishment of an orderly representation of the tive is to represent the visual world in the brain, that is, peripheral sense organs within the brain has long been to reconstruct a topographic representation of the world of interest to systems and developmental neurobiolothat projects onto the retina and is remapped in the gists. Classically, the projection from the retina to the tectum. To carry out this function requires the maintebrain has served as the model system for understanding nance of a precise spatial ordering of axonal connechow precise neural connections are formed. More retions within the tectum that reflects their origins in the cently, the molecular cloning of olfactory receptors retina. In contrast, in the olfactory system, since odors (ORs) has provided valuable insights into the functional have no relevant spatial component, there is no overridand anatomical organization of the olfactory system, ing need to maintain spatial continuity, either between including the projection of olfactory neurons (ONs) from cells expressing a given OR, and presumably respondthe olfactory epithelium to the olfactory bulb (OB). The ing to the same odors, or between glomeruli in the OB. mechanisms involved in establishing this projection, as This functional difference relates to differences in the well as its organization, are atypical and make for revealmapping strategies employed in the two systems. ing comparisons when juxtaposed to the development This article will review current knowledge of the mechof order in the visual system. anisms and molecules proposed to control mapping in Both the visual and olfactory systems represent senthe visual and olfactory systems and attempt a synthesis sory information within the brain through the use of to highlight differences and similarities in their organizasensory maps. The projection of sensory axons to the tions and the molecular mechanisms that may control brain forms these maps through the spatial segregation their development. We have focused almost exclusively and orderly termination of their axonal connections in on the projection of RGCs to the chick optic tectum, or specific target tissues. However, the visual map is funits equivalent in rodents, the superior colliculus (SC), as damentally different from the olfactory map in that it is well as the main olfactory system of mice. We have not strictly topographic: a two-dimensional sheet of retinal discussed the large bodies of excellent work on the ganglion cells (RGCs) in the retina is rerepresented in retinotectal system of fish and amphibians, the main the brain as more or less the same two-dimensional olfactory system of lower vertebrates or C. elegans, nor sheet through the orderly terminations of RGC axons. do we consider the projection from the vomeronasal In contrast, the olfactory map is formed by the converorgan to the accessory olfactory bulb (for reviews of gence of the axonal projections of a specific set of functhese topics see Roskies et al., 1995; Bargmann, 1997; tionally similar ONs that are randomly distributed in the Karlstrom et al., 1997; Ebrahimi and Chess, 1998). olfactory epithelium onto specific glomeruli, and in doing