Choosing axonal real estate: Location, location, location

In this issue of The Journal of Comparative Neurology (pages 6–27), Leake, Snyder, and Hradek examine the sequence of events whereby axons from spiral ganglion cells establish precise tonotopic maps in the three subdivisions of the cochlear nucleus (CN) of the kitten. The careful, quantitative work of this group provides some important new insights about the formation of sensory maps in the developing brain. Three take-home lessons are to be learned. First, comparison of this work with studies on the development of retinotopic maps make it unclear whether similar developmental sequences of events occur. If the events are similar, then we might hope to find similar mechanisms. If the sequences of events are dissimilar, then it is highly unlikely that the developmental principles and mechanisms will be the same. Second, single auditory nerve axons must form three distinct and separate tonotopic maps with three different target cell groups (divisions of the cochlear nuclei) that have quite different cytoarchitectures. It is of interest to ask if the process has the same characteristics in each region. The answer appears to be yes, although small differences in timing may occur. Finally, their careful quantitation allows an evaluation of the relative contributions of initial specificity and subsequent growth vs. overproduction and activity-dependent (or activity-independent) pruning of terminal arbors. Here the lesson appears clear: neighbor relations are established early and maintained. Topographic maps are a fundamental organizing feature of the nervous system. In most sensory systems, the neighbor relationships of receptor cells in the sensory epithelium are preserved in a series of projections into the central nervous system, and throughout both ascending and descending pathways. Similarly, efferent projections have motor maps related to muscles, and spinal cord motor pools are ordered with topography relative to muscles. Because these maps were first discovered during the middle of the past century, two questions have dominated much of developmental neurobiology.