Four major classes of nerve fibers make specific and characteristic connections with the inner or outer hair cells in the mouse organ of Corti. Each class of nerve fibers develops during a defined window of time, growing between and along supporting cells to reach the hair cell partners. When mature, these neuronal connections accurately link tonotopically arranged regions in the organ of Corti with corresponding regions in the brain stem. Such specificity during development is of prime importance in assuring normal auditory function, but the cellular and molecular processes which guide the growth of nerve fibers in the cochlea are largely unknown. It is the working hypothesis of this project that interactions between adhesive molecules on nerve fibers and those in the microenvironments surrounding nerve fibers facilitate neuronal outgrowth and pathfinding in the mammalian cochlea. The long range strategy is: 1) to identify likely molecular candidates in the cochlea; and 2) to assess the roles of these molecules in neurite outgrowth, bundling, pathfinding and target recognition. In the previous project period, the distributions of extracellular matrix (ECM) proteins and cell surface adhesion molecules in the cochlea were mapped. The developmental distributions in regions where afferent growth cones change association or directions suggested specific roles for each of the molecules under study. In this project period, it is proposed to further test the predictions of the working hypothesis by examining cochlear neural development in three model systems: in knockout mice lacking specific adhesion molecules, in perturbation (blocking) studies in cochlear organ culture, spiral ganglion explant and dissociated cell cultures; in spiral ganglion explant and dissociated cell cultures maintained on different artificial substrates. The project will also continue to examine, by immunohistochemistry, the distributions of cell surface adhesive molecules in the cochlear regions where directional choices are made by neuronal growth cones. The questions addressed in this project are important for understanding normal and abnormal neural development in the cochlea, and thus are important for understanding the mechanisms of congenital deafness. But in addition, the results will be relevant to questions of neural regeneration and bear on experimental attempts to encourage re-establishment of functional nerve fiber networks in the cochlea after nerve damage.
