Molecular Basis of Inner Ear Induction

The induction of the vertebrate inner ear is a complex developmental process that has been under investigation for decades. The traditional tools of embryology used in most of these studies have provided a wealth of information on the subject; however, they are generally limited in focus to morphological features. Recent advances in molecular biology have provided the opportunity to study inner ear induction in ways not possible in previous years. The capacity for visualizing and manipulating gene expression in combination with more traditional embryological techniques has changed the focus of inner ear induction from morphology to changes in gene expression. This chapter provides a critical review of recent studies relating to the molecular induction of the inner ear. A major theme to emerge from these studies is the dependence of inner ear induction on fibroblast growth factor (FGF) signaling. The source(s) of these FGF signals is still not entirely clear, though the available data are consistent with a mesodermal and{plus 45 degree rule}or hindbrain origin as has been long proposed. Numerous early otic marker genes have been identified as well, and their conservation in ear induction is quite clear. Functional data regarding these genes are still largely incomplete, although a role for several of these genes in zebrafish inner ear induction has been demonstrated. As a whole, these studies have made exciting and provocative contributions to the field, thus creating a more complete and precise picture of inner ear induction.