INTRODUCTION The organ of Corti, the sensory organ responsible for hearing in mammals, differentiates within the cochlear epithelium to produce a complex array of sensory hair cells and non-sensory supporting cells whose organization is crucial for normal ear

The organ of Corti, the sensory organ responsible for hearing in mammals, differentiates within the cochlear epithelium to produce a complex array of sensory hair cells and non-sensory supporting cells whose organization is crucial for normal ear function (Rueda et al., 1996). Remarkably, many of the molecular mechanisms involved in the development of these sensory arrays appear to be conserved during the development of functionally related and highly patterned sensory organs in invertebrates (Eddison et al., 2000; Hassan and Bellen, 2000). In particular, these studies have indicated the importance of homologs of proneural and neurogenic genes involved in developmental patterning of sensory structures in Drosophila (Hassan and Bellen, 2000). These include members of the proneural group of basic helix-loop-helix (bHLH) transcription factors such as Achaete-scute and atonal (Bermingham et al., 1999; Zheng and Gao, 2000; Itoh and Chitnis, 2001), as well as several elements in the Notch signaling pathway (Eddison et al., 2000; Haddon et al., 1998; Lanford, 1999; Lanford et al., 2000; Lewis et al., 1998; Shailam et al., 1999; Zhang et al., 2000; Zheng et al., 2000; Zine et al., 2000; Zine et al., 2001). Proneural genes in Drosophila function in at least two steps in the development of the sensory elements of the Drosophila nervous system. First, they specify a cluster of cells in the naïve ectoderm of the embryo, endowing these cells with the competence to become neural/sensory precursors (reviewed by Jarman and Ahmed, 1998; Lewis, 1996). Proneural genes function a second time during the selection of a specific cell within the proneural field, the sensory organ precursor (SOP), which will go on to divide through a fixed lineage and differentiate into supporting cells and neurons of the Drosophila sensory organ. Once selected, under the influence of the relevant proneural gene, the SOP actively inhibits the differentiation of other cells in the prosensory domain through Notch-dependent lateral inhibition. Loss of the proneural gene leads to the loss of the entire lineage associated with the sensory structure in question (Jarman and Ahmed, 1998). Similar to the development of sensory arrays in Drosophila, development of the organ of Corti appears to be separated into 2495 Development 129, 2495-2505 (2002) Printed in Great Britain © The Company of Biologists Limited 2002 DEV1807