Once specific regions of the epithelia are specified as “sensory” by Sox2 and/or Pax genes, the process of neurogenesis begins in these domains, and several different bHLH transcription factors become important in the production and differentiation of the sensory receptor cells in these regions. The proneural gene, Ascl1, is expressed in the developing retina and olfactory epithelium and is necessary for providing a neural competence in the progenitor cells (Cau et al., 2002, Baf-A1 concentration Cau et al., 1997, Jasoni et al., 1994 and Nelson et al., 2009). The proneural neurogenins are also expressed in the olfactory, retinal, and inner ear epithelia

and play important roles in the production of specific types of mTOR inhibitor neurons in each region. Loss of Neurogenins in the inner ear, for example, causes the failure of spiral ganglion neurons to develop (Ma et al., 2000). In addition to these proneural factors, other

bHLH transcription factors are required for differentiation of the sensory receptor cells or their associated neurons. NeuroD1 is expressed in the photoreceptors in the retina, and targeted deletion of this gene in mice leads to a failure of normal cone photoreceptor differentiation and the degeneration of the rod photoreceptors (Liu et al., 2008). In the inner ear, NeuroD1 is required in the ganglion neurons that synapse with the hair cells (Jahan et al., 2010 and Liu et al., 2000). One of the most important genes for hair cell development, Atoh1 (Bermingham et al., Mephenoxalone 1999), is another member of the bHLH family of transcription factors and is required for hair cell development. Targeted deletion of this gene results in the absence of hair cells in all the inner ear sensory epithelia, and overexpression

of Atoh1 during development induces hair cells in nonsensory regions of the inner ear epithelium. Although not required for the sensory receptors in the retina, the related Atoh7/Math5 is necessary for the development of the retinal ganglion neurons (Brown et al., 1998). The similarity in the expression of the proneural and neural differentiation bHLH genes during development of the specialized sensory organs is quite striking and supports the idea that these systems have well conserved developmental mechanisms. In addition to the transcription factors discussed above, the development of the specialized sensory structures is regulated by many different signaling factors. One of the most important is Notch signaling. Notch is required in all these systems and functions at several different stages of their development. For example, in the inner ear, Notch is initially required in the early specification of the Sox2 expressing presumptive sensory domain of the epithelium (Brooker et al., 2006, Daudet and Lewis, 2005, Kiernan et al., 2001 and Kiernan et al., 2006).