Inner Ear Sensory Epithelia Development and Regulation in Zebrafish

The inner ear is a complex sensory organ of interconnected chambers, each with a sensory epithelium comprised of hair cells and support cells for detection of sound and motion. This dissertation focuses on the development and regulation of sensory epithelia in zebrafish and utilizes loss of function, gain of function and laser ablation techniques. Hair cells and support cells develop from an equivalence group specified by proneural genes encoding bHLH transcription factors. The vertebrate Atoh1 bHLH transciption factor is a potential candidate for this role. However, data in mouse has led some researchers to conclude it does not have a proneural activity, but, rather, is involved in later stages of hair cell differentiation. In addition, the factors regulating Atoh1 are mostly unknown. We address these issues in zebrafish and show that the zebrafish homologs atoh1a and atoh1b are required during two developmental phases, first in the preotic placode and later in the otic vesicle. They interact with the Notch pathway and are necessary and sufficient for specification of sensory epithelia. Our data confirm atoh1 genes have proneural function. We also go on to show Atoh1 works in a complex network of factors, Pax2/5/8, Sox2, Fgf and Notch. Misexpression of atoh1 alters axial patterning and leads to expanded sensory epithelia, which is enhanced by misexpression of either fgf8 or sox2. Lastly, we examine the role of sox2 in sensory epithelia development and regeneration. Sox2 has been implicated in maintainence of pluripotent stem cells as well as cell differentiation. In the inner ear, Sox2 is initially expressed in the prosensory domain and is required for its formation. Eventually, Sox2 is downregulated in hair cells and maintained in support cells; however, its later role has not been determined. We show that in the zebrafish inner ear, sox2 is expressed after sensory epithelium development has begun and, like in mouse, expression is down regulated in hair cells and maintained in support cells. Our data demonstrate a role for sox2 in maintenance of hair cells and in transdifferentation of support cells into hair cells after laser ablation. Additionally, sox2 is regulated by Aoth1a/1b, Fgf, and Notch