Fig. 10. Summary of atoh1 regulation and function. fgf-foxi1-pax8 and dlx pathways induce expression of atoh1b (red) in medial preotic cells, specifying the prosensory equivalence group. By 12 hpf, the domain is restricted into two intermediate groups by Dl-N activity, which is activated by atoh1b function. Tether cells are specified around 14 hpf as atoh1a is activated (blue, coexpression with atoh1b, purple). At 20 hpf, N and Fgf activate a wider domain of atoh1a associated with later-forming hair cells. Tether cells (asterisks) terminally differentiate. atoh1a is required to maintain or activate atoh1b in differentiating cells, and atoh1b helps maintain high levels of atoh1a. At 24 hpf and thereafter, later-forming hair cells begin to differentiate and coexpress atoh1 genes, and N activity limits atoh1 expression. Mature tether cells and hair cells downregulate atoh1 expression.