Fig. 9. A model for neuronal subtype specification in the zebrafish epiphysis. (A) Schematic representation of the neuroepithelium. (1) Neural progenitors (dark blue) are selected from a pool of neuroepithelial cells (light blue) through Notch signaling (yellow arrow). Selected cells migrate to the basal side of the epithelium (blue arrow indicates the direction of movement) where they encounter new neighbors. (2) Selected neural progenitors again communicate via Notch to establish their respective identities. (3) Specified neural progenitors finish their last cell cycle. Progenitors for photoreceptors are in red, progenitors for projection neurons are in green. Apical and basal are labeled a and b, respectively. (B) Neuronal progenitors communicate via Notch (orange), thereby inhibiting both the projection neuron program and the expression of Delta genes. In parallel, a photoreceptor-inducing signal (red arrow) activates the photoreceptor program.