FIG. 10.

Involvement of Otx2 in photoreceptor and bipolar-cell development. (A) In normal development, Otx2 is required for the cell fate determination of photoreceptors. Otx2 is also involved in the terminal differentiation of photoreceptors, but the contribution is relatively small compared to that of Crx. In contrast, in bipolar cells, Otx2 is required not for cell fate determination but instead for terminal differentiation cooperatively with Crx. (B) When Otx2 is misexpressed, a larger number of postmitotic precursors is directed to choose the photoreceptor lineage. As a result, the number of photoreceptors increases and the numbers of other retinal cells, including amacrine and bipolar cells, decrease. (C) When Otx2 is ablated, postmitotic precursors originally fated to become photoreceptors are forced to change their cell fate to the amacrine lineage. Crx, which is induced by Otx2, is also downregulated. As a result, the number of photoreceptors decreases and the number of amacrine cells increases. In the bipolar-cell lineage, cell fate determination is not affected but terminal differentiation is impaired. Therefore, the number of bipolar cells also decreases. PR, photoreceptor; AM, amacrine cell; BP, bipolar cells.