Fig. 6. A model summarizing Pax6 functions in retinal progenitor cells. (A) In the central retina, close to the differentiation front (arrows), Pax6 is required for the normal expression profile of transcription factors that play a role in the execution of specific retinal lineages but is dispensable for the completion of neurogenesis. (B) In the peripheral RPCs, Pax6 inhibits Crx expression and at the same time is essential for proneural gene expression and completion of the neurogenic program. The involvement of Pax6 in the regulation of Crx and Otx places Pax6 upstream in the transcriptional network that regulates PR specification and differentiation. The requirement for Otx and Crx encompasses the determination and specification of both rod and cone photoreceptors. Later, the terinoids and thyroid hormone (e.g. Trβ2) nuclear receptors function within the cone precursors for the distinction between the M and S cone types (Hennig et al., 2008). nnp, non-neuronal progenitors; nbl, neuroblast layer; prp, photoreceptor precursor layer.