Fig. 7. A model of the specification, organization and maintenance of vertebrate retinal cells. (A) Retinal cell type specification occurs prior to the OV stage. By the OV stage depicted here, retinal cells are already biased to become either RPE or neuroretinal cells, as shown by the striped green and yellow region. (B) When the surface ectoderm comes in close contact with the OV, FGF1 and/or FGF2 from the surface ectoderm signals through Chx10 to organize the NR, adjacent to the future lens. Chx10 then represses the expression of Mitf (directly or indirectly) to maintain neuroretinal cell identity, perhaps through the regulation of Fgf8, Fgf9 and/or Fgf15 and other neuroretinal genes. Cells that are distant from the surface ectoderm, and thus from FGF1 and/or FGF2, do not express Chx10, allowing Mitf expression to continue, thus organizing the RPE at the back of the developing eye. The RPE, by contrast, appears to be organized by activin signals from the posterior ocular mesenchyme (Fuhrmann et al., 2000), signals that may act through Mitf and other genes essential for RPE formation, such as Otx1, Otx2 (Martinez-Morales et al., 2001), Pax2 and Pax6 (Baumer et al., 2003). Mitf appears to maintain RPE cell identity by the activation of downstream genes, such as pigmentation enzyme-encoding genes. Finally, although the mechanism is unknown, Mitf may also negatively regulate Chx10 to maintain RPE cell identity.