Fig. 7. Proposed model for the action of Wnt2b in the marginal retina. (A) Gene cascade leading to the inhibition of cellular differentiation by Wnt signaling. See the text for details. (B) During early retinal development, Wnt2b sends a signal to the peripheral region of the optic vesicles to keep the progenitor cells undifferentiated. Centrally located progenitor cells do not receive the Wnt signals and express proneural genes as well as Notch, which triggers the differentiation cascades. (C) At later stages, the marginal retina differentiates into iris and ciliary epithelium, induced by factors coming from surrounding tissues such as the lens or cornea.