Figure 1. (a) Illustration of three diverse eye types based on a typical mammalian eye, a cephalopod (squid) eye and Drosophila eye. (b) The developmental processes which give rise to the mammalian eye and the Drosophila eye are very different. The expression of Pax6 (eyeless in Drosophila) is shown in red. In the mammalian eye Pax6 is expressed in the region of the forebrain which buds out to form the optic cup; the inner layer of this cup will become neural retina whereas the outer layer gives rise to the retinal pigmented epithelium (RPE). Invagination and differentiation within the optic cup is mediated by interactions with the head surface ectoderm which itself invaginates to form the lens. Pax6 expression is downregulated first in the RPE and then in the neural retina. In Drosophila eye development Pax6 is expressed in undifferentiated epithelium anterior to the morphogenetic furrow which sweeps across the eye imaginal disc in a posterior-to-anterior direction. The photoreceptors differentiate on the posterior side of the furrow.