|1st Induction||Specific positions along the anterior-posterior axis of the archenteron (cells arising from the organizer to form dorsal mesoderm and endoderm) induce specific regions of the brain to form. For the developing eye, the diencephalon is induced along with its outpatching, the optic vesicle.|
|2nd Induction||The optic vesicle induces the lens placode in the overlying head ectoderm. This is evidenced by: surgical or genetic removal of the optic vesicle cases failure of lens formation; optic vesicle transplantation causes formation of a new, ectopic lens. The optic vesicle then folds back on itself to form the optic cup: the outer optic cup layer becomes the pigmented retina; the inner optic cup layer becomes the neural retina. The lens placode also invaginates to form the lens vesicle. Axons from the neural retina travel down the optic stalk; once travelled by axons, the optic stalk becomes the optic nerve. Next, the lens cells differentiate.|
|3rd Induction||The lens vesicle induces the overlying ectoderm to differentiate as cornea.|
A critical gene that is expressed throughout the developing eye is Pax6. Mutants of Pax6 have aniridia (no iris), small eyes and even no eyes. Pax6-/- homozygotes lack eyes and nasal cavities. Pax6 is required in the ectoderm, as shown by tissue recombination studies using Pax6+/+ and Pax6-/- ectoderm and optic cups. Also, ectopic expression of eyeless (the Drosophila homolog of Pax6) in Drosophila larvae causes ectopic eyes to develop on the legs and wings; murine Pax6 has a similar effect, showing its strong conservation.