<P> Only the epidermis in the head is competent to respond to the signal from the optic vesicles . Both the optic vesicle and the head epidermis are required for eye development . The competence of the head epidermis to respond to the optic vesicle signals comes from the expression of Pax6 in the epidermis . Pax6 is necessary and sufficient for eye induction . This competenece is acquired gradually during gastrulation and neurulation from interactions with the endoderm, mesoderm, and neural plate . </P> <P> Sonic hedgehog reduces the expression of Pax6 . When Shh is inhibited during development, the domain of expression for Pax6 is expanded and the eyes fail to separate causing cyclopia . Overexpression of Shh causes a loss of eye structures . </P> <P> Retinoic acid generated from vitamin A in the retina plays an essential role in eye development as a secreted paracrine signal which restricts invasion of perioptic mesenchyme around the optic cup . Vitamin A deficiency during embryogenesis results in anterior segment defects (particularly cornea and eyelids) that lead to vision loss or blindness . </P> <P> There is some evidence that LMX1B plays a role in periocular mesenchymal survival . </P>

The last structures of the eye to develop are