Fig. 9. (A) A molecular gradient in the RPE of Xenopus tadpole embryos. The most peripheral region of the RPE expresses X-Smo, Gli2 and Gli3. BrdU-positive cells have been found in this zone 1. More centrally, in zone 2, these genes are inactivated and Hh genes are not activated. In zone 3, the Hh genes are expressed: X-bhh and X-chh strongly and X-shh weakly. Xotx5 is also expressed in zone 3. The XAR1 antibody also gives a staining in zone 3. (B) Model for the activity of Hh signalling during several steps of retinogenesis. From the midline, Hh signalling is required in eye field separation. This source of Hh signalling is then required in the establishment of the proximodistal axis of the retina. Later during retinogenesis, a source of Hh signalling emanating from the RPE is required for the proper RPE differentiation. The eye development is represented in blue, the mature RPE in black, Hh signalling gradients are in pink and areas affected when Hh signalling is blocked are indicated with yellow stars.