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Fig. 4. The retinotopic organisation of retinal projections requires Wnt
activity in both vertebrates and invertebrates. (A) Schematic of
the chick retinotectal projections under normal (left) or experimental
conditions (right). In the chick, ventral retinal axons expressing both Ryk
and Fz (red) project to the medial tectum, whereas dorsal axons expressing
only Fz (blue) terminate in the lateral tectum. This organisation is achieved
in part through the graded medial-high distribution (graded blue staining) of
Wnt3a. Fz-positive dorsal retinal cell axons are attracted by low doses of
Wnt3a. Misexpression of a dominant-negative (DN)-Ryk form in the dorsal RGCs
(green staining) induces a medial shift of their terminal arborisations.
(B) Schematic of the Drosophila visual system. Photoreceptor
cells differentiate behind the morphogenetic furrow (mf) and project in an
organised manner to the lamina after crossing though the optic stalk: ventral
axons expressing Dfz2 (orange) are attracted by Dwnt4 (yellow) expressed in
the ventral lamina. Iro expression (red) in the dorsal retina
attenuates the competence of Dfz2-positive dorsal photoreceptors (grey) to
respond to Dwnt4. Ectopic expression of Dwnt4 in the dorsal lamina
attracts ventral axons to the dorsal lamina.
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