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Fig. 8. Model for the segregation of the telencephalic and eye anlage during
zebrafish forebrain patterning. Patterning boundaries are indicated by
black arrows. During gastrulation (left panel), a prevalent view is that the
forebrain is patterned as a whole by the opposite activities of a posterior
Wnt source located at the level of the posterior diencephalon (green gradient)
and Wnt antagonists (purple gradient) located at the ANB
(Wilson and Houart, 2004).
`High Wnt' defines the presumptive diencephalon (green), whereas `low Wnt'
defines an anterior domain (purple) including the presumptive telencephalon
and eyes. At the tail-bud stage (middle panel), a posterior patterning
boundary (black arrow with star) is set-up in front of the diencephalon,
isolating the anterior forebrain. Within the latter domain, Rx3 activity (red)
represses tlc expression (blue gradient) and foxg1 (blue
bar) and attributes retinal fate to its expressing cells at the expense of a
telencephalic fate, thereby segregating retinal and telencephalic identities
(top panel). In the absence of Rx3, retinal precursors acquire a telencephalic
fate (bottom panel), leading at 24 hpf to the absence of eyes and an enlarged
telencephalon (right panel).
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