Figure 3. Model for regulation of mesoderm formation in zebrafish.
Sox2-expressing NMP cells are in a zone of moderate Wnt signaling (light orange) that maintains Brachyury. As cells leave this region, if they are not exposed to continued Wnt signaling, they express Sox2 but not Brachyury and become neural. Instead, if they are exposed to high levels of Wnt (dark orange) they express Tbx16, which represses Sox2. Tbx16 also represses Wnts so as cells continue to move anteriorly (left side), they shut off Brachyury and turn on mesodermal differentiation genes, thus locking in the mesodermal choice.