Fig. 9. A proposed mechanism can generate a differential gradient of Fz8a-mediated Wnt8b signalling required for the anterior brain patterning. The spatial expression pattern of fz8a, wnt8b, pax6 and eng2 in the presumptive forebrain and midbrain domains of the 90% and 100% epiboly stage embryos is schematically illustrated. At the 90% epiboly stage, the posterior expression domain of fz8a overlaps that of wnt8b expression. At the 100% epiboly stage, fz8a expression within this overlapping domain gradually lost its expression by the repression Wnt8b that is secreted from presumptive midbrain. This repression might decrease Fz8a/Wnt8b signal activity in the presumptive posterior diencephalon, thus generating a sharp gradient of Fz8a/Wnt8b signal activity at 100% epiboly stage. As results, eng2 (light yellow) expression was probably induced by higher Fz8a/Wnt8b signal activity in the presumptive midbrain during late gastrulation. However, anteriorly transferred Wnt8b can establish lower Fz8a/Wnt8b signalling activity, thus defines territory of pax6 expression in the presumptive diencephalon (grey). Broken and unbroken brown lines indicate hypothetical gradient of Fz8a/Wnt8b signal activity at the 90% and 100% epiboly stage, respectively. T, telencephalon; E, eye primordium; vd, ventral diencephalon; M, midbrain.

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