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Fig. 9
Model of dorsal-ventral axis formation in zebrafish.
The zebrafish DV axis specification can be divided to four phases based on the dynamic alteration of the dorsalizing activity of lithium treatment. SW1 designates 0–10 mpf in which lithium treatment can cause dorsalization of the embryos. In this phase, fertilization initiates a GSK-3 dependent mechanism regulating the orientation but not stablization of vegetal microtubules which is critical for the dorsalward transport of DDs like Wnt8a mRNA. UW1 designates the first unresponsive window of lithium treatment, from 10 mpf to the 32-cell stage, in which, especially in the early period, lithium treatment fails to efficiently cause dorsalization. In this period, dorsal determinants are transported from the vegetal pole to the perspective dorsal side. The transduction of Wnt/β-catenin pathway is probably blocked by some unknown mechanism in SW1 and UW1. SW2 designates the period from the 32-cell stage to the mid-blastula stage. In this period, dorsally located DDs are able to inhibit GSK-3, causing the stabilization and nuclear localization of β-catenin, and lead to the expression of dorsal organizer genes. In UW2, lithium treatment loses its ability to dorsalize zebrafish embryos and the organizer gene expression is translated gradually by cell movement to morphologically distinguished dorsal-ventral axis. Arrow head at the lower-left corner indicates the shield.