Box 1. Defining the axes of Xenopus embryos
The first axis of the Xenopus embryo is the animal-vegetal axis, which passes through the animally localized egg pronucleus, the center of the egg and the vegetal pole (Fig. 1B, Fig. 2B). The second axis is defined by the sperm-entry point and by the position of maximal movement of the cortical cytoplasm away from the sperm-entry point (Fig. 1B, Fig. 2B). At gastrulation, the dorsal lip of the blastopore forms opposite the sperm-entry point. Although described as `dorsal', this Spemann Organizer region (Fig. 1A) contains anterior precursors, including prechordal mesoderm (Dale and Slack, 1987; Keller, 1975; Lane and Sheets, 2000; Moody, 2000; Shook et al., 2004).
As the term `dorsal lip of the blastopore' is established in the literature, it continues to be used here, accepting the fact that the region includes non-dorsal precursors. As shown in Fig. 2, the embryonic dorsoventral axis describes the position of cells relative to the sperm-entry point and to the site of future formation of the dorsal lip blastopore (Fig, 2B). The definitive dorsoventral axis refers to the axis at right angles to the anteroposterior axis, which is established at the end of gastrulation (see Fig. 2B).
Box 2. Factors regulating chordamesoderm formation
Convergence extension (CE) movements of the chordamesoderm close the blastopore during gastrulation (see Fig. 2A and Fig. 6). Several factors have essential roles in this process:
- The establishment of two domains in the chordamesoderm, marked anteriorly by chordin and posteriorly by Xbra expression. Mixing chordin-and Xbra-expressing cells prevents convergence extension until the two populations are sorted (Ninomiya et al., 2004).
- Xbra and FGFR1 activity. Their abrogation causes CE defects and reduces notochord and somite formation, illustrating the importance of the FGF/Xbra loop in gastrulation movements (Amaya et al., 1991; Conlon et al., 1996).
- Zygotic Wnt11mRNA expression, which is regulated by Xbra (Tada and Smith, 2000). Wnt11 causes Xdsh localization at the cell cortex in a PAR1/PKC dependent fashion (Habas et al., 2003; Kusakabe and Nishida, 2004; Tada and Smith, 2000) and also mobilizes the cytoskeleton by activating Rac and RhoA (Habas et al., 2003; Habas et al., 2001).
- The Wnt target gene Xnr3, which activates the dorsal Xbra expression domain (Yokota et al., 2003). Xnr3 underexpression causes CE defects and head reduction, indicating that it has an additional role in prechordal mesoderm migration/specification.
- Xlim1, a VegT-target LIM-domain transcription factor specifically expressed in the organizer. Its depletion causes head reduction and CE defects (Hukriede et al., 2003). Xlim1 regulates the dorsal paraxial protocadherin expression, which is required for dorsal RhoA and Jun kinase activation and for CE movements. Xlim1-depleted embryos still make posterior notochord, indicating that the anterior chordamesoderm is Xlim1 dependent, while posterior chordamesoderm may be regulated by Xbra/Wnt11/Dsh.