Over the past 5 years, the usefulness of the Xenopus model organism has grown considerably as a result of the Xenopus Genome Initiative (see www.xenbase.org/). This endeavor has provided a quantum increase in the amount of information available on Xenopus genes and the resources with which to study them. The development of loss-of-function technology has also increased our knowledge of individual gene function (Heasman et al., 2000). The result is that many more molecules have been shown to control early Xenopus development. The challenge for the modern developmental biologist is to stay abreast of this information. In this review, I summarize these new findings and incorporate them with the old. Inevitably, this survey will be incomplete. [For further information, see De Robertis and Kuroda (De Robertis and Kuroda, 2004), and for a comparison with zebrafish axis patterning, see Schier and Talbot (Schier and Talbot, 2005). For research into germ-line establishment, see also Zhou and King (Zhou and King, 2004).] For example, the nuts and bolts of development, including the cytoskeletal and adhesion machinery, many components of signaling pathways, transcriptional and cell cycle regulators are incompletely covered. The question that drives this review is, `What insight have recent functional studies given us on the mechanisms that pattern the early Xenopus embryo?'.
An overview of early Xenopus development
After fertilization, Xenopus embryos undergo cell cycles that have characteristic features (Fig. 1). During the first, 90-minute cell cycle, cortical cytoplasmic movements and male and female pronuclear fusion occur. The next eleven divisions occur at 20- to 30-minute intervals with no gap phases, while the embryo forms a ball of 4000 cells, which encloses a fluid-filled blastocoel cavity. This mid-blastula embryo has three regions, the animal cap (which forms the roof of the blastocoel), the equatorial or marginal zone (the walls of the blastocoel) and the vegetal mass (the blastocoel floor) (see Fig. 1B). Although all mid-blastula cells are pluripotent (Heasman et al., 1984), explants of the animal cap form ectodermal derivatives in culture, while equatorial explants form mesoderm and vegetal explants form endoderm. At the end of the twelfth cycle, gap phases reappear, the cell cycle lengthens to 50 minutes and zygotic transcription starts (this is called the mid-blastula transition, MBT). In the 15th cycle, the dorsal lip of the blastopore forms, the cell movements of gastrulation begin and mitosis stops. Gastrulation converts the embryonic ball into three layers, and establishes definitive anteroposterior and dorsoventral axes (Fig. 2, Box 1). In this review, I retrace this developmental pathway and ask how cells become committed to specific fates.
Pre-patterning by maternally stored mRNAs and proteins
To what extent does embryonic patterning rely on mRNAs and proteins inherited from the oocyte, or upon intercellular signaling downstream of zygotic gene transcription? For Xenopus development, it was predicted that oocyte stores would be essential for embryonic patterning, because zygotic transcription does not begin until the 4000-cell stage and because newly expressed zygotic genes have localized expression patterns. Recent studies have confirmed this prediction. Included in the essential maternal pool are: genome-wide transcriptional repressors, such as Xkaiso and the LEF/TCF family member Xtcf3 (Houston et al., 2002; Ruzov et al., 2004); transcriptional activators, including forkhead proteins (e.g. FoxH1, Foxi1E) (Kofron et al., 2004a; Suri et al., 2005); the T box protein VegT (Zhang et al., 1998); and cAMP response element-binding protein (CREB) (Sundaram et al., 2003). TATA-binding components of basal transcriptional complexes, TBP and TBP2, are also essential for normal development, and their depletion reduces the transcription of specific zygotic target genes and disrupts gastrulation (Jallow et al., 2004).
A simple strategy that provides a blueprint for development is the localized positioning of maternal mRNAs in the oocyte so that they are inherited by specific areas of the embryo. Transcripts of the transcription factors Zic2 and Xenopus grainyhead 1 (Xgrhl1) are localized to the animal hemisphere of the oocyte and early embryo (Houston and Wylie, 2005; Tao et al., 2005a). By contrast, VegT transcripts are localized in the oocyte vegetal hemisphere (Zhang and King, 1996), and VegT protein is inherited by only vegetal cells (Stennard et al., 1999).
The list of vegetally localized mRNAs continues to grow and includes transcripts of the signaling molecules Vg1 (Weeks and Melton, 1987) and Wnt11 (Ku and Melton, 1993), of the transcription factor Otx1 (Pannese et al., 2000) and of the RNA-binding protein bicaudal C (Wessely and De Robertis, 2000). The cortical cytokeratin filament network is likely to hold these transcripts in place, as antibodies specific for cytokeratin disruption dislodge localized mRNAs (Kloc et al., 2005). Unexpectedly, the degradation of two of the localized mRNAs themselves, VegT mRNA and the non-translated mRNA Xlsrts, also dislodges other mRNAs (Heasman et al., 2001; Kloc and Etkin, 1994; Kloc et al., 2005) and disrupts the cytokeratin network. These effects are rescued by VegT mRNA, suggesting that it has an architectural role, although the mechanism is unresolved (Kloc et al., 2005).
Vegetally localized mRNAs do not all fall into one spatial group. For example, transcripts of the RNA-binding protein Xdazl (Houston et al., 1998) and Xpat mRNAs (Machado et al., 2005) localize to the germplasm and remain in primordial germ cells, while VegT mRNA localizes to presumptive endodermal cells (Stennard et al., 1999). Vg1 mRNA becomes enriched in the dorsal vegetal quadrant of the early embryo compared with the ventral vegetal quadrant (Birsoy et al., 2006; Tao et al., 2005b). Thus, several distinct mechanisms of partitioning probably exist.