235antisense oligonucleotide mRNA depletion of maternal ctnnb1 mRNA in Xenopus
oocytes, leading to embryos lacking axial structures and dorsal-specific gene expression
(Heasman et al. 1994 ). Additionally, in Nieuwkoop conjugate experiments, late blas-
tula vegetal masses from ctnnb1-depleted embryos fail to induce dorsal mesoderm in
animal caps, suggesting that maternal Wnt/beta-catenin is essential for the genera-
tion of the Nieuwkoop signal and acts upstream of other axis-inducing molecules
(Wylie et al. 1996 ). Analysis of heterochronic Nieuwkoop conjugates pre- and post-
midblastula transition also showed that dorsal and general mesoderm induction are
primarily zygotic events (Wylie et al. 1996 ). Beta-catenin is present in dorsal nuclei
prior to major zygotic genome activation in the Xenopus morula and blastula as well
as in the zebrafish dYSL and dorsal marginal blastomeres (Schneider et al. 1996 ;
Jesuthasan and Stähle 1997 ; Kelly et al. 2000 ; Dougan et al. 2003 ) demonstrating
that Wnt/beta-catenin signaling is active in the relevant region of the embryo.
In addition to theses data, genetic studies in zebrafish identified a requirement for
a maternally expressed beta-catenin in normal axis formation (ctnnb2/ichabod;
Kelly et al. 2000 ). Furthermore, in the mouse, genetic deletion of Ctnnb1 results in
embryos lacking axial structures and anteroposterior polarity, resulting from lack of
all mesoderm and failure to form the anterior visceral endoderm (AVE) (Haegel
et al. 1995 ; Huelsken et al. 2000 ; Morkel et al. 2003 ). Mouse beta-catenin is pre-
dominantly required in the epiblast, as shown in chimeric embryo experiments
(Huelsken et al. 2000 ) and is not required maternally (De Vries et al. 2004 ), reflect-
ing a different mode of activation in mammals. The main Wnt ligand expressed at
this time, Wnt3, is primarily expressed in the posterior epiblast and is required for
embryonic axis and mesoderm formation (Liu et al. 1999b). Interestingly, the for-
mation of the AVE is normal in Wnt3 null mice, indicating a differential role for
beta-catenin in the development of this tissue. A Wnt ligand-independent role for
beta-catenin in anteroposterior patterning has been proposed (Morkel et al. 2003 ),
possibly through regulation of Tdgf1 (teratocarcinoma-derived growth factor 1,
alias Cripto/frl1) expression and subsequent effects on Nodal activity (see Sect. 6.5).
Although genetic manipulations are less tractable in the chicken, studies using
extracellular Wnt inhibitors suggested that Wnt signaling is required for experimen-
tal axis induction (Skromne and Stern 2001 ).
6.3.3 Asymmetric Activation of Wnt/Beta-Catenin Signaling
in Early Amphibian and Fish Embryos
6.3.3.1 Xenopus
Despite the well-documented roles for beta-catenin in axis formation in Xenopus,
and more recently in zebrafish, it remains relatively unclear how Wnt/beta-catenin
signaling is initiated in early embryos, as well as the extent that these activating
mechanisms are conserved. Cytoplasmic transplantation studies in Xenopus identi-
fied the presence of a cytoplasmic, transplantable dorsalizing activity in the vegetal
6 Vertebrate Axial Patterning: From Egg to Asymmetry