175Establishment of the DV axis of the embryo is governed by maternal factors
localized to the vegetal pole of the egg (Fig. 5.1b) (see Chap. 6 ) (De Robertis and
Kuroda 2004 ; Langdon and Mullins 2011 ). Among these, the factors that determine
the site of the dorsal organizer of the embryo are called dorsal determinants, and
they are specifically localized in the egg during oogenesis (Langdon and Mullins
2011 ). In both Xenopus and zebrafish, the activity of the dorsal determinants results
in the nuclear localization of the Wnt pathway effector, β-catenin, specifically in a
group of blastomeres on the future dorsal side of the blastula embryo (De Robertis
and Kuroda 2004 ; Langdon and Mullins 2011 ). This local activity of β-catenin
cooperates with broader Nodal and FGF signals to induce dorsal gene expression
(De Robertis and Kuroda 2004 ; Langdon and Mullins 2011 ).
Evidence indicates that the dorsal determinants are Wnt ligands. Albeit implied
from the role of β-catenin in dorsal organizer specification, their entity had been
illusive for some time. In Xenopus the dorsal determinant is a unique combination
of Wnt11 and Wnt5a ligands, each traditionally considered a non-canonical Wnt
ligand (Cha et al. 2008 , 2009 ; Tao et al. 2005 ). Surprisingly, Wnt11 and Wnt5a are
connected via O-sulfate bonds and function as a dimerized ligand to activate the
canonical Wnt pathway, inducing β-catenin nuclear localization on the future dorsal
side of the embryo (Cha et al. 2008 , 2009 ; Tao et al. 2005 ). In zebrafish Wnt8 is the
implicated dorsal determinant (Lu et al. 2011 ).
Despite using different Wnt ligands, dorsal determinant activity is regulated by
similar principles in Xenopus and zebrafish. The wnt ligand mRNAs are initially
EggDorsal DeterminantsAnimalVegetalFertilization Ventral DorsalGerm plasm accumulation
(Animal view)BuckyballZygote
Germ Plasm2-cell stage 4-cell stageab cMicropyleFig. 5.1 Function of localized maternal products. Establishment of the AV axis in the egg defines
the (a) sperm entry region, (b) dorsal-ventral (DV) axis, and (c) the location of the germ plasm,
which specifies the PGCs. (a) In frog, fish, and mouse, the oocyte nucleus migrates to the animal
pole during oocyte maturation and extrudes the first polar body. Frog and fish eggs are fertilized in
the animal half, and in zebrafish the sperm enters through the micropyle that forms at the animal
pole. Following fertilization, the second polar body is also extruded at the animal pole. At the egg
vegetal pole, the germ plasm (red) and dorsal determinants (green) are localized. (b) Upon fertil-
ization, dorsal determinants are transported from the vegetal pole to the future dorsal side of the
embryo. Here, activation of the Wnt pathway specifies the dorsal fate. (c) The germ plasm, includ-
ing Bucky ball, accumulates at the cleavage furrows of the 2- and 4-cell stage embryo, illustrated
for the zebrafish embryo here
5 Localization in Oogenesis of Maternal Regulators of Embryonic Development