177axis of the early embryo. In this setting, the ectoderm forms from cells in the animal
region, the endoderm from the most vegetal region, and the mesoderm from the margin
(see Chap. 7 ) (De Robertis et al. 2000 ; Kimelman 2006 ). All animal embryos generate
their germ layers during the process of gastrulation (see Solnica-Krezel and Sepich
2012 ). The outcome of this massive reorganization is the formation of the endoderm as
the innermost tissue, the ectoderm as the outermost, and the mesoderm between them.
The major inducer of mesoderm is the Nodal signaling pathway (reviewed in
Kimelman 2006 ; Schier 2003 ). Nodal ligands are secreted from vegetal cells and
activate the pathway vegetally as well as in marginal cells of the embryo (De Robertis
et al. 2000 ; Kimelman 2006 ). This marginal zone is specified as mesoderm in
Xenopus and mesendoderm in zebrafish (De Robertis et al. 2000 ; Kimelman 2006 ;
Schier and Talbot 2005 ). In addition to the uniform secretion of Nodal along the DV
axis, on the dorsal side of the embryo, nodal ligands are expressed slightly earlier,
resulting in a brief transient higher accumulation of Nodal signal in this region
(Kimelman 2006 ). In addition, the nodal ligand squint (sqt) mRNA was described in
zebrafish to exhibit localization dynamics that enrich it dorsally at the 4-cell stage
(Gore et al. 2005 ), which could further reinforce a Nodal gradient. In Xenopus the
endoderm forms in the vegetal hemisphere, while in zebrafish, the mesendoderm
comprises the marginal cells, with the endoderm specified in the most vegetal cell
tier (Ober et al. 2003 ; Schier and Talbot 2005 ). This setting puts the endoderm pre-
cursor cells in both animals in greater proximity to the vegetal Nodal signal source,
and this vegetal-to-animal Nodal signaling gradient is thought to play a role in dis-
tinguishing between mesodermal and endodermal cell fates (De Robertis et al. 2000 ;
Kimelman 2006 ; Schier and Talbot 2005 ; van Boxtel et al. 2015 ).
The expression of nodal in Xenopus is regulated by the vegetal pole localized fac-
tor VegT, a T-box transcription factor (Clements et al. 1999 ; Kimelman 2006 ; Xanthos
et al. 2001 ). Veg T mRNA is released from the vegetal cortex upon fertilization and
slowly diffuses in the vegetal cytoplasm. During the third cell division in Xenopus, a
cleavage furrow forms along the embryo equator, perpendicular to the AV axis, and
this cell division confines Veg T transcripts to the vegetal hemisphere. VegT protein is
then only produced in the vegetal hemisphere, which induces expression of nodal
ligands specifically in this region (Clements et al. 1999 ; Kimelman 2006 ; Xanthos
et al. 2001 ). In zebrafish, Nodal signaling from the extraembryonic yolk syncytial
layer (YSL), which underlies the blastoderm, is required to induce nodal ligand
expression in the marginal blastoderm cells (Fan et al. 2007 ; Hong et al. 2011 ).
Within the YSL, another maternal T-box gene, Eomesodermin, together with the Mix
homeodomain transcription factor Mxtx2, induces nodal ligand expression (Du et al.
2012 ; Xu et al. 2014 ). The eomesodermin mRNA localizes to the zebrafish oocyte
cortex, similarly to vasa, although it is not known if it localizes there via the Bb like
vasa (Bruce et al. 2003 ; Kosaka et al. 2007 ).
The vegT mRNA is localized to the oocyte vegetal pole not via the Bb but by a
later pathway in oogenesis (see Sect. 5.4). In this pathway, the Kinesin-1 and
Kinesin-2 motors bind and transport mRNPs, including vegT mRNA, to the vegetal
cortex, utilizing specialized microtubules that radiate their plus-ends vegetally
(King et al. 2005 ; Messitt et al. 2008 ). vegT mRNA remains anchored to the vegetal
pole cortex throughout oogenesis and in the egg, before being released in the early
5 Localization in Oogenesis of Maternal Regulators of Embryonic Development