55near their original sites of localization. After fertilization the new cells that form during
the initial cell divisions in cleavage-stage embryos capture these mRNAs and their sur-
rounding cytoplasm. As a consequence, vegetally localized mRNAs become concen-
trated in vegetal cells that will give rise to endoderm, while animally localized mRNAs
become concentrated in animal cells that will give rise to ectoderm.
The importance of localized maternal mRNAs for the formation of the germ lay-
ers is supported by loss-of-function experiments that deplete specific maternal
mRNAs from developing embryos, as discussed above (see Sect. 2.2). A role for the
vegetally localized maternal VegT mRNA was discussed above. Another vegetally
localized mRNA, the maternal Vg1 mRNA, encodes a secreted ligand that can acti-
vate TGFβ signaling. Vg1-depleted embryos exhibit defects that indicate that Vg1
is required for endoderm and mesoderm formation and anterior cell types (Birsoy
et al. 2006 ). In contrast, the Foxi2 maternal mRNA encodes a transcription factor
and is localized to animal cells. Depletion of Foxi2 causes defects in ectoderm for-
mation by disrupting the normal activation of zygotic genes important for the ecto-
derm (Cha et al. 2012 ). Thus, specific localized mRNAs provide important links
between the asymmetries formed in the oocyte and the establishment of germ layers
during embryogenesis.
Finally, localized mRNAs provide the molecular basis for establishing the asym-
metries that form during the initial stages of embryogenesis and organize the verte-
brate body plan (Houston 2012 , 2013 ). During the earliest steps of Xenopus
development, there are at least two types of embryonic asymmetries that must form
animal-vegetal and dorso/anterior. mRNA localization in the oocyte creates an
unequal distribution of molecules in the animal-vegetal dimension that persist after
fertilization to contribute to the animal-vegetal axis of the embryo (Fig. 2.2a). The
second asymmetries form as a result of cortical rotation and are keys for establish-
ing the early signaling centers of the embryo: the Nieuwkoop center and the
Spemann organizer that are located to one side of the embryo (Gerhart et al. 1989 ,
1991 ; Houston 2012 ) (Fig. 2.2b). While the importance of cortical rotation is
unequivocal, our understanding in molecular terms of exactly what it accomplishes
remains incomplete. The challenge is to explain, at the level of molecular mecha-
nism, how both type asymmetries influence development.
2.4 Signaling Pathways and Their Activation
During Maternal Xenopus Development
Multiple signal transduction pathways are present in Xenopus embryos, and their
regulated activation is necessary to guide the normal events of development. These
pathways include the fibroblast growth factor (FGF), Wnt, bone morphogenetic
protein (BMP), and Vg1/Nodal pathways. Each of these pathways functions in
different regions of the embryo to direct the formation and patterning of the
embryonic germ layers (Heasman 2006b; Schohl and Fagotto 2002 ; Harland and
2 Controlling the Messenger...