51Growing Xenopus oocytes are transcriptionally active and accumulate mRNAs
as they develop (oogenesis stages 1–6). Oocytes are thus accumulating the maternal
mRNA population required to drive the later stages of oogenesis, the initial cell
divisions following fertilization, and the subsequent maternal stages of embryonic
development. Some maternal mRNAs translated in oocytes generate proteins used
by oocytes and eggs, while others are stored in translationally inactive states until
later in maternal embryogenesis when their encoded proteins are needed. These
stored mRNAs include a specialized cohort that are transported and anchored to the
vegetal cortex of the oocyte (Medioni et al. 2012 ; King et al. 2005 ; Houston 2013 )
and another group that are transported and concentrated in the animal hemisphere
(Fig. 2.2a). These localized mRNAs encode proteins that help establish animal and
vegetal cell identity that defines the animal-vegetal axis of the oocyte. This animal-
vegetal axis is carried over into the fertilized egg and early embryo. Thus the com-
Fig. 2.2 Formation of asymmetries and specific cell types during Xenopus early development. (a)
Animal-vegetal polarity is established during oogenesis by the partitioning of molecules to animal
and vegetal hemispheres. These partitioned molecules are exemplified by specific mRNAs that
become localized during oogenesis to vegetal cortex and the animal hemisphere (Medioni et al.
2012 ; King et al. 2005 ; Houston 2013 ). After fertilization the cells of the embryo inherit these
molecular asymmetries when they contribute to the formation of the ectoderm, the mesoderm, and
the endoderm germ layers. (b) After fertilization asymmetry in the dorso/anterior to ventral/poste-
rior dimension is established as a result of cortical rotation (Gerhart et al. 1989 ; Houston 2012 ).
Wnt11 is translocated from the vegetal pole of the egg during cortical rotation to create the embry-
onic asymmetry in the dorso/anterior to ventral/posterior dimension (Schroeder et al. 1999 ).
Wnt11 activates Wnt signaling to direct the accumulation of beta-catenin protein. The high levels
of beta-catenin cause the dorso/anterior cells to induce and give rise to the organizer (Heasman
et al. 1994 ). At the gastrula stage, the organizer produces extracellular signals that influence and
pattern the adjacent cells of each germ layer (Gerhart et al. 1991 ; Harland and Gerhart 1997 )
2 Controlling the Messenger...