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embryo (King et al. 2005 ). Therefore, the initial positional information that sets up
the embryonic germ layers and body axis is stored in and provided by the vegetal
pole. Oocyte polarization during early oogenesis is therefore critical for these fun-
damental processes of embryogenesis.
Interestingly, the final destinations of vegetally localized transcripts in the Xenopus
egg differ following fertilization. While vegT and wnt11 are released from the cortex
and diffuse in the cytoplasm, syntabulin and grip2a transcripts localize to the cleavage
furrows with the germplasm and are inherited specifically by the primordial germ cells
(PGCs) (Claussen et al. 2011 ; Colozza and De Robertis 2014 ; Ge et al. 2014 ; Kaneshiro
et al. 2007 ). Despite its mRNA localizing to the PGCs, Syntabulin protein is expected
to localize more broadly in Xenopus, as it does in zebrafish, to fulfill its role in dorsal
axis determination (Colozza and De Robertis 2014 ; Nojima et al. 2010 ). The distinct
post fertilization localization of vegT and wnt11 versus syntabulin and grip2a may
reflect their distinct localization mechanisms during oogenesis via the late localization
pathway versus the early Bb pathway, respectively. It is possible that distinct vegetal
cortical docking mechanisms cause the distinct postfertilization cortical release or
cleavage furrow localization, or alternatively different regulatory information in their
respective mRNAs deliver them to distinct locales following fertilization.
5.2.3 Germ Cell Determinants Originate in the Polarized
Oocyte
Animals have developed two primary strategies to specify the germ line (see Chap.
8 for more details). In mammals, the germ cell lineage is induced in the epiblast by
the action of BMP4 produced in the extraembryonic ectoderm (Lawson et al. 1999 ).
In other vertebrates, and in most insects, germ cells are specified by the inheritance
of the germplasm (GP), a cytoplasmic aggregate containing RNA and proteins suf-
ficient to determine the primordial germ cell (PGC) fate (Kobayashi et al. 1994 ;
reviewed in Extavour and Akam 2003 ). In zebrafish and Xenopus, GP accumulates
at the 2- and 4-cell stages at the ends of the cleavage furrows (Fig. 5.1c) (Hashimoto
et al. 2004 ; Kloc et al. 2001 ; Yoon et al. 1997 ). Removal of GP diminishes or ablates
PGC formation in fish and frogs (Ikenishi et al. 1974 ; Smith 1966 ; Whitington and
Dixon 1975 ; Yoon et al. 1997 ; Zust and Dixon 1975 ). Conversely, GP transplanta-
tion induces PGC formation (Kobayashi et al. 1994 ), thus providing compelling
evidence for GP function in germ cell specification.
In Xenopus the GP becomes localized during oogenesis to the oocyte vegetal cor-
tex, where it remains localized in the egg (Figs. 5.1a and 5.2b) (Heasman et al. 1984 ;
Fig. 5.2 (continued) molecular motors (late pathway). Thus, at this stage early and late localized
RNAs reside at the vegetal cortex (orange). During this stage, additional mRNAs (e.g., Vg1 in zebraf-
ish) are localized animally (light blue), and the micropyle becomes evident at the animal pole. In stage
IV, the oocyte matures, the nucleus migrates to the animal pole, and the nuclear envelope disassembles
in frogs, fish, and the mouse. At this stage, the oocyte transitions from diplotene of meiosis I to meta-
phase of meiosis II, extruding the first polar body at the animal pole in all three species
M. Escobar-Aguirre et al.