407terms of direction remains unclear. There is evidence for three different cell signal-
ing systems operating to guide PGCs to their targeted location: CXCR4/SDF1
(Takeuchi et al. 2010 ), Notch/Delta2 (Morichika et al. 2010 ), and KIF13B-PIP3-
GRIP2.1 (Tarbashevich et al. 2007 ). Notch 1 and Delta-1 are expressed in the endo-
derm and PGCs, but only delta-2 and Serrate-1 are expressed in PGCs at tailbud
stages. CXCR4 is expressed in PGCs and its ligand, SDF1 is expressed in the dorsal
mesentery, implicating this signaling system in providing the directional cues for
correct migration. Whether the endoderm secretes any signal that repulses migrat-
ing PGCs from the endoderm is unknown. PGC proteins Dead-End and Xdazl have
been implicated in normal PGC migration (Horvay et al. 2006 ; Houston and King
2000a). These RNA-binding proteins are involved in promoting mRNA translation,
suggesting that level of control is important.
8.3.3 Mechanisms Preserving Full Potential
Through Protection
As previously described, during oogenesis both germ cell and somatic cell determi-
nants become localized at the vegetal cortex. After fertilization, cellular divisions par-
tition somatic determinants equally to the vegetal blastomeres, but germ plasm is
asymmetrically inherited by only one of the two dividing daughter cells. To compli-
cate matters, some of these localized RNAs, such as Fatvg, have functions in both the
endoderm and PGCs (Chan et al. 2001 , 2007 ; Table 8.2). Given that the genetic pro-
grams for endoderm are not activated in PGCs, despite the presence of the endoderm
determinant VegT, repressive mechanisms must be operating. In Xenopus, as well as
in other organisms, both translational repression and the transient suppression of tran-
scription are critical to preventing expression of somatic fate in pPGCs (Lai et al.
Fig. 8.7 Migration of Xenopus Primordial Germ Cells within the endoderm. Migration within the
endoderm has been divided into four steps: clustering at stage 24, dispersing laterally at stage 28,
directionally migrating dorsally at stages 33/34 and aggregating at the dorsal-most aspect of the
endoderm at stage 40. Migration stages according to Terayama et al. ( 2013 ). Inset shows schematic
of PGCs within somatic gonadal tissue
8 Mechanisms of Vertebrate Germ Cell Determination