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expressing Blimp1 become lineage restricted PGC precursors (Ohinata et al. 2005 ).
A subsequent analysis using multicolored chimeric embryos estimated that the
number of germ cell founders is ~4 cells in the mouse (Ueno et al. 2009 ). Blimp1 is
thought to act as a transcriptional repressor of genes that promote somatic differen-
tiation in PGCs (Kurimoto et al. 2008 ). More recently, it has been shown that Hox
Fig. 8.11 Development of the germ line in mouse and pig. (a) Mouse PGC precursors are speci-
fied from proximal epiblast cells at E6.25. Between E7.25 and 7.5, the pool of PGCs expands
forming a cluster of ~40 cells (red dots) located at the base of the allantois. By E8, PGCs start their
migration towards the gonadal ridges reaching their destination by E9.5–E10. (b) Pig primordial
germ cells (red dots) are first identified as a cluster in the posterior primitive streak at day 12 (E12).
Later, they migrate as single cells through the gut mesentery reaching the gonad by day 22 (E22)
T. Aguero et al.