421without radically affecting the development of somatic cells. The paradigm estab-
lished with the axolotl system is defined by the Last Cell Standing Hypothesis, which
proposes that PGCs are derived from the last cells from the animal cap to engage in
lineage commitment. This model for germ line development contrasts with that of
experimental systems that contain germ plasm: In these organisms PGCs are the first
cell lineage established, not the last. However, the Last Cell Standing Hypothesis is in
accord with the late passage of PGCs over the blastopore, demonstrated independently
by both Nieuwkoop ( 1947 ) and Smith ( 1964 ). Moreover, axolotl PGCs develop within
a signaling niche that is conserved in large, non-rodent mammals, suggesting that the
Last Cell Standing Model describes a conserved paradigm for vertebrate germ cell
development (Johnson and Alberio 2015 ).
8.6 Germ Cell Development in Mammals
The first detailed accounts of PGC development in mammals were based on histo-
logical studies of early human and domestic animal embryos (Fuss 1913 ; Allen
1904 ). Although these reports showed that germ cells migrated through the gut mes-
entery before entering the primitive gonad, they provided details of where the cells
originated (Fig. 8.11a). A few decades after these initial studies were published,
Witschi described the presence of PGCs in 3.5-week-old human specimens in the
posterior end of the embryo close to the base of the allantois (Witschi 1948 ). These
histological observations were further validated when alkaline phosphatase activity
was identified as a reliable method for identifying PGCs, and enabled researchers to
track their origin back to the posterior primitive streak of E7 embryos (Chiquoine
1954 ; Ginsburg et al. 1990 ). Despite this progress the question of the mechanisms
by which germ cell are specified in the mammalian embryo remained unexplained
(Eddy et al. 1981 ). The intricate process of germ cell specification and epigenetic
reprogramming of the mammalian germ line have been thoroughly studied in mice,
and some progress has been made in other mammalian species. Here we present a
summary of the current understanding of these processes and highlight some of the
critical differences between species.
8.6.1 Germ Cell Specification Genes
Early lineage tracing experiments suggested that PGC did not originate from epi-
blast cells in E6.5 mouse embryos (Lawson and Hage 1994 ). Transplantation exper-
iments also showed that when cells from the distal epiblast were introduced into the
posterior proximal epiblast in E6.5 embryos they could contribute to the germ line,
indicating that cells can be recruited to this lineage after the onset of gastrulation
(Tam and Zhou 1996 ). However, in 2005, Ohinata et al. ( 2005 ) showed using a
genetic labeling technique, that approximately six cells in E6.25 proximal epiblast
8 Mechanisms of Vertebrate Germ Cell Determination