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extraembryonic mesoderm of mid-primitive streak stage embryos
(
Fig. 5.1
B). Thereafter, they are found in extraembryonic mesoderm at
the base of the developing allantois and then subsequently in the developing
hindgut endoderm (
Ginsburg et al., 1990
). With the advent of transgenic
technologies, PGCs have been traced back to the epiblast at E6.25 when
a few cells positive for Blimp1 (see later) can be identified (
Fig. 5.1
C).
Recent lineage tracing experiments suggest that the whole germ-cell lineage
may be descended from as few as three initial PGCs (
Ueno, Turnbull, &
Weissman, 2009
).
Although the relevance of the high AP activity in PGCs remains unclear,
it appears to be well conserved, marking PGCs not only in mice but also rats
(
Kemper & Peters, 1987
), humans (
Fujimoto, Miyayama, & Fuyuta, 1977
),
cows (
Cherny et al., 1994
), sheep (
Ledda, Bogliolo, Bebbere, Ariu, &
Pirino, 2010
), and pigs (
Takagi, Talbot, Rexroad, & Pursel, 1997
), although
the enzymatic activity has been reported to be lower in goats (
K¨hholzer,
Baguisi, & Overstr
¨
m, 2000
).
Rat PGC development seems to follow a similar gross embryological
time course to that of the mouse apart from differences in timing, due to
their longer gestation. At E10, rat PGCs are found at the base of the allantoic
mesoderm, entering the developing hindgut (equivalent to E8.5 in the
mouse). By E13.5, colonization of the genital ridges is essentially completed
(equivalent to E11.5 in the mouse) (
Kemper & Peters, 1987
).
More recently, PGC development in the pig has been studied using spe-
cific marker genes, such as Oct4 (see later). PGCs can first be detected in
early primitive streak stage embryos (E12) at the posterior pole of the epiblast
(
Wolf, Serup, & Hyttel, 2010
). In late primitive streak stage embryos
(E13-14), PGCs are found in the yolk sac/allantoic mesoderm and endo-
derm, presumably after passing through the streak (
Vejlsted, Offenberg,
Thorup, &Maddox-Hyttel, 2006; Wolf et al., 2010
). The early stages of por-
cine PGC development therefore appear to be largely similar to mouse and
human. The E15 pig embryo appears comparable to the E8.5 mouse, with
germ cells commencing migration into the developing gut. At around E18,
colonization of the genital ridge commences (
Hyldig, Ostrup, Vejlsted, &
Thomsen, 2011
). There appears to be disparity between timing of develop-
ment of the genital ridge/gonad and PGCs in the pig (in comparison with the
mouse), with PGCs having left the hindgut by E21-22 but morphological
genital ridges not being evident until E25 (
Hyldig, Ostrup, et al., 2011
). Overt
sexual differentiation of the gonad occurs by E28 (
Pilon, Behdjani, Daneau,
Lussier, & Silversides, 1998
).
