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and Zhao 2001 ; Ohinata et al. 2009 ), and it was shown that their signals are trans-
duced via phosphorylated Smads (Chang and Matzuk 2001 ; Tremblay et al. 2001 ).
Using an in vitro culture system it was also established that BMP4 is a more potent
inducer than BMP2, and that BMP8b indirectly potentiates germ cell induction by
inhibiting signals produced by the visceral endoderm (Ohinata et al. 2009 ). In other
mammalian species less is known about the roles of BMPs in germ cell induction.
However, it is worth noting the important anatomical difference between mice and
other mammals at this stage of development. After the formation of the ICM, the
mouse embryo undergoes cavitation of the epiblast which results in the formation of
the egg cylinder. The mechanisms directing this process are mediated by BMP4
secreted by the extraembryonic ectoderm, which promotes cavitation via apoptosis
(Coucouvanis and Martin 1999 ). In species in which cavitation is not apparent, like
in human and other mammalian embryos, the polar trophoblast is lost during the
formation of the epiblast (or embryonic disc). What is the relationship between the
differences in embryological configurations for germ line development?
Previous studies showed that BMP4 can induce the activation of germ cell mark-
ers from (Fig. 8.11b) stem cells (Kee and Reijo Pera 2008 ; Alberio et al. 2010 ), and
BMP2 and -4 have been shown to efficiently induce hPGCL cells from hESC (Irie
et al. 2015 ). The role of BMPs as inductive signals in embryonic disc forming species
is consistent with evidence from studies in rabit and pig embryos showing that BMPs
are highly expressed in the primitive endoderm as well as in early mesodermal cells
at this stage of early gastrulation (Valdez Magana et al. 2014 ; Hopf et al. 2011 ).
Interestingly, in both species, BMP2 expression in the primitive endoderm precedes
BMP4 expression, which is enriched in the mesodermal layer in early gastrulating
embryos (Hopf et al. 2011 ; Valdez Magana et al. 2014 ). Furthermore, BMP expres-
sion in the posterior streak region is mediated via phosphorylated Smad1/5/8 (Valdez
Magana et al. 2014 ), similar to what is known in mice. Thus, these observations sug-
gest that the activation of BMP4 in mice extraembryonic ectoderm had consequences
not only for the formation of the egg cylinder, but it may also have resulted in the
precocious activation of germ line specification genes from proximal epiblast cells.
8.6.3 Pluripotent Gene Expression During PGC Specification
Intriguingly, during germ cell specification pluripotent gene expression is restored
following the reactivation of Nanog and Sox2 in Oct-4 expressing PGC precursors.
Expression of Nanog, Oct-4 and Sox2 is essential for PGC development, as shown
by the loss of PGCs as a result of reduced proliferation (Sox2 mutants) and death by
apoptosis (Nanog and Oct-4 mutants) after conditional repression of these genes
(Campolo et al. 2013 ; Kehler et al. 2004 ; Chambers et al. 2007 ). It is not clear how-
ever, what are the specific roles of these factors during germ cell development, but
it is thought that their expression confers latent pluripotency to the germ line. This
latent pluripotency was demonstrated by the derivation of embryonic germ cells
from PGCs (Labosky et al. 1994 ; Matsui et al. 1992 ). Embryonic germ cells can
T. Aguero et al.