Biology Reference
In-Depth Information
2. PGC SPECIFICATION
2.1. Induction of PGCs from epiblast
Gene knockout studies in mice have revealed the critical role of bone mor-
phogenetic protein (BMP) signaling in PGC induction from the epiblast.
Bmp4
and
Bmp8b
are expressed in the extraembryonic ectoderm and both
knockouts exhibit loss of PGCs (
Lawson et al., 1999; Ying, Liu, Marble,
Lawson, & Zhao, 2000
). Mice heterozygous for either
Bmp
also have
reduced numbers of PGCs, as do
Bmp2
knockout mice (
Ying & Zhao,
2001
), emphasizing the importance of BMP dosage for PGC induction.
Similarly, the BMP type 1 receptor
Alk2
(
de Sousa Lopes, 2004
) and the
intracellular BMP-signaling transducers
Smad5
(
Chang & Matzuk, 2001
)
and
Smad1
(
Tremblay, Dunn, &Robertson, 2001
) all exhibit reduced num-
bers of PGCs when heterozygous and loss of PGCs in homozygous mutants.
Induction of PGCs from cultured E5.5 epiblasts requires the presence of the
extraembryonic ectoderm, but not of the visceral endoderm (VE;
Yoshimizu, Obinata, & Matsui, 2001
). However, PGCs do emerge from
the isolated E6.5 epiblast in culture (
Yoshimizu et al., 2001
). Exposure to
BMP4 or 8b increases the induction of PGCs in cultured E6 epiblasts, with
or without an intact VE, demonstrating the direct effect of these molecules
on the epiblast (
Ying, Qi, & Zhao, 2001
). These combined results suggest
that BMP molecules released from the extraembryonic ectoderm are
required to induce PGCs but not for subsequent specification events.
A recent comprehensive study of the signaling pathways required to
induce PGCs in cultured epiblasts has confirmed and expanded upon these
previous findings (
Ohinata et al., 2009
). The authors report that BMP4 is
sufficient to induce PGCs in cultured epiblast in the absence of the VE.
BMP2 can also induce PGCs although less efficiently than BMP4, while
BMP8b does not act directly to induce PGCs. However, when epiblasts
are cultured in the presence of the VE, addition of BMP8b is necessary
for BMP4-mediated induction. The proposed mechanism is that BMP8b
restricts inhibitory signals emanating from the anterior VE. In keeping with
this,
Bmp8b
null embryos have an expanded
Cer1
expression domain, and
their defect in PGC specification can be rescued by culturing
Bmp8b
null
epiblasts in the absence of VE. Interestingly,
FoxH1
null embryos, which
have a defect in anterior VE formation, appear to have increased numbers
of PGCs. Critically, PGCs induced in this manner exhibit an appropriate
gene expression pattern and, after further culture in a cocktail of supportive
