Biology Reference
In-Depth Information
(
Borgel et al., 2010; Guibert, Forne, & Weber, 2012
). Although
Dazl
,
Mvh
,
and
Sycp3
appear not to be required during PGC development in the mouse,
they play an essential role in later germ cells.
Dazl
is required for gametogen-
esis in both males and females (
Ruggiu et al., 1997
), whereas
Mvh
and
Sycp3
play a critical role in spermatogenesis (
Tanaka et al., 2000; Yuan et al., 2000
)
In the rat, Mvh expression is detected in migrating PGCs prior to their col-
onization of the genital ridges, while GCNA and SSEA-1, PGC markers in
the mouse, are not detected in rat PGCs (
Encinas et al., 2012
). Thus, even
between rodents there appear to be some differences in the expression profile
of classic late PGC marker genes. As previously discussed, SSEA-1 and c-Kit
are detected in human PGCs although the onset and duration of their expres-
sion has not been rigorously determined. DAZL, VASA, and SYCP3 are
expressed in human gonadal germ cells (
Anderson, Fulton, Cowan, Coutts,
& Saunders, 2007; Castrillon, Quade, Wang, Quigley, & Crum, 2000; Liu
et al., 2007
). Similar to rats, VASA is detectable in human migratory PGCs
at the gut and mesenteric root (
Castrillon et al., 2000
). Interestingly, the
CpG islands associated with
Dazl
and
Vasa
in the mouse appear to be con-
served in humans, and they remain hypomethylated in sperm but not other
somatic tissues (
Chai, Phillips, Fernandez, & Yen, 1997; Sugimoto et al.,
2009
). Whether regulation of germline-specific genes by promoter methyla-
tion is a general feature in humans remains to be elucidated.
A summary of the expression of a range of PGC marker genes is pres-
ented in
Table 5.1
. We hope that in the coming years this will be completed
and expanded upon. A more detailed analysis of the expression of germ-cell
marker genes in a range of mammals, including human, would greatly
enhance our understanding of the most conserved, and therefore perhaps
most critical, progressions in gene expression during mammalian PGC
development.
4. EPIGENETIC REPROGRAMMING
Epigenetic reprogramming is a key feature of PGC development. Per-
haps most notably, this allows erasure of the DNA methylation marks asso-
ciated with imprinted genes, allowing establishment of sex-specific imprints
during gametogenesis. There is also extensive reprogramming of histone
modifications both prior to and during imprint erasure. These chroma-
tin changes may be necessary to facilitate DNA demethylation but may
have other functions such as the erasure of “somatic” epigenetic marks
established during postimplantation development, to allow X-chromosome
