Biology Reference
In-Depth Information
First wave of
reprogramming
Second wave of
reprogramming
E6.25
E7
E8
E9
E10.5
E11
E12
E13.5
Mouse
PGC induction
and
specification
PGCs reside
in yolk sac
mesoderm at
the base of
the allantois
PGCs migrate
along the
hindgut and
travel through
the dorsal
mesentery
PGCs
colonize the
genital ridge
from the
dorsal
mesentery
Appearance
of sex cords
in male
gonad,
indicating
gonadal sex
Female germ
cells enter
meiosis
??
Human
Wk 3
Day 15-21
Wk 4
Day 22-28
Wk 5
Day 29-35
Wk 6
Day 36-42
Wk 7-9
Day 43-63
Wk 10
Day 64-
(CS6 -10)
(CS10 -13)
(CS14 -15)
(CS15 -16)
(CS17-24)
First wave of
reprogramming??
Second wave of
reprogramming??
Figure 5.2 Timeline of mouse and human PGC development. Established embryological
landmarks are compared between mouse and human. The exact timing of human PGC
specification is unknown (indicated by
“
??
”
). The timing andmolecular features of epige-
netic reprogramming inhumans has not beenestablished. Note that the timelines arenot
drawn to scale, and themouse timeline is aligned to the human timeline according to the
timing of various embryological events. CS, Carnegie stage; E, embryonic day; Wk, week.
comparison with large domestic animals and, if information emerges, non-
human primates will be particularly informative in combination with direct
studies in the human. We hope that further studies will allow a more rigor-
ous comparison of PGC development and epigenetic reprogramming
between mouse and man in the future (
Fig. 5.2
).
5. PGC CULTURE AND EMBRYONIC GERM-CELL
DERIVATION
Efficient
in vitro
culture of PGCs has long been a goal of researchers as a
tool to study germ-cell development in greater molecular detail. Investiga-
tions aimed at improving
in vitro
culture of PGCs also led to the serendip-
itous discovery of embryonic germ (EG) cells (
Matsui, Zsebo, & Hogan,
1992; Resnick, Bixler, Cheng, & Donovan, 1992
). Although derived from
PGCs, EG cells exhibit growth requirements and properties indistinguish-
able from ES cells and are thus classified as pluripotent stem cells. As a result,
in vitro
investigations have focused largely on two different goals; continued
