Making Germ Cells from Human Embryonic Stem Cells - Male Germline Stem Cells Developmental and Regenerative Potential - page 53

BOULE , NANOS , PUMILIO ( PUM ), and VASA genes, and functional data indicates
that these genes are required for the establishment, maintenance, and differentiation
of germ cells in various organisms (Eberhart et al. 1996 ; Houston et al. 1998 ;
Houston and King 2000 ; Johnson et al. 2001 ; Karashima et al. 2000 ; Lin and Page
2005 ; Maegawa et al. 1999 ; Reijo et al. 1995 ; Ruggiu et al. 1997 ; Tsuda et al .
2003 ). Notably, in both germ plasm inheritance in non-mammalian species and
inductive signaling in mammals, the germ cells are maintained by mechanisms that
prevent them from differentiating into somatic cells. Indeed, the observation that in
many animals the PGCs arise outside of the embryo proper during the time that the
somatic cell program is being established may have functional significance in nor-
mal germ cell formation.

3.2.2 The Morphological Features of Gamete Development

From limited human embryological studies and studies in animal models focused
on germ cell development we have gained some understanding of the complex
temporal and spatial sequences of early germ cell development and gamete differ-
entiation in vivo . The formation of the PGCs is an early event during human devel-
opment. The PGCs arise in the proximal epiblast immediately outside of the
extra-embryonic ectoderm. Within the first 2-3 weeks of embryonic life, after gas-
trulation of the embryo, the germ cell lineage begins as a small population of PGCs
located just outside of the embryo in the developing yolk sac (Larsen 1997 ; McKay
et al. 1953 ; Moore and Persaud 1998 ; Motta et al. 1997 ). During the first 5 weeks
of gestation, the PGCs begin to migrate from the yolk sac into the developing
embryo (Park et al. 2009 ). At this time the genital ridges begin developing from the
intermediate mesoderm, which ultimately forms all the somatic cells of the gonad.
The PGCs migrate along the hindgut to the genital ridges, undergoing massive
proliferation along their journey.
Once PGCs arrive in the nascent gonads, between 5 and 6 weeks of gestation,
the PGCs continue to proliferate, remaining connected by intercellular cytoplasmic
bridges (Motta et al. 1997 ; Heyn et al. 1998 ). In the female fetus, within the ovaries
these PGCs, termed gonocytes, begin oogenesis, differentiating into some 5-7 million
oogonia. Each oogonium then enters meiosis and develops within the supportive
surrounding granulosa and thecal cell layers and connective tissue that make up the
follicle. The development of oocytes is then halted at meiotic prophase I during the
12th week of gestation (Motta et al. 1997 ). Subsequent follicle loss by atresia or
cell death begins, resulting in approximately one million follicles by the time of
birth. By the time of sexual maturation or puberty, roughly 500,000 follicles remain
within the ovaries. At this time oocytes are either recruited to resume meiosis,
mature, and are ovulated, or they continue to undergo atresia. With no oogonial
stem cells, over time the oocyte population is depleted until approximately 1,000 or
less remain, menstrual cycling ceases, and menopause ensues. In fact, the total
oocyte pool or “ovarian reserve” can be correlated with reproductive potential and