Making Germ Cells from Human Embryonic Stem Cells - Male Germline Stem Cells Developmental and Regenerative Potential - page 62

stages of follicular development. One antral follicle gives rise to the large dominant
follicle, which contains the oocyte that will be ovulated. AFC is correlated with
oocyte number and may be an accurate reflection of the number of remaining
oocytes in a woman's ovaries and a noninvasive window into the oocyte reserve
(Giacobbe et al. 2004 ; Kwee et al. 2007 ; Morris et al. 2002 ). Ongoing studies are
focusing on the determination of the genes and environmental factors that may
impact the oocyte pool as assessed by AFC in a large group of women of several
racial/ethnic groups (Schuh-Huerta et al., unpublished).
Pertaining to the molecular functions of the DAZ genes, it is interesting to note
that DAZ and DAZL associate with RNAs (Fox et al. 2005 ; Venables and Eperon
1999 ), ribosomes (Tsui et al. 2000b ), and several other RNA-binding proteins
including, Pumilio-2 (PUM2), BOULE, NANOS, DAZ-Interacting Protein 1
(DZIP1), and DAZ-Associated Protein-1 and -2 (DAZAP1 and DAZAP2) (Fox
et al. 2007 ; Moore et al. 2003, 2004 ; Tsui et al. 2000a ; Urano et al. 2005 ). Several
of these proteins are also required for fertility and germ cell development in diverse
organisms (Eberhart et al. 1996 ; 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 ). Homologs of PUM2 and NANOS proteins
act as translational repressors in germ cells in several species and are required for
germ cell migration and proliferation. Notably, when these genes are knocked out
in worms, flies, and mice the phenotype is a reduced number of germ cells in the
gonads and complete infertility (Tsuda et al. 2003 ; Forbes and Lehmann 1998 ;
Subramaniam and Seydoux 1999 ). It has been discovered that along with DAZL and
associated proteins, PUM2 and NANOS3 are expressed at various stages of oocyte
and follicular development in the woman and spermatogenesis in the man
(Fig. 3.3A ) (Moore et al. 2003 ). In fact, many of the proteins that associate with
DAZ/DAZL are homologs of germ plasm components found in invertebrates and
non-mammalian vertebrates. We propose that these molecules may interact in a
complex or “germ cell particle” that is evolutionarily conserved and functions in
germ cell development in diverse organisms from worms to humans (Fig. 3.3B ).
DAZL , along with several interacting proteins, may function in this germ cell
complex to regulate RNA stability and translation of key proteins in early germ
cells, eggs, and sperm. Early repression by this complex may ultimately determine
quantitative and qualitative characteristics of the germ cell population. The lack of
scientific tools to investigate this hypothesis in vivo as well as better understand the
various developmental programs of the germ cells, necessitate studies using human
embryonic stem cells in vitro .

3.4 The Generation of Germ Cells from ES Cells In Vitro

Accumulating recent work indicates that human ES cells can be used as a human
genome-based model system to generate germ cells and more mature gametes and to
study the molecular events of germ cell development and differentiation in vitro .