Download The constitution and the control of the ovarian

The constitution and the control of the ovarian reserve is of importance in mammals and women. In
particular, the number of primordial follicles at puberty is positively correlated with the number of
growing follicles and their response to gonadotropin treatments. The size of this ovarian reserve
depends on genes involved in germ cell proliferation and differentiation, sexual differentiation,
meiosis, germ cell degeneration, formation of primordial follicles, and on a potential mechanism of
self-renewal of germ stem cells.
In this review, we present the state of the art of the knowledge of genes and factors involved in all
these processes. We first focus on the almost 70 genes identified mainly by mouse invalidation
models, then we discuss the most plausible hypothesis concerning the possibility of the existence
of germ cell self-renewal by neo-oogenesis in animal species and human, with a special interest for
the role of corresponding genes in evolutionary distinct model species. All of the genes pointed out
here are candidates susceptible to explain fertility defects such as the premature ovarian failure in
human.
Formation of primordial follicles
In mice, before establishment of the pool of primordial follicles, following their proliferation and
startup of meiosis as described above, nests of oogonia are observed. Then the breakdown of
nests leads to massive apoptosis and formation of primordial follicles containing germ cells
arrested at the diplotene stage. One main factor involved in ultimate stage of ovarian follicle
formation is FIGLA, a germ cell-specific bHLH transcription factor (Fig. 1). Female mice lacking
Figla are sterile due to the absence of follicles and oocytes (Soyal et al., 2000).
The defect appears after birth, ovaries from Figla null and wild-type female fetus at E18 have
similar numbers of germ cells clusters. The first known targets of FIGLA are the three zona
pellucida genes Zp1, Zp2, and Zp3 present in mouse genome (Liang et al., 1997). This suggests
that FIGLA is important not only for the initial formation of primordial follicles, but also at later
stages of folliculogenesis and embryo development.
Several in vivo experiments in the mouse strongly suggest that estradiol and phytoestrogens inhibit
the breakdown of germ cell clusters and the formation of primordial follicles (Chen et al., 2007).
This suggests that in fetal mice, high levels of maternal steroids prevent cluster breakdown, and at
birth, their drop allows it (Chen et al., 2007; Kezele and Skinner, 2003). However, this should not
be extrapolated to non rodent species, as in primates, estrogens seem to stimulate rather than
inhibit the formation of primordial follicles (Zachos et al., 2002).
Degeneration of germ cells by apoptosis
Several factors known to be involved in apoptosis, play a role in degeneration of ovarian germ cells
in the ovary. For example, invalidation of the antiapoptotic gene Bcl2 results in a strong reduction
of oocytes and primordial follicles at six week of age, with no differences in the number of primary
and preantral follicles (Ratts et al., 1995). Moreover, deletion of Bax, a proapoptotic factor, leads to
increased number of healthy primordial follicles compared to wild-type controls and a prolonged
reproductive lifespan (Greenfeld et al., 2007).
Other factors are involved in degeneration of PGCs and/or oocytes, although neither the
mechanisms underlying this loss, nor the stage at which germ cells degenerate have been yet
identified. For example, as stated above, the disappearance of oocytes following invalidation of
Figla is only visible after birth (Soyal et al., 2000). This strongly suggests a perinatal loss of
follicles, however the mechanisms involved remain unknown for the moment. Similarly, invalidation
of ZFX, an X-linked gene encoding a zinc finger protein, leads to reduction of almost 75% germ
cells in the mouse at birth, which a picture of premature ovarian failure, female being fertile at 17
weeks but sterile at 54 weeks PN, ovaries being devoided of any follicles (Luoh et al., 1997).
However, the stage and the mechanism of action of this factor remain unknown.
Recruitment of primordial follicles
The recruitment of primordial follicles into the growing pool remains poorly understood. However,
very recently, several transcription factors that might regulate this early step of folliculogenesis
have been identified, illuminating key signaling pathways responsible for the maintenance at the
resting stage or the recruitment of primordial follicles. Currently, four main transcription factors
have been identified by using mutant mice: NOBOX (newborn ovary homeobox), SOHLH-1 and -2
(spermatogenesis and oogenesis helix–loop–helix 1), and LHX8 (Rajkovic et al.,2004; Pangas et
al., 2006; Hao et al., 2008; Fig. 1). Expression of these genes is restricted to germ cell clusters and
primordial follicles, and it decreases in primary and secondary follicles, except Nobox whose
expression persists until the preovulatory follicle stage. Invalidation of each of these genes leads to
the sterility of females, with a blockade at primordial stage, except a few Sohlh2_/_ follicles that
escape the blockade. Target down-regulated genes in Nobox_/_ ovaries are Zp genes.
Other elements required for recruitment of primordial follicles and follicular growth, are KIT and KIT
ligand (KL), expressed by oocyte and pre-granulosa cells then granulosa cells, respectively. The
KIT system has first been identified in several mouse mutants carrying natural polymorphism in the
receptor (the White spotting locus) or ligand sequence (the Steel locus). For example, mice
homozygous for the Steel Panda or Steel Contrasted mutations have a reduced expression of KL
transcript in the gonads, a decreased number of growing oocytes, and most of them being arrested
at the primary follicle stage (Huang et al., 1993; Bedell et al., 1995;Kuroda et al., 1988).
Similarly, oocyte-specific inactivation of Pten (Phosphatase and Tensin Homolog deleted on
chromosome 10), results into a exhaustion of the primordial follicle stock (John et al., 2008; Reddy
et al., 2008).
In particular, ovaries from Foxl2_/_ mice have primordial follicles, but proliferation of granulose
cells, and their differentiation from the squamous to cuboidal state are blocked, being unable to
support growing oocytes (Schmidt et al., 2004; Uda et al., 2004). By two months of age, there is no
more primordial follicles in the ovary due to massive atresia.
Self-renewal of primordial follicles
A dogma in biology of reproduction states that the pool of nonrenewable
primordial follicles serves as a source of developing follicles and oocytes that declines with age.
This doctrine has been challenged by the Jonathan Tilly’s team in 2004 and 2005, who claimed
that a neo-oogenesis can occur in the ovary of adult mouse. According to their data, neooogenesis would originate from female germline stem cells present in the ovarian surface
epithelium and bone marrow.
Elabore hipóteses para testar a existência da neo-oogênese.