Ovarian follicle activation
Ovarian follicle activation can be defined as primordial follicles in the ovary moving from a quiescent to a growing phase. The primordial follicle in the oocyte is what makes up the “pool” of follicles that will be induced to enter growth and developmental changes that change them into pre-ovulatory follicles, ready to be released during ovulation. The process of development from a primordial follicle to a pre-ovulatory follicle is called folliculogenesis.
Activation of the primordial follicle involves the following: a morphological change from flattened to cuboidal granulosa cells, proliferation of granulosa cells, formation of the protective zona pellucida layer, and growth of the oocyte.
It is widely understood that androgens act primarily on preantral follicles and that this activity is important for preantral follicle growth. Additionally, it is thought that androgens are involved in primordial follicle activation. However, the influence of androgens on primordial follicle recruitment and whether this response is primary or secondary is still uncertain.
Activation of Primordial Follicle Development
Primordial follicles are activated to grow into antral follicles. Communication between the oocytes and the surrounding somatic cells, such as the granulosa cells and the theca cells, is involved in the control of primordial follicle activation. There are various activator signalling pathways that are involved in the control of ovarian follicle activation, including: Neurotropin, nerve growth factor and its tyrosine receptor kinase, neurotrophin 4, brain-derived neurotrophic factor and their receptor NTRK2. Additional ligands have a role in facilitating primordial follicle activation such as transforming growth factor-beta, growth differentiation factor 9 and bone morphogenic protein 15.GDF9
Follicular activation rate is increased in experiments where recombinant GDF9 is added. Additionally, in vitro addition of GDF9 to human ovarian cortical tissue causes enhanced activation and follicular survival. Removing GDF9 from mice, through knock-out experiments, halts follicle progression beyond the first stage, and prevents granulosa cell proliferation. However, these GDF9 null mice have accelerated oocyte growth, suggesting that GDF9 is partially responsible for granulosa cell recruitment, as well as inhibiting oocyte growth. GDF9 promotes follicular survival and growth as a result of dampened granulosa apoptosis and follicular atresia.TGF-β
As discussed above TGF-β ligands, for example BMP4 and 7 have a role in follicular activation. SMADS are downstream molecules of the TGF-β signalling pathway, hence rely on TGF-β for activation. In the absence of SMADs, mice have decreased folliculogenesis, with decreased quantities of primordial follicles, as well as developed adult follicles at both developmental stages. BMP15 has been shown to stimulate granulosa cell growth by encouraging proliferation of undifferentiated granulosa cells. This is not dependent on FSH. It was shown that two proliferation markers, Ki-67 and proliferating cell nuclear antigen, are regulated by these factors. Additionally, PCNA has been suggested to act as a key regulator of ovarian follicle development. The temporal expression of PCNA in oocytes is coincident with the start of primordial follicle formation. PCNA promotes apoptosis of oocytes, which regulates primordial follicle assembly.Foxl2">Forkhead box L2">Foxl2
Another molecule that has been implicated in the activation of oocyte follicles is Forkhead boxL2. In knock out studies, it has been shown that Foxl2 may be responsible for the cuboidal transition of the pre-granulosa cells. Hence, when Foxl2 is removed, the primordial follicles are unable to develop into secondary follicles.Sohlh1
Spermatogenesis-and-oogenesis-specific basic helix-loop-helix containing protein 1 is expressed within germ cell clusters and in new primordial follicles. Knock out studies of this protein in mice show a reduced number of oocytes present at 7 weeks post birth and a malfunction in the transition from primordial to primary follicle.Repression of Primordial Follicle Activation
PTEN">PTEN (gene)">PTEN
Phosphatase and tensin homolog is a tumour suppressor gene whose actions directly affect the activation of primordial follicles. It does this by negatively controlling the PI3K/AKT/mTOR pathway. This particular action of PTEN was initially discovered in an experiment using PTEN knockout mice. The absence of PTEN within the primordial follicles lead to an increase in AKT phosphorylation. This then creates a subsequent rise in FOXO3 export, as AKT is no longer inhibiting its production. This led to over-activation of the primordial follicles, which resulted in a premature decline of the primordial follicle pool.Foxo3">FOXO3">Foxo3
When Foxo3 is KO in mice models a huge uncontrolled activation of follicles is seen thus the mouse ovaries are deficient of the entire pool of primordial follicles because they have been prematurely activated. This action is regulated by phosphorylation, the unphosphorylated form is transcriptionally active in the nucleus. However, when phosphorylation occurs the protein is transported to the cytoplasm and loses its transcriptional activity. Pelosi et al. noted that the timing and level of the Foxo3 expression is very important to regulate ovarian follicle activation.AKt- PTEN-AKt and Foxo3 are all involved in the same pathway. PTEN is situated upstream of AKt. Therefore, if PTEN is deleted specifically from an oocyte this causes an increase in AKt activity resulting in large numbers of dormant ovarian follicles resuming their growth and differentiation. The TSC complex also plays an important role in this pathways by suppressing the activity of mTOR which has been proven to be essential for maintaining dormancy.
TSC and mTOR
Tuberin/tuberous sclerosis complex is also thought to be important in the regulation of primordial follicle activation. TSC negatively controls the function of mTOR. TSC knockout mice have a raised level of mTORC1 activity. Suppressing mTORC1 is a necessary process to prevent primordial follicles from being prematurely activated and therefore premature ovarian insufficiency.AMH
AMH is a member of the transforming growth factor beta, that has a very important role in regulating both testicular and ovarian function. In the first instance AMH inhibits the initial enrollment of the resting primordial follicles. Secondly AMH prevents the regulation of preantral/small antral follicle growth by reducing their responsiveness to FSH.Cyclin-dependent kinase (Cdk) inhibitor p27
P27 inhibits cell cycle progression at the G1 phase by preventing the action of cyclin E-Cdk2. Due to its important role in the cell cycle, it is found within the nucleus of mice oocytes in primordial and primary follicles. During puberty of p27 knock out mice, all primordial follicles are activated and leads to POF. This indicates that p27 is a vital regulator in maintaining a quiescent state in primordial follicles.Medical Consequences
Premature Ovarian Failure (POF)
, or premature ovarian insufficiency, is a female reproductive disorder characterised by at least 4 months of primary or secondary amenorrhea, before the age of 40. It is caused by either a decrease in the primordial follicle pool, accelerated atresia of follicles or altered maturation or recruitment of primordial follicles and is associated with menopausal levels of follicle stimulating hormones, exceeding 40 Ul/L., Specific activator and suppressor genes are implicated in ovarian follicle activation and recent research suggests that POF may be the consequence of a genetic mutation in one or more of these genes.FOXL2 - FOXL2 knockout mouse models showed failure of granulosa cell differentiation, which led to the premature activation and depletion of primordial follicles, characteristic of POF. Two different variations of mutations in the FOXL2 gene, which cause different forms of POF, one with earlier onset and the other with later onset and incomplete penetrance, have been identified. Additionally, mutations in the FOXL2 gene have been found in approximately 5% of nonsyndromic POF patients, which suggests that FOXL2 mutations are also associated with idiopathic POF.
BMP15 and GDF9 - Mutations in BMP15 and GDF9 genes can be involved in POF, but are not major causes of the disease. For example, low GDF9 mutation frequency has been found in a large cohort of Indian cases of POF.
SOHLH1 - Little is known about the causative association of SOHLH1 and POF, however three novel SOHLH1 variants have been found to potentially cause the disease and when studied, they were absent in controls.
AMH - A decrease in AMH expression in POF antral follicles leads to defective antral development.
mTORC1 and PI3K - Deregulation of mTORC1 and PI3K signaling pathways in oocytes results in ovarian pathological conditions, including POF and subsequent infertility.
PTEN - Studies of mice with a deletion in PTEN in the oocytes showed early activation of the entire pool of primordial follicles, leading to a lack of primordial follicles in adulthood, resulting in a POF phenotype.
Foxo3a - Studies into mice with complete and partial Foxo3a deletions also showed premature activation of the entire primordial follicle pool, destroying the ovarian reserve and leading to oocyte death. This led to a POF phenotype, seen in studies in a range of countries.,
TSC - In the oocytes of Tsc2 knockout mice, elevated mTORC1 activity causes the pool of primordial follicles to become activated prematurely. This results in follicle depletion in early adulthood, causing POF.
[Chemotherapy] and Ovarian Follicle Activation
As well as having many genetic causes, premature ovarian failure has been shown to be a side effect of many chemotherapeutic agents. The damage suffered by ovaries appears to be dose-dependent, and a class of chemotherapy drugs known as alkylating agents, seem to cause the most damage to the ovary and follicles. There are two ways in which this damage occurs:- By direct damage to the primordial follicle, causing cell death by toxicity
- By indirect damage to the stromal cells which surround the follicle and support it, to allow it to grow. Loss of these supporting cells then leads to death of the follicle.