preantral folliculogenesis Flashcards
Folliculogenesis - overview
Where do eggs come from?
At 3-4 week human embryo
Epiblast cell in yolk sac at base of allantois differentiate into PGC (primordial germ cells)
What happens at 5-6 week human embryo?
Mitotically dividing PGCs migrate along dorsal mesentery of hind gut to colonise genital ridge → chemotactic substance secreted by ridge to attract PGCs.
It maybe Kit ligand (KL) as the receptor cKit is present on surface of PGCs
cytoplasmic bridges between mitotically-dividing oocytes to form..
its role?
syncitia or “nests”
Role: the exchange of organelles eg. ER and mitochondria
Retinoic Acid (the biologically active variant of Vitamin A) has been identified as
key extrinsic regulator of germ cell entry into meiosis
DAZL expression increases (when?)
before meiosis at 9-14 weeks gestation.
DAZL expression in mice - mice germ cells do not develop past PGC
Primordial follicle formation occurs (in humans - before or after birth?)
before birth in humans.
Mice: Primordial follicle formation occurs after birth
How are Primordial Follicles formed?
Syncytia breakdown & somatic cells invade to surround oogonia to form primordial follicle (PF)
Primordial follicle formation is regulated through:
Numerous transcription factors identified in mice & human:
FIGLA (no figla no PF)
Nobox
Activin βA
Co-ordination of signalling pathways: KIT, Notch and TGFβ
Hormones – FSH promotes E2 and Progesterone oppose
Which receptor is downstream of Activin βA?
TRKβ receptor
(Activin βA)
If TRKβ receptor is knocked out this can lead to..?
-> loss of oocytes
->“streak” ovaries
In males, you can get testes with no sperm
Activin βA expression decreases just before..
Nest breakdown
ovarian reserve
Once formed primordial follicles represent the entire pool of germ cells available during reproductive life of the female
“Germ Cell Selection” theory is the reason as to why not all primordial follicles formation survive
Ovaries are to select oocytes of highest quality
One layer of Cuboidal granulosa cells - primary follicle
Secondary follicle - two layers of granulosa cells, theca cell formation and basement membrane formation
Primordial-Primary Transition Morphological changes
1) Change in granulosa cells (~15 cuboidal granulosa cells)
2) Massive increase in oocyte growth & activity
3) Controlled & very slow process
Zona Pellucida (ZP) Formation is a marker of..
ZP formation is a marker of follicle/oocyte growth
What is the. Zona Pellucida (ZP)
ZP is a thick extra-cellular coat separating the egg from surrounding gc
Human follicles made up of four ZP proteins:
ZP1, ZP2, ZP3, ZP4
Permeable to large macromolecules
Follicle extensions continue through it
How does Intracellular communication between oocyte and GC take place?
Via gap junctions that penetrate ZP
and Connexins (another card)
Which specifc connexin between Granulosa cells and which between GC and oocyte?
Cx43 between GC and Cx37 between GC & oocyte
Primordial Follicle has 3 possible fates:
To remain quiescent and die out directly at dormant stage
To begin development but arrest and later undergo atresia
To develop, mature & ovulate
What causes initiation of follicle growth?
2 main ideas:
1) initiation is regulated by loss of an inhibitor:
Resting follicles under constant inhibitory influence (local paracrine/autocrine factors) to remain dormant.
2) initiation is regulated by stimulatory factor/s:
From the microenvironment (other follicles, stromal cells) and/or blood
Gradient of diffusion from centre to periphery
3) Combination of both – inhibition & stimulation:
As inhibitory factors decline then you get an increase in the effects of stimulatory ones. It is also dependent on the size of the PF pool (ovarian reserve) and the ratio at which it enters the growing pool.
“Production-line” hypothesis:
those that enter meiotic arrest first..
..those that enter meiotic arrest first in foetal ovary, will initiate growth first
Extracellular Matrix (ECM) consists of:
consists of collagen, lamimin, fibronectin, proteoglycans & polysaccharides
Importance of Extracellular Matrix (ECM)
ECM turns over and remodelled during folliculogenesis to allow for growing follicle
May regulate follicle growth especially interactions between gc & oocyte
Mechanical stimuli are communicated rapidly throughout follicle as various cell types are physically connected eg via connexins
Genes & Primordial Follicles
Nest breakdown & Primordial Follicle Assembly
FIGLA (human & mice)
Zona Pellucida 1-4 (human & mice)
Activin βA & BDNF (human)
AMH? (mouse)
Oestrogen? (baboons, human)
Genes & Primordial Follicles
Primordial Follicle Activation
Granulosa-derived factors (stimulatory):
- KL (KIT ligand aka stem cell factor SCF) secreted from granulosa cells evidence that KL may inactivate Foxo3a
Oocyte-derived factors (stimulatory):
- cKIT (KIT ligand tyrosine kinase receptor) in oocytes –> necessary for follicle activation
Primordial Follicle Maintenance/Repression
Oocyte-derived factors (inhibitory):
- PTEN (tumour suppressor gene) –> inhibits signalling by Akt/PI3K signalling pathway.
- FOXO3a (transcription factor) -> also part of PI3K and restrains follicle activation
- SDF-1 (stromal-derived factor) chemokine inhibits follicle activation in autocrine/paracrine fashion
Granulosa-derived factors (inhibitory):
- AMH (anti-Müllerian hormone) –> acts in paracrine fashion to inhibit primordial follicle initiation
Primary → Preantral Growth
FOXL2
FOXL2 (forkhead transcription factor) ⇒ granulosa cell proliferation
NO FOXL2
–> have Type 1 BPES and POF (premature ovarian failure) – no progression of follicles to secondary stage
Factors relevant in progression of Primary Follicle Growth
Three groups: oocyte derived, granulosa derived, Extra follicular
Oocyte-derived factors:
- GDF-9 (growth differentiation factor-9)
- BMP-15 (bone morphogenetic protein-15)
- Cx37 (connexin 37) gap junction protein (between oocyte & gc)
Granulosa-derived factor:
- Cx43 (connexin 43) gap junction protein (between gc & gc)
Extra-follicular factors:
- Insulin & IGF-1 & IGF-II –>
increase primary stage follicles in cultured human ovarian cortex
NGF (Nerve Growth Factor)
Androgens & Early follicle growth: Do androgens play a part?
Testosterone rapidly increases intra-oocyte PI3K/Akt/FOXO3 pathway in mouse follicles → increasing >2-fold ratio of primary:primordial follicles
Inhibiting AR in bovine ovaries prevents primary to secondary follicle transition
Importance of gonadotrophins during basal follicular growth?
Physiological & pathological states where circulation gonadotrophin levels are low still see follicular growth:
- FSHß & FSHR k/o mice have normal preantral growth
- inactivating mutations of the FSH receptor
~follicle growth to antral stage, but less follicles - FSHR have been found on primary stage follicles
Summary of Preantral folliculogenesis
