Preantral Folliculogenesis Flashcards
What has to be achieved to reproduce?
- differentiation into male or female
- sexual maturation
-production, storage and release of sufficient supply of eggs and sperms - correct number of chromosomes in eggs and sperm
- eggs and sperm have to meet
-creation of new individual with genes from both parents
Where do eggs come from?
3-4 weeks human embryo
Epiblast cell in yolk sac differentiate into PGC
5-6 weeks human embryo
Mitotically divinding PGC migrate to genital ridge , genital ridge releases chemotherapy factors such as KIT which guides the PGCs
Formation of oocytes and follicles
primary germ cells -> oogonia -> primary oocytes
germ cells become oogonia in ovary, oogonia are egg precursors, once mitosis stops they enter meiosis they are known as primary oocytes
- During migration, PGC undergoes epigenetic re-programming, including genome-wide DNA de methylation and ensures genomic imprinting ~ 5 weeks
- cytoplasmic bridges between mitotically -dividing oocytes to form syncytia or nests (role of nests may be to exchange organelles) ~ 11 weeks
- Retinoic acid (biologically active variant of vitamin A) derived from somatic cells drives germ cell into meiosis -> induces Stra8 gene expression in oogonia.
formation of primordial follicle
- syncytia breakdown and somatic cells invade to surround oogonia to form primordial follicle (PF) .
- PF formation is regulated through a complex network of molecular signals between oocytes and somatic cells, ie KIT, NOTCH and TGF beta all coordinate the signaling pathway.
- numerous transcription factors in formation of primordial follicle identified in mice and human: FIGLA, Nobox, Activin beta A
KO mouse model = FIGLA knockout mice cannot form primordial follicles and loose oocyte rapidly after birth
++ 100 Chinese women study with POF , 4 had mutations in the gene coding for FIGLA , but 4/100 isn’t sufficient evidence other factors also involved, also the sample had only Chinese women so ethnicity bias ,finding less representative
Why do you get massive loss of oocytes and follicles?
- if the follicle isnt formed properly the oocyte isnt relesed in ovulation,
For an oocyte to develop properly and be able to get relesed into the menstrual cycle during ovulation it needs to be surrounded by granulosa and theca cells well, and be well vascularised in order to gain nutrients and oxygen to grow, the growing follicle enter the menstrual cycle but only the follicle at the right stage to become the dominant follicle releases oocyte into the menstrual cycle, the rest of the growing follicle are lost through apoptosis.
This process of losing primordial follicles occurs since birth till menopause.
From birth to puberty we lose a few.
Post puberty in each menstrual cycle we lose a lot more , as many growing follicles enter menstrual cycle but only the dominant one survives and releases its oocyte.
In this way our ovarian reserve is slowly and by menopause all the oocytes are depleted
What is ovarian reserve?
- once formed primordial follicles represent the entire pool of germ cells available during reproductive life of female - known as ovarian reserve
- predicted to be ranging between 35, 000 - 2,500,000 primordial follicles
+ mathmatical modelling and histological counting predicts the range but these methods of counting are prone to errors hence why there is such a large range
Where are primordial follicles primarily found?
ovarian cortex - edge of ovary so it can be picked up by fimbria into the fallopian tube
anatomy of ovary
- fimbriae are free and flexible to allow the, to swing around and collect egg so it can enter the uterine tube easily, hence why any fibroids or adhesions or surgery causes infertility
-blood vessels are in the centre of ovary and outer cortex region is avascular - primordial follicles are located in avascular ovarian cortex , as follicle grow they migrate inwards towards blood supply and once there is one dominant follicle , the DF has to move back outwards towards the ovarian cortex so it can get picked up by the fimbrae and ovulate.
What are stages of follicle growth?
1.resting (0.02mm) - gonadotrophin independent
2. preantral follicle(0.2mm) - gonadotrophin independent
3. antral follicle (2mm) - gonadotrophin dependent
4. ovulatory (20mm) -gonadotrophin dependent
Resting to preantral transition = initiation
preantral to antral = recruitment
antral to ovulatory = selection
What happens in the preantral stage- how do primordial follicles develop into secondary follicles?
- primordial follicle stage
resting follicle - comprises of flattened granulosa cells - oocyte in meiotic arrest - transitional follicle stage
as they start to grow we see a distinct change in morphology - granulosa cells go from flat to cuboidal and multiply in number, in this stage we see a mixture of cuboidal and flattened cells - primary follicle stage
- single layer of cuboidal granulosa cells - secondary follicle stage
-when you have two layers of cuboidal granulosa cells
-oocyte is growing and increasing in size and it secretes proteins known as zona pellucida proteins which form zona pellucida (so ZP is a mark of growth of oocyte)
-precursor theca cells in ovarian cortex condense around follicle
-basement membrane separates granulosa from theca - multilaminar follicle
secondary follicle grows multiple layers of granulosa cells.
When talking about preantral and antral follicle its important to remember ….
pre- antral follicle include all primordial, transitional, primary, secondary and multilaminar follicles - all at different developmental stages
antral/ tertiary - no multiple different cells, anything with antrum is antral cell
primordial to primary transition - morphological changes
- change in granulosa cells (~ 15 cuboidal granulosa cells)
- massive increase in oocyte growth and activity (still in meiotic arrest)
- controlled and very slow process
4 . as follicles grow they move from collagen-rich , avascular cortex to the medullary zone of the ovary where there is rich blood supply
(once they become secondary follicle - the follicle in the right stage will become DF and this will move back into the ovarian cortex to be taken up by the fimbriae in ovulation)
Zona pellucida formation
- thick extra-cellular coat separating the egg from surrounding granulosa cell
- ZP formation is a marker of follicle/oocyte growth
- sperm binds to the zona pellucida(zp2) during fertilisation - prevents poly-spermia
What 4 proteins make up human follicles?
-ZP1, ZP2, ZP3 ZP4
- permeable to large macromolecules
- follicle extensions continue through it
What is the structure of preantral follicle?
How does communication occur between oocyte and granulosa and why is this important?
- intracellular communication between oocyte and GC via gap junctions that penetrate ZP
- communication via connexin37 between oocyte and GC
(connexin43 allows communication between GCs)
this communication is essential for the initiation and progression of oocyte and follicle growth
What is the consequence of no blood supply in ovarian cortex where primordial follicles are found?
- not subject to any blood bourne influence so blood supply not needed = basal lamina around the follicle creates microenvironment for gc and oocyte- everything that is happening is within the follicle ie. not in contact with other cells in the ovary, so theca and surrounding cells important to maintain that microenvironment.
primordial follicle to primary follicle - follicle activation
Primordial follicle (unactivated)=
-Oocytes = lots of FOXO3 (involved in maintaining pool of primordial cell) suggest oocyte is at rest
-Granulosa cell = likey AMH binds to AMH receptor activating SMAD3 which binds to DNA increasing Ccnd2 which keeps primordial follicle in cells cycle arrest
Hippo signaling phosphorylates YAP inhibiting it
Activated primary follicle
-TGF beta binds to TGF receptors , activates mTOR pathway which inhibits SMAD3 by phosphorylating it , this enables activation of cell cycle and removes cell cycle arrest
-Hippo signaling is also turned off so YAP desphosphorylated and translocate to the nucleus to aid the process
Ooctye = KIT ligand protein is also produced and binds to cKIT receptor on oocyte and activates PI3K pathway and AKT phosphorylates FOXO3 and removes it, allowing cell cycle to be activated.
Do androgens play a part in early follicle growth?
testosterone rapidly increases intra-occyte PI3K /Akt/FOXO3 pathway in mouse follicles -< increasing -> 2 folds ratio of primary : primordial follicle
Monkeys treated with androgens have more primary follicles and increased FSH receptors
inhibiting AR in bovine ovaries prevents primary-to secondary-follicle transition- can we apply this to humans?
Reproductive characteristics of humans ewes, cows and rodent are all different - vary in ovulatory cycle, ovulations per cycle, length of follicular and luteal phase and gestation duration.
- when picking an animal model need to chose according to what you are studying .
if looking at menstrual cycle women 24-30 days and cows 17-24 days are much more similar than rodents 4-6 days making cows a more comparable model.
so depending on what is investigated you can pick a suitable model.
Mice have very little similarity in reproductive charcateristics, eg. duration of gestation is 21 days but this is an advantage when looking at environmental factors or K/O effects so we can see the whole process quickly unlike other animal models which take longer.
Also mouse ovary culture survive very well in vitro unlike human ovary culture so better for studying.
