antral folliculogenesis Flashcards
Preantral to Antral Follicle Transition – Formation of Theca is crucial
- Theca of follicle is envelope of connective tissue → differentiates into theca interna & externa containing vascular tissue, immune cells and matrix factors
- Theca is critical for maintaining structural integrity of follicle and delivering nutrient to avascular GC layer
Formation and differentiation of theca extremely important for preantral to antral progression. Why?
- GDF9 k/o mice (& GDF9 mutations in human & sheep) fail to develop theca layer and follicles arrest → oocyte-derived GDF9 regulating formation of theca cell layer.
- Neo-angiogenesis, hence follicle interaction with systemic endocrine factors
- Acquisition of steroidogenic function
Theca cells derived from 2 different sources in the embryonic gonad:
- Mesenchymal (from mesonephros) cells become steroidogenic cell
- Stromal cells (indigenous to medullary region) become fibroblasts, perivascular smooth muscle cells and interstitial ovarian tissue
Preantral to Antral Follicle Transition – Antrum formation
When the follicle reaches a diameter of 200-400µm, surrounded by a vascularized theca, hence subject to circulating influences.
Fluid-filled spaces appear between the granulosa cells which soon coalesce together to form a single, large, fluid-filled cavity or “antrum”.
follicular fluid
fluid formed as exudate of plasma containing secretory products of oocyte & GC
KL and Cx37 are they essential for antrum formation
k/o of these genes result in no antral follicles at all
As the fluid volume increases,
the follicle continues to expand greatly in size.
The Ovarian Follicle structure
Theca externa is
Concentrically arranged smooth muscle cells; innervated by autonomic nerves; lymphatic vessels; important during ovulation
Theca interna are..
Steroid-producing cells; contain LH-r & Insulin-r; richly vascularized
Granulosa cells is invovled in
They differentiate into two mature cell lineages:
involved in endocrine feedback control; express FSHr, P450arom, LHr
GC differentiate into 2 mature cell lineages: mural and cumulus cells.
cumulus oophorus
Remain in contact with oocyte & interact with oocyte via gap junctions; mitotically active; no LHr
COC (cumulus-oocyte complex)
How do they respond so rapidly after LH surge if no LH receptors on cumulus gc?
GC produce:
- EGF-like ligands that bind LH
–>
allow for secretion of hyaluronan and a complex of hyaluronan cross-linking proteins that cause expansion of COC
Inter-cycle rise in FSH crucial for recruitment of AF into the menstrual cycle.
This is crucial for the following reasons: (3)
-Progression of antral follicles
-Selection of dominant follicle
-Fate of remaining AF
In response to LH, theca expresses key
..steroidogenic enzymes to make androgens from cholesterol.
Likewise granulosa cells respond to FSH by up-regulating..
..aromatase (CYP19A1) and 17β-HSD to make oestrogens
Role of FSH in antral follicles
↑ granulosa cell proliferation
↑ aromatase
↑ induce and maintain FSHr
↑ induce and maintain LHr
Interact with paracrine factors
Autocrine Control of FSHR Expression
Androgens role in antral follicles
Androgens act on granulosa cells (paracrine interaction) to upregulate AR & FSHR
AMH role in antral follicles
AMH (produced by GC of small antral follicles) acts as a brake on FSH recruitment of antral follicles by:
- decreasing FSH sensitivity
- decreasing FSH-stimulated aromatase expression
Its important to counter-balance effect of AMH, Androgens and FSH to ensure against..
- premature depletion of PF pool and/or
- premature selection of follicles by FSH
Which follicle will be selected (which feature should it have as a bare minimum)
follicle with the lowest threshold will be recruited
largest follicle is NOT always selected
Factors which assist in dominant follicle selection
Dominant follicle survives fall in FSH by:
- increased sensitivity to FSH –> increased FSH receptors
- increased numbers of granulosa cells
- acquisition of LH receptors
the LHR gene is switched on by FSH - possible involvement of insulin-like growth factors 1&2 (IGF-2 particularly important in humans)
DF & IGFs
Make this make sense (revise)
Importance of LH in folliculogenesis
Role of LH in Antral Follicles
↑ theca function of CYP11a, CYP17
↑ growth & steroidogenesis in dominant follicle
- Withdrawal of gap junctions between gc & oocyte and resumption of meiosis
- Expansion of COC
- Ovulation & luteinization
LH signalling in theca interna
LH and FSH have same 2nd messenger - cAMP
How does the cell distinguish between them?
- FSH produces low cAMP levels
- LH produces high cAMP levels
LH and FSH have same 2nd messenger - cAMP
Role of cAMP apart from signalling?
- provides energy for biosynthetic activity
- mediates effects of FSH and LH on protein production eg. aromatase, SCC, LHr, proteolytic enzymes
Why do we need ovarian angiogenesis?
Constant re-modelling to allow for growth of follicle (2-20mm) through the ovarian tissue
- angiogenesis of CL- tissue repair etc.
Angiogenic factors are stimulated by primarily
primarily by androgens but also oestrogens – theca, gc, stroma all involved
Basic fibroblast growth factor (bFGF)
endothelial cell mitogen, most potent angiogenic factor
Vascular endothelial growth factor (VEGF) (angioigenic factor)
endothelial cell mitogen, enhances vascular permeability
VEGF-R3
Oversees Ovarian lymphatic vessels recruited to theca and stroma layers around growing follicle,
Working model for androgen action on endothelial cells:
liganded AR (androgen receptor) induces HIF-1 expression which is transcription factor for VEGF
AMH secretion is maximal during..
small antral follicle stage (≤4mm) and decreases to undetectable levels later
Hence serum AMH reflect small AFs
The number of antral follicles (AFC) in the early follicular phase correlates with..
numbers of growing follicles only - the ones that have been recruited!
Low numbers of antral follicles are a sign of..
ovarian ageing
Intriguingly, right ovary has been shown to be larger and have higher AFC than left ovary → thought to be due to
larger PF pool in right ovary formed in fetal life
Premature Ovarian Failure/Primary Ovarian Insufficiency (POI) is defined as and its feature:
Defined as ovarian dysfunction <40yrs → oligomenorrhoea or amenorrhoea
Overarching feature is infertility resulting from accelerated depletion or reduced follicle reserve
Premature Ovarian Failure/Primary Ovarian Insufficiency (POI)
Aetiology is poorly understood, however this is what we have so far as to causes:
- Environmental genotoxins induce DNA damage eg chemo/radio-therapy for cancer treatment
- Mutations in genes e.g. BRCA1 and BRCA2that repair DNA double-stand breaks, resulting in diminished ovarian reserve
- Altered hormonal signalling
- Chromosomal defects e.g. Turner’s syndrome (XO) → have streak ovaries
- Autoimmune diseases including thyroiditis & Addison disease
To support follicle growth ex-utero, in vitro, two types of environment
-Collagen
- Alginate (product of seaweed)
Alginate gels able to produce antral follicles from 2° in monkey & human
