antral folliculogenesis Flashcards by Umar Hashim

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

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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

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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

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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”.

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follicular fluid

fluid formed as exudate of plasma containing secretory products of oocyte & GC

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KL and Cx37 are they essential for antrum formation

k/o of these genes result in no antral follicles at all

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As the fluid volume increases,

the follicle continues to expand greatly in size.

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The Ovarian Follicle structure

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Theca externa is

Concentrically arranged smooth muscle cells; innervated by autonomic nerves; lymphatic vessels; important during ovulation

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Theca interna are..

Steroid-producing cells; contain LH-r & Insulin-r; richly vascularized

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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.

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cumulus oophorus

Remain in contact with oocyte & interact with oocyte via gap junctions; mitotically active; no LHr

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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

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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

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In response to LH, theca expresses key

..steroidogenic enzymes to make androgens from cholesterol.

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Likewise granulosa cells respond to FSH by up-regulating..

..aromatase (CYP19A1) and 17β-HSD to make oestrogens

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Role of FSH in antral follicles

↑ granulosa cell proliferation
↑ aromatase
↑ induce and maintain FSHr
↑ induce and maintain LHr

Interact with paracrine factors

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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