Antral Folliculogenesis Flashcards
What is formed for the Preantral to Antral Follicle Transition? (3)
Theca:
Theca of follicle is envelope of connective tissue → differentiates
into theca interna &
externa containing vascular tissue, immune cell + matrix factors
- critical for main. structural integrity of follicle + delivering nutrient to avascular GC layer
- Form. and differ. of theca extremely important for
preantral to antral progression.
Theca formation (3)
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 + interstitial ovarian tissue
Antrum formation (3)
follicle reaches a diameter of 200-400μm,
surrounded by a vascularized theca, hence subject to circulating influences.
Fluid-filled spaces appear b/w GC
which soon coalesce together to form a single, large, fluid-filled cavity or “antrum”
- As the fluid volume increases the follicle continues to
expand greatly in size
Antrum (2)
– Contains fluid formed as exudate of plasma containing secretory
products of oocyte & GC
Known as FOLLICULAR FLUID
images
Change in GC & FF volume
As the fluid volume increases, the follicle continues to expand greatly in size.
Ovarian Follicle elements (4)
Theca externa
Theca Interna
GC
Cumulus Oophorus
Theca externa
Concentrically arranged smooth muscle cells; innervated by autonomic nerves; lymphatic vessels;
important during ovulation
Theca interna(3)
Steroid-producing cells;
contain LH-r & Insulin-r;
richly vascularized
GC (3)
Mural Granulosa – involved in endocrine feedback control; express FSHr, P450arom, LHr
Remain in contact with oocyte & interact with oocyte via gap junctions; mitotically active; no 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
How do COC respond so rapidly after LH surge if no LH receptors on cumulus gc?
GC produce EGF-like ligands that bind LH and allow for secretion of hyaluronan + a complex of hyaluronan cross-linking proteins that cause expansion of COC
Why in the FSH inter-cycle rise important? (4)
crucial for recruitment
of AF into MC
Progression of antral follicles
* Selection of dominant follicle
* Fate of remaining AF
Two-cell, Two-Gonadotrophin Concept (3)
- HPG axis acts to control antral follicle growth at this stage
-In response to LH, theca expresses key steroidogenic enzymes to make androgens from cholesterol.
-GC respond to FSH by up-regulating aromatase
(CYP19A1) and 17β-HSD to make oestrogens
What determines survival of follicles?
FSH:
↑ granulosa cell proliferation
↑ aromatase
↑ induce and maintain FSHr
↑ induce and maintain LHr
Interact with paracrine factors
Autocrine Control of FSHR Expression
image
Androgens + antral Follicles (2)
Androgens act on granulosa cells (paracrine interaction)
to upreg.AR & FSHR
– AR k/o mice have reduced FSHR mRNA
AMH + antral Follicles (4)
(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
Counter-balancing effect of AMH, Androgens and FSH to ensure against.., (2)
– premature depletion of PF pool and/or
– premature selection of follicles by FSH
FSH threshold & DF selection (9)
amount of FSH required to recruit one follicle
»follicle w/ the lowest threshold will be recruited
» largest follicle is not always selected
»FSH receptors
– Increased no,’s
– Coupled more effectively to down-stream signalling
»Growth and Oestradiol production
– increased cell division (2-5 million GC in EFP and 50-100 million at ovulation)
– Size ~5.5-8.2mm in EFP and 18-20mm in LFP
– increased aromatase (200x more E2 than other follicles)
»Androgens & Oestrogens
»Intra-follicular cAMP
»Increased area of theca vasculature
Dominant follicle (5)
Dominant follicle survives fall in FSH by:
»increased sensitivity to FSH = increased FSHr
»increased no. of GC
»acquisition of LHr
‒ 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 (bovine)
» IGF-2 enhances FSH effects; stimulates androgen output and
hence oestrogen
» IGF activity suppressed by IGFBP (IGF-binding protein)
» IGF cleaved from IGFBP by PAPP- A (pregnancy-associated plasma
protein A)
» PAPP-A expression high in DF
» Thought that other AF in cohort may have higher levels of IGFBP
hence preventing co-stimulatory effect of IGF & FSH
Importance of LH in folliculogenesis (6)
»inactivating mutations of LH receptor = normal *EFP E2 , anovulatory, multiple cysts, morphologically normal antral follicles
»hypogonadotrophic women:
- FSH treatment effective as long as some LH
present
- E2 is sig.reduced but detectable, why?
* can some A be accessed from adrenal?
»LH k/o mice
* Antral stage growth blocked
Role of LH in Antral Follicles
diagram
LH signalling in theca interna
diagram
one message, 2 signals - cAMP
LH and FSH have same 2nd messenger - cAMP
FSH produces low cAMP LH produces high cAMP
Difference in density of FSHr & LHr (LHr>FSHr or LHr more effectively coupled to cAMP generation)
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.
Ovarian angiogenesis (4)
- Angiogenic factors stimulated by primarily by androgens but also oestrogens – theca, gc, stroma all involved
- Basic fibroblast growth factor (bFGF)
– endothelial cell mitogen, most potent angiogenic factor
*VEGF
– endothelial cell mitogen, enhances vascular permeability
- Ovarian lymphatic vessels recruited to theca and stroma layers around growing follicle, under control of VEGF-R3
Androgens & VEGF (2)
image
Working model for
androgen action on
endothelial cells:
liganded AR induces
HIF-1 expression
which is transcription
factor for VEGF
AMH, Antral Follicle Count (AFC) & Ovulatory Potential (7)
AMH sec. =maximal during small
antral follicle stage (≤4mm) and
dec. to undetectable levels later
- = serum AMH reflect small AFs
- no. of antral follicles (AFC) in the EF phase correlates w/numbers of growing follicles only
- Use ultrasound to count number of 2-8mm follicles at start of cycle &
correlate ≈ AMH serum levels - Low numbers of antral follicles are a sign of ovarian ageing
- Observable earlier than a rise in FSH serum level
- right ovary has been shown to be larger +
have higher AFC than left ovary → thought to be due to larger PF pool in right ovary formed in fetal life
Fertility & Follicle Reserve
AFC & serum markers FSH, AMH, E2 and
Inhibin B used to determine “functional
ovarian reserve”, but does not indicate
true PF reserve
ie what you are born with
Premature Ovarian Failure/Primary
Ovarian Insufficiency (POI)
term for premature ovarian failure
- Affects 1% of women worldwide
- Defined as ovarian dysfunction <40yrs → oligomenorrhoea
or amenorrhoea - Overarching feature is infertility resulting from
accelerated depletion or reduced follicle reserve. - Aetiology is poorly understood:
Causes of POF/POI (5)
– 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
