Antral Folliculogenesis- reproduction

8/12/2019 Antral Folliculogenesis- reproduction

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Antral Folliculogenesis

Antral Follicle

Characterised by a cavity or antrum Contains fluid formed as exudate of plasma containing secretory products of oocyte & GC Known as follicular fluid In lab animals Kit Ligand and Cx37 proteins essential for antrum formation

Antrum Formation:

Definitions:


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Change in GC & FF volume

As the fluid volume increases the follicle continues to expand greatly in size.

(McNatty KP: Hormonal correlates of follicular development in the human ovary. Aust J Biol Sci

34:249, 1981)

The Ovarian Follicle

Two-cell, Two-Gonadotrophin Concept

This follicular fluid is formed by filtration of thecal blood, and its composition is different from that of

plasma. The follicle can now respond to factors/hormones etc in the blood.

Differentiation of the granulosa cells - those just below the basement membrane known as the mural GC

and those surrounding the oocyte known as the cumulus aka COC (cumulus-oocyte complex).

Theca and GC also begin to differentiate in terms of function Inner theca=steroid secreting and express LHR

Mural GC near the basal membrane stop proliferating, express P450 aromatase activity and later LHR

Cumulus GC are mitotically active and have no LHR

At ovulation the COC released as a mucified complex

COC has gap junctions which couple the GC to each other and to the oocyte via processes, which withdraw

when meiosis resumes.


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The HPG axis acts to control antral follicle growth at this stage

In response to LH, theca expresses key steroidogenic enzymes to make androgensfrom cholesterol.

Likewise granulosa cells respond to FSH by upregulating aromatase (CYP19A1) and17-HSD

Lecturers notes:

The Two-Cell, Two-Gonadotropin Concept.To understand the underlying mechanisms of follicle estradiol

production, we need to consider steps that occur in the two-cell, two-gonadotropin concept. In response to

the LH, which is delivered to the follicle through the theca vasculature, there occurs an increase in the

expression of specific steroidogenic genes in the TICs, which increases the synthesis of androstenedione.

The level of androgen secretion reflects the presence within the theca of plus and minus factors, including

insulin, lipoproteins, activin, and inhibin. Some of the TIC-derived androstenedione diffuses across the basal

lamina and enters the follicular fluid and granulosa cells. In response to the P450arominduced by FSH in the

granulosa, the androstenedione is aromatized to estrone, which then is converted to estradiol by 17-HSD.

These synthetic activities of the granulosa cells can also be modulated by a wide variety of regulatory

molecules, such as the stimulators insulinand IGF-I and the inhibitors epidermal growth factor and tumor

necrosis factor-. The sum total of the ligand controls that operate to determine the activities of P450arom

and 17-HSD in the granulosa cells determines when and how much estradiol is produced by the dominant

follicle. The estradiol molecules enter the follicular fluid and then diffuse into the theca vasculature, where

they enter the systemic circulation.


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Role of FSH in antral follicles


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Autocrine Control of FSHR Expression

AFC & Ovarian Reserve

Antral Follicle Count (AFC)

The number of antral follicles in the early follicular phase correlates withovarian reserve

Low numbers of antral follicles are a sign of ovarian ageing

Lecturers Notes:

Diagram of the proposed mechanism for the autocrine control of follicle-stimulating hormone

receptor expression in granulosa cells of preantral follicles.(From Erickson GF: Dissociation of

endocrine and gametogenic ovarian function. In Lobo R (ed): Perimenopause.New York:

Springer-Verlag, 1997.)


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Use ultrasound to count number of 2-8mm follicles at start of cycle Observable earlier than a rise in FSH serum level Used together with FSH, AMH and Inhibin to determine ovarian reserve

Menstrual cycle & follicles

What causes the intercycle rise in FSH? The fall in progesteronereleasing negative

feedback

The window of opportunity


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FSH threshold

amount of FSH required to recruit one follicle follicle with the lowest threshold will be recruited size? Serial USS has revealed that largest follicle is not always selected GC in selected follicle divide at faster rate than others in cohort growing follicle prevents growth of others

Menstrual cycle & follicles

Lecturers Notes:

From a size of 2mm follicles become more dependent on FSH. The granulosa cells of the

selectable follilces increase their rate of proliferation as circulating levels of FSH increase. It

is from among these follicles that the follicle destined to ovulate in the subsequent cycle

will be selected (dominant follicle)

The increase in FSH in the follicular phase is an absolute requirement for selection of the

DF since in its absence there is no DF and no ovulation


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What are the requirements and function of the selected follicle?

growth and oestradiol production increased cell division (2-5 million GC in EFP and 50-100 million at ovulation) increased aromatase (200x more E2than other follicles)

FSH receptors androgens and oestrogens intra-follicular cAMP

Increasing oestradiol from the

dominant follicle then feedsback to

inhibit FSH and cause other follicles

to die off

As E2 levels increase and are

sustained then it switches to

positive feedback and get LH surge


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increased area of theca vasculature consequence?FSH signalling

Lecturers Notes:

Steroidogenic activity increases as its maturation progresses

In the pre-ovulatory follicle the fraction area of the theca layer occupied by blood vessels is significantly

greater than in other follicles within the same or contralateral ovary. This would allow paracrine growth

factors like IGF-1 to reach follicle, increasing GC proliferation along with EGF, activin and FSH

Oestradiol production by the DF (200x that of other selectable follicles) suppresses plasma FSH level

causing the concentration of FSH in developing cohort follicles to fall below the threshold. Basis for

infertility therapy using the anti-oestrogen clomiphine citrate, which blocks estrogen, increases

gonadotrophin levels and allows for maturation of a single, preovulatory follicle.

In fact the rate of proliferation of GC in the DF increases whereas the rate in the non-DFs slows down, this

is probably related to high threshold levels of FSH in the microenvironment of follicle.

The DF size increases from a range of 5.5-8.2mm in early follicular phase to about 18-20mm in late

follicular phase. Mean no of GC increase from 2-5million in early follicular phase to 50-100million at time

of ovulation


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FSH-regulated signaling pathways


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Hunzicker-Dunn M & Maizels ET (2006) Cellular Signalling 18:1351-1359

Dominant follicle


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how does the dominant follicle survive the fall in FSH?

Dominant follicle survives fall in FSH by

increases sensitivity to FSH increased FSH receptors

increased numbers of granulosa cells

acquisition of LH receptors the LHR gene is switched on by FSH


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Folliculogenesis

What happens to the rest of the growing antral follicles in the ovary?

Role of LH

What is the role of LH in folliculogenesis?


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Importance of LH

inactivating mutations of LH receptor normal EFP E2 , anovulatory, multiple cysts, morphologically normal antral

follicles (Toledo, 1996)

hypogonadotrophic women FSH treatment effective as long as some LH present E2is significantly reduced but detectable, why? can A be accessed from adrenal?

LH k/o mice Antral stage growth blocked

Role of LH in Antral Follicles

Theca function is to produce high levels of androstenedione and testosterone whichthen aromatised to oestradiol.

Activins decline as follicle grows.


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LH signalling in theca interna

One message, 2 signals


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Folliculogenesis: what else will be needed to support follicle growth?

Ovarian angiogenesis

Basic fibroblast growth factor (bFGF) endothelial cell mitogen, most potent angiogenic factor

Vascular endothelial growth factor (VEGF) endothelial cell mitogen, enhances vascular permeability

Which cells in the ovary would make angiogenic factors?

The follicle itself (2- ovulatory)

Why do we need angiogenesis?

After selection of the dominant follicle?

LH surge and ovulation!

JC Lousse & J Donnez (2008) Fertil. Steril. 90:833-834

Lecturers notes:

Angiogenic factors can originate from follicles of different sizes from secondary to pre-

ovulatory to act locally or via general circulation to enhance thecal vascularizaiton

Angiogensis required because of extensive re-modelling of follicle during itsgrowth - it

goes from 2mm to 20mm. The basement membrane is being re-modelled continuously,

oocyte enclosed in cumulus cells rel. anaerobic conditions in middle of follicle etc.


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After selection of the dominant follicle?

luteinisation formation of the CL CL maintenance CL demise menstruation

Lecturers Notes:

These are the clearest pictures ever taken of what is the starting point of every human life: ovulation

occurring inside a woman's body.

"The release of the oocyte from the ovary is a crucial event in human reproduction," saysJacquesDonnezat the Catholic University of Louvain (UCL) in Brussels, Belgium. "These pictures are clearly

important to better understand the mechanism."

Observing ovulation in humans is extremely rare, andprevious imageshave been fuzzy. Donnez

captured the event by accident while preparing to carry out a partial hysterectomy on a 45-year-old

woman. The release of an egg was considered a sudden, explosive event, but his pictures, show it

taking place over a period of at least 15 minutes.
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Summary - Follicle Growth & Feedback


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Conclusion

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Antral Folliculogenesis- reproduction