antral folliculogenesis Flashcards
Describe the transition from pre-antral to antral follicles and explain why the formation of theca is important in follicles
Studies in 1970s showed that when radio-labelled LH/hCG injected into adult female rats localized specifically to the theca layer of small preantral, antral and pre-ovulatory follicles but not primordial follicles
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
Where are theca cells formed from (embryonic origin)
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
Describe the formation of the antrum from the pre-antral to antral stages
Formation of antrum occurs 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”.
Contains fluid formed as exudate of plasma containing secretory products of oocyte & GC
Known as follicular fluid
KL and Cx37 essential for antrum formation in lab animals – as 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.
Structure of the ovarian follicle
theca externa-Concentrically arranged smooth muscle cells; innervated by autonomic nerves; lymphatic vessels; important during ovulation
theca interna-Steroid-producing cells; contain LH-r & Insulin-r; richly vascularized
GC differentiate into 2 mature cell lineages: mural and cumulus cells.
Mural granulosa cells-Mural Granulosa – involved in endocrine feedback control; express FSHr, P450arom, LHr
Cumulus oophorus cells-Remain in contact with oocyte & interact with oocyte via gap junctions; mitotically active; no LHr
How do cumulus oocytes respond so fast if they have no LH receptors
GC produce EGF-like ligands that bind LH and allow for secretion of hyaluronan and a complex of hyaluronan cross-linking proteins that cause expansion of COC
Describe the 2 cell 2 gonadotrophin concept
The 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.
Likewise granulosa cells respond to FSH by up-regulating aromatase (CYP19A1) and 17β-HSD to make oestrogens
Inter-cycle rise in FSH crucial for recruitment of AF into the menstrual cycle
Progression of antral follicles
Selection of dominant follicle
Fate of remaining AF
Describe the role of FSH in antral follicles
↑ granulosa cell proliferation ↑ aromatase ↑ induce and maintain FSHr ↑ induce and maintain LHr Interact with paracrine factors
Effect of androgen, AMH and FSH on antral follicles
Androgens act on granulosa cells (paracrine interaction) to upregulate AR & FSHR
AR k/o mice have reduced FSHR mRNA
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
Counter-balancing effect of AMH, Androgens and FSH to ensure against premature depletion of PF pool and/or premature selection of follicles by FSH
Describe the FSH threshold and dominant follicle selection
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
FSH receptors
Increased numbers
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 consequence?
How does the dominant follicle survive the fall in FSH
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)
Effect of IGF on dominant follicle
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
How can we investigate the importance of LH in folliculogenesis
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
E2 is significantly reduced but detectable, why?
can some A be accessed from adrenal?
LH k/o mice
Antral stage growth blocked
Describe the pathway for the role of LH on 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 and FSH have the same secondary messenger- cAMP. So how does the cell distinguish between them
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
Yong, Baird, Hillier (1992) Clin.Endocrinol.(Oxf)
provides energy for biosynthetic activity
mediates effects of FSH and LH on protein production eg. aromatase, SCC, LHr, proteolytic enzymes
One of the things necessary to support follicle growth is ovarian angiogenesis. Explain how this works and why it’s necessary
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
Vascular endothelial growth 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
Why do we need angiogenesis?
Constant re-modelling to allow for growth of follicle (2-20mm) through the ovarian tissue, angiogenesis of CL, tissue repair etc.
