Menstrual Cycle II Flashcards
What are the threshold requirements for the DF to be selected?
at least >15mm diameter on ultrasound
Onset of LH surge precedes ovulation by 36h
Peak precedes ovulation by 10-12 hours.
When does the +ve feedback switch occur in the menstrual cycle?
At end of follicular phase E2 feedback becomes positive & persistent (300pM, 48h)
What is the effect of the +ve E2 feedback?
Causes exponential rise in LH that has to exceed a threshold
How long does the LH surge last?
LH surge lasts for 36-48 hours & triggers ovulation (timing varies from species-species)
What is a good way to predict ovulation?
LH surge relatively precise predictor of timing of ovulation
How efficiently is LH cleared from serum after the surge?
LH rapidly cleared from serum, in contrast to hCG which is cleared slowly & binds with great affinity to LHCGR
Where are LHr found at the end of the LH surge?
LHr found on both theca and granulosa cells of the follicle at this stage
Outline the structure of the pre-ovulatory follicle prior to LH surge
Oocyte surrounded by zona pellucida and cumulus granulosa cells that connect to mural granulosa cells lining interior of the follicle
How is the granulosa and theca cell layer separated in the pre-ovulatory follicle?
GC compartment separated from TC compartment by basal lamina
Describe the structure of the theca cell compartment of the pre-ovulatory follicle
TC compartment composed of:
- inner theca interna
- outer theca externa
theca externa blends into connective tissue layer that is separated from ovarian surface epithelium by a basal lamina
Describe the vasculature of the pre-ovulatory follicle
Unlike GC compartment, TC layer is highly vascularized
Circulating leukocytes are present in the vessels
What happensto the pre-ovulatory follicle after the LH surge and just before ovulation?
- Loss of OSE
- basal lamina breakdown
- GC basal lamina disruption
Why does OSE breakdown before ovulation?
Loss of OSE & breakdown of underlying basal lamina, GC & TC at apex to allow for rupture
Why is the GC basal lamina disrupted prior to ovulation?
Allows extension of blood vessels into GC layer and for infiltration of TC & leukocytes into GC compartment
COC detaches from surrounding GC to expand.
What factors are responsible for holding the oocyte in meiotic arrest?
- High cAMP
- cGMP
- H2O2/NO/calcium
- Other cells
- Ovarian environment & integrity of follicle (cumulus / mural cells)
How does cAMP cause meiotic arrest in the oocyte?
keeps maturation promoting factor (MPF) inactive
How does cGMP keep oocyte in meiotic arrest?
cGMP enters oocytes from cumulus cells via gap junctions to inhibit oocyte cAMP phosphodiesterase PDE3A activity
(PDE3A normally degrades cAMP)
What is the effect of the LH surge on the COC?
Detachment of COC from surrounding mural GC
Followed by cumulus cell expansion – formation of unique extracellular matrix between cumulus cells (aka “mucification”)
Describe the extracellular matrix structure of mucification that occurs after LH surge
Comprised of long chains of hyaluronan
Visco-elastic properties of CC matrix important for successful ovulation, ovum pick up by oviducts and penetration of sperm
What is the effect of the LH surge on cGMP?
decreased cGMP production and closure of gap junctions
How does LH surge affect PDE3A activity?
Activation of PDE3A → ↓cAMP → activation of pathways leading to breakdown of nuclear membrane in primary oocyte aka germinal vesicle breakdown (GVBD)
What is the effect of the LH surge on the oocyte’s meiotic arrest?
Resumption of meiosis in oocyte → completion of Meiosis I & release of 1st polar body
Arrests again in Metaphase II
Where in the oocyte is cAMP found?
cAMP is produced endogenously in the oocyte via Gs stimulation by GPR3
Where is cAMP stored in the oocyte?
Transported into oocyte from adjacent cumulus cells, and/or held by PDE3A inhibitor(s) in the follicular environment
What is the proposed model of meiotic arrest before the LH surge?
Before LH surge:
oocyte nucleus held in arrest by inactivation of maturation promoting actor (MPF) due to high cAMP
PDE3A inactivation due to high cGMP presence, maintains high cAMP levels
cGMP enters through gap junctions from surrounding cumulus cells
What is the proposed model of resumption of meiosis after the LH surge?
During LH surge:
- gap junctions between cumulus cells close
- cGMP levels decline within oocyte
- PDE3A activated causing cAMP levels to decrease
- MPF activated due to low cAMP causing germinal vesicle breakdown (GVBD)
Meiosis I completed and oocyte enters meiosis II
How are early oocytes classified?
Early oocytes classified as immature i.e. at germinal vesicle (GV) or metaphase 1 stage
When does meiosis resume?
Germinal Vesicle breakdown indicates meiosis resumption
How do we know 1st meiotic division is complete?
Extrusion of first polar body (1 PB) indicates completion of 1st meiotic division in human oocytes
What is the result of oocyte undergoing meiosis I?
Meiosis I is completed with half chromosomes but nearly all cytoplasm remaining in secondary oocyte
What is the polar body?
Remaining chromosomes move with small bag of cytoplasm to form discarded polar body (PB)
Why does the oocyte undergo unequal cell division?
Unequal division of cytoplasm to conserve essentials for oocyte - all material synthesised earlier → takes fertilized zygote through growth and implantation
What happens to the chromosomes of the secondary oocyte?
Chromosomes of secondary oocyte immediately enter 2nd meiotic division - form 2nd metaphase spindle and arrest
How is the 2nd meiotic arrest maintained?
This arrest is maintained by cytostatic factor (protein complex)
Egg ovulated in this arrested state
What effect does the LH surge have on receptor expression on the oocyte?
Induces progesterone receptor (PR) expression in GC
Results in luteinisation of DF cells (both GC and TC)
What happens to E2 and P levels during the LH surge?
E2 production falls and P is stimulated (P & 17α-OHP)
Describe how the LH surge affects vascularisation of the oocyte
Blood flow to follicle increases & new vessels appear in avascular GC
Why is there increased enzyme activity during LH surge?
Prostaglandins + proteolytic enzymes (e.g. collagenase and plasmin) are increased in response to LH and progesterone to digest collagen in follicle wall
What visible characteristic indicates LH surge?
Appearance of apex or stigma on ovary wall
What is the stigma in ovulation?
Point of DF closest to ovarian surface where digestion occurs
When in the menstrual cycle does ovulation occur?
Ovulation occurs ~12-18 hours after the peak of LH surge
Describe the acute inflammatory response that occurs during ovulation
Increased secretion of chemokine/cytokines from GC & TC triggers massive infiltration of leukocytes from circulation → acute inflammatory response
What event directly precedes ovulation?
Release of COC → Ovulation
Which ovary does ovulation occur from?
In humans – ovulation occurs randomly from either ovary during a given cycles, some indication more common from right ovary
Which hormone is essential for ovulation?
Progesterone
- Progesterone inhibitor (RU486) suppress ovulation
What other factors surge during ovulation?
Prostaglandins-E and -F and hydroxyeicosatetraenoic acid (HETE metabolite of arachidonic acid) reach a peak level in follicular fluid just prior to ovulation
What is the role of prostaglandins in ovulation?
Prostaglandins stimulate proteolytic enzymes (proteases)
What is the role of HETEs in ovulation?
HETEs may stimulate angiogenesis and hyperemia (↑blood flow)
Describe the intra-follicular pressure during ovulation
No increase in intra-follicular pressure
What causes the follicle rupture and ovulation?
Progressive weakening of stigma region and OSE overlying follicle prior to rupture – fundamental aspect
Allows release of COC once stigma formed
What is the OSE?
Ovarian Surface Epithelium
simple layer of epithelial cells (squamous / cuboidal / columnar depending on location)
How is the OSE held up?
OSE supported by basement membrane lying over Tunica Albuginea (held together by desmosomes + gap/tight junctions)
How does the oocyte move towards the OSE?
Preferential growth of the DF brings it in close apposition with the OSE
What factor contributes to the breakdown of the OSE?
LH stimulates meiosis resumption and secretion of Plasminogen Activator (PA) via LHR binding
Plasminogen → Plasmin
What is the effect of plasmin production during ovulation?
Activates collagenase (via MMP-1 formation) to disrupt fibril network of theca & tunica albuginea
- promotes digestion of basement membrane of follicle and OSE
What is the role of TNFa during ovulation?
TNF(𝛼) induces cell death, proteolysis via MMP-2 formation, stigma formation and eventual follicular rupture
What happens to the secondary oocyte after ovulation?
Secondary oocyte (arrested in metaphase II) with cumulus cells is extruded from the ovary
What can happen to follicular fluid after ovulation?
follicular fluid may pour into Pouch of Douglas
What happens to the oocyte that is ovulated?
egg ‘collected’ by fimbria of uterine tube
progresses down tube by peristalsis and action of cilia
What controls the ciliated cells movement?
Ciliated cells are controlled by hormones
What happens to the residual follicle after the oocyte is ovulated?
Residual part of follicle collapses into space left by fluid – a clot forms and whole structure become corpus luteum
Describe the inflammation that occurs during ovulation
Follicular fluid is “inflammatory”
Inflammation is definitely present, but too much is detrimental
What is the consequence of high inflammation during ovulation?
Higher “inflammation markers” in FF associated with decreased pregnancy rate (specifically C Reactive Protein, CRP)
Gingivitis associated with poorer IVF outcomes
How does the ovary repair the ovulatory wound?
Ovary faces monumental task of repairing damage caused by follicle rupture after each ovulation
?steroidogenic environment helps – mitogenic (oestrogen)
Recently identified stem cell/progenitor population - may contribute to OSE maintenance
What are the signs of ovulation?
- Rise in basal body temp, ~0.5-1 degree, measured by thermometer
- Tender breasts
- Abdominal bloating
- Light spotting
- Changes in cervical mucus
- Slight pain / ache on one side of abdomen
Describe the appearance of the cervical mucus immediately after menstruation
Immediately after menstruation = scant and viscous
What does the cervical mucus look like during late follicular phase?
Late follicular phase, ↑ E2 levels, = becomes clear, copious and elastic
- Quantity ↑ 30 fold compared to EFP
How does the cervical mucus appear after ovulation?
Increased P levels = again becomes thick, viscous + opaque
↓quantity produced
What happens to the corpus luteum post-ovulation?
- remaining granulosa enlarge
- vacuolated in appearance
- accumulate a yellow pigment (lutein)
How does the corpus luteum form?
Massive angiogenesis to form new capillaries
Luteinized GC combine w/ newly formed theca-lutein cells + surrounding stroma in ovary to become corpus luteum (CL)
What hormones does the CL produce?
- Progesterone
- Inhibin A
- Androgens
- Oestrogens
What is the corpus luteum?
A transient endocrine organ that predominantly secretes progesterone
What is the function of the corpus luteum?
To prepare oestrogen primed endometrium for implantation of fertilized ovum
How does the CL achieve its ufnctions?
- Basal lamina dissolves
- Angiogenic factors secreted by GC and TC
- Capillaries invade into granulosa layer of cells
- peak vascularisation 8-9 days after ovulation
Why does vascualrisation of CL occur 8-9 days after ovulation?
~time of expected implantation
This time also corresponds to peak serum levels of P and E2
What is the CL lifespan dependent on?
Life span of CL depends on continued LH support or hCG from pregnancy (luteotrophic support)
What happens to the CL if no pregnancy occurs?
CL undergoes luteolysis if no pregnancy and forms a scar tissue called the corpus albicans.
Cell death occurs, vasculature breakdown, CL shrinks
removal of CL essential to initiate new cycle
How does the CL survive during pregnancy?
High preovulatory surge + low levels of LH are enough to support CL until hCG from trophoblast takes over
What happens to progesterone levels when no pregnancy occurs after ovulation?
Progesterone withdrawal results in increased coiling and constriction of spiral arterioles
How does the endometrium react to no pregnancy?
Endometrium releases prostaglandins that cause contractions of uterine smooth muscle and sloughing of degraded endometrial tissue
How can menstrual bleeding be lessened?
Use of prostaglandin synthetase inhibitors decreases amount of menstrual bleeding
What is the average duration of menses?
Average duration of menstrual flow is 4-6 days (range 2-8 days)
How much blood is lost during menstruation?
Average amount of menstrual blood loss is 30ml with >80ml abnormal
What is anovulation?
no ovulation
common cause of infertility in women – affecting up to 40% of infertile women
What are the non-ovarian causes of anovulation?
Obesity, thyroid disorders
What are the ovarian causes of anovulation?
Primary ovarian insufficiency (POI) aka premature ovarian failure due to loss of follicles
OR due to disorders that prevent ovulation:
- Luteinized unruptured follicle syndrome (LUF)
- Effect of non-steroidal anti-inflammatory drugs (NSAIDs)
- Polycystic Ovary Syndrome (PCOS)
What is Luteinized unruptured follicle syndrome (LUFs)?
Normal follicle growth in follicular phase and normal hormonal profile but absence of follicle rupture and no release of oocytes
Form a CL with trapped oocyte and luteal phase length is normal
How is LUF diagnosed?
Diagnose using repeated transvaginal ultrasound
Who is affected by LUF?
LUF occurs in women with normal menstrual cycle at rate of 5% but in infertile women at rate of >25%
What are the proposed causes of LUF?
Linked to dysregulation of ovulation associated inflammatory changes
- reduction in prostaglandin synthesis/action
- The lack of cytokine CSF3
What is the evidence that reduced prostaglandin activity leads to LUF?
Patients treated with high dose prostaglandin synthetase inhibitors (eg Indomethacin) => block in prostaglandin production and follicular rupture
What are the 2 prostaglandin synthases expressed by ovarian follicles?
PTGS1 (constitutive)
PTGS2 (inducible)
