Lecture 7: Menstrual Cycle 2 Flashcards
What does the inter cycle rise and then fall in FSH cause?
- Selection
- & exponential growth of the dominant follicle (dominant follicle -> preovulatory follicle)
How does the switch to positive feedback occur?
- End of FP, E2 feedback becomes +ive
- & persistant - 200pM, 48hrs
= exponential rise in LH that has to exceed a threshold
Once the +ive feedback is achieved, what does it result?
- LH surge
- Lasts for 36-48 hours
- & THEN triggers ovulation (timing varies from species-species)
What does the LH surge cause?
- Ovulation
(E2 300pM, 48hrs –> +ive feedback –> LH surge for 36-48hrs –> OVULATION)
For ovulation to occur, what must happen?
- LH surge must exceed threshold
How quickly is LH cleared from serum compared to hCG?
- LH rapidly cleared from serum,
- hCG cleared slowly & binds with great affinity to LHCGR
Where is hCG produced from?
- Blastocyst
- Only during pregnancy
Where is hCG produced from?
- Blastocyst
What is an indicator of pregnancy?
- hCG
Mid-cycle (during the LH surge/ovulation), where are the LHr found on the follicle?
- Theca
- & Granulosa cells
ONLY ON DOMINANT FOLLICLE
What size should a follicle be to be considered a dominant follicle?
- at least >15mm diameter on ultrasound
How many hours does onset of LH surge precede ovulation?
- 36hrs
How many hours does LH peak precede ovulation?
- 10-12hrs
Does the whole follicle ovulate?
- No
- Only the cumulus-oocyte complex (COC)
As the antral follicles grow, where do they move to in the ovary?
- Middle of the ovary
- Due to rich blood supply
When the selection and growth of dominant follicle occurs, does it move?
- Moves back out to surface of ovary
- Ready to be ovulated
Summarise what needs to happen for ovulation to occur?
- Loss of OSE & breakdown of underlying basal lamina and GC & TC at apex to allow for rupture.
-GC basal lamina disrupted allowing extension of blood vessels into GC layer & for infiltration of TCs & leukocytes into GC compartment. - COC detaches from surrounding GC to expand and move out
Describe in detail the preovulatory follicle prior to LH surge
- Oocyte surrounded by zona pelucida & cumulus GCs that connect to mural GCs that line interior of follicle.
- GC compartment separated from TC compartment by basal lamina.
- TC compartment composed of inner theca interna & outer theca externa. Unlike GC compartment, TC layer is highly vascularized (red).
- Circulating leukocytes present in vessels.
- Theca externa blends into a layer of connective tissue, separated from ovarian surface epithelium by basal lamina.
Describe in detail the preovulatory follicle immediately prior to ovulation
(- Preovulatory follicle following LH stimulation immediately prior to ovulation.)
- Disruption of GC basal lamina allows extension of vessels into GC compartment.
- TC & leukocytes also enter into GC compartment.
- CO Complex detaches from surrounding GCs & undergoes cumulus expansion.
- At follicular apex (top of image), there is loss of ovarian surface epithelium, the breakdown of the underlying basal lamina, & a loss of TCs & GCs.
- Rupture will occur at follicle apex.
Do the GCs typically have a blood supply?
- No
(- blood vessels infiltrate GCs just before ovulation)
Where are the blood vessels typically situated in the follicle?
- Theca externa
Preovulatory follicle prior to the LH surge vs following LH stimulation immediately prior to ovulation
When the blood vessels invade & infiltrate into GC layer just before ovulation, what do they bring in and why?
- Inflammatory markers
- Cytokines
- Leukocytes etc.
- Why? For ovulation to occur & for changes in COC to allow for release of egg which is within the follicle
What happens to the remainder of the follicle after ovulation? i.e. after COC leaves
- lutenised &
- becomes CL
Briefly summarise the state of the oocyte until just before being recruited into the MC (oogenesis)
Mitosis -> Increase in germ cells -> differentiate when at gonadal ridge -> oocytes -> mitosis stops -> enter meiosis 1 -> meiotic arrest
If a female ovulates at the age of 32 years and 3 months, how long had the oocyte ovulated been in meiotic arrest?
-32 years and 3 months (longer if taking gestation into account)
What factors are responsible for holding the oocyte in meiotic arrest?
- High cAMP
- cGMP
- H2O2/NO/calcium
- other cells/ ovarian environment & integrity of the follicle?
How does high cGAMP hold the oocyte in meiotic arrest?
- keep maturation promoting factor (MPF) inactive
How does cGMP hold the oocyte in meiotic arrest?
- cGMP enters oocytes from CC via gap junctions to inhibit oocyte cAMP phosphodiesterase PDE3A activity
(PDE3A normally degrades cAMP)
What enzyme degrades cAMP?
- cAMP phosphodiesterase (PDE3A)
What inhibits phosphodiesterase 3A (PDE3A)
- cGAMP
How do the cumulus cells and oocyte communicate?
- Projections between cumulus cells and oocyte
What is the effect of the LH surge within 3-12hrs?
- Detachment of COC from surrounding mural GC
- Cumulus cell expansion: formation of unique extracellular matrix between cumulus cells (aka “mucification”)
-↓cGMP production and closure of gap junctions - Activation of PDE3A → ↓cAMP → activation of pathways leading to breakdown of nuclear membrane in primary oocyte aka germinal vesicle breakdown (GVBD)
- Resumption of meiosis in oocyte → completion of Meiosis I & release of 1st polar body
- Arrests again in Metaphase II
What is mucification 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 properties does the CC matrix have and what is its importance in the MC?
- Visco-elastic properties of CC matrix
- important for successful ovulation, ovum pick up by oviducts and penetration of sperm
What is contained in the first polar body?
- 23 chromosomes (that are chromatids)
- small amount of cytoplasm
What is CC expansion?
- formation of unique extracellular matrix between cumulus cells (aka “mucification”)
What are early oocytes classified as?
- Immature
- ie at germinal vesicle (GV) or metaphase 1 stage
The cAMP is produced endogenously in oocyte through the stimulation of the Gs G-protein by the GPR3, transported into oocyte from adjacent cumulus cells, and/or held by PDE3A inhibitor(s) in the follicular environment.
1b: A proposed model for maintenance of meiotic arrest at diplotene stage in follicular oocyte. The NO produced through nitric oxide synthases in cumulus-granulosa cells stimulate generation of cGMP through GCs pathway. The cGMP from encircling somatic cells is transferred though gap junctions to the oocyte. An increased level of intraoocyte cGMP level may inactivate PDE3A in the oocyte. The NO is also produced by oocyte itself through iNOS-mediated pathway and possibly inhibits PDE3A through cGMP pathway. The inhibition of oocyte PDE3A prevents cAMP hydrolysis and increase intraoocyte cAMP level. The increased cAMP level may activate PKA which in turn inactivate CDC25B phosphatase and thereby MPF. The inactive MPF does not induce meiotic resumption and diplotene arrest is maintained. The reduced production of H2O2 and Ca2+ release from mitochondria may also maintain meiotic arrest at diplotene arrest.
2b: A proposed model of LH/hCG-induced meiotic resumption from diplotene arrest in in preovulatory oocyte. LH/hCG reduces iNOS activity and induces disruption of gap junctions between cumulus-granulosa cells and oocyte. The interruption of communication between cumulus-granulosa cells and oocyte may block the transfer of cGMP produced through NO–GCs pathway. The reduced iNOS activity and thereby decreased intraoocyte NO level further decreases oocyte cGMP level. The net reduction in cGMP level may activate PDE3A that reduces cAMP level generated by oocyte itself through GPR3/AC pathway. The decrease in the cAMP level results in the inactivation of PKA activity, which in turn stimulates CDC25B phosphatase in the oocyte. The activated CDC25B phosphatase induces MPF activity that finally induces resumption of meiosis. Generation of tonic level of ROS and Ca2+ release from mitochondria may also be associated with the induction of meiotic resumption from diplotene arrest.
When meiosis 1 is completed, what does the secondary oocyte contain?
- half chromosomes
- nearly all cytoplasm
What is in the discarded polar body?
- other half of the chromosomes
- small bag of cytoplasm
What does the breakdown of the germinal vesicle indicate?
- resumption of meiosis
What does the the extrusion of the first polar body (1 PB) indicate?
- completion of the first meiotic division in human oocytes
Why the unequal division of cytoplasm at the end of meiosis 1?
- Need to conserve for the oocyte all the materials synthesised earlier –> takes fertilized zygote forward in growth and implantation
What happens to the secondary oocyte once it is formed?
- Immediately enter 2nd meiotic division
- Form 2nd metaphase spindle and arrest
How is the arrest maintained in the secondary oocyte?
- cytostatic factor (protein complex)
In what state is the egg ovulated?
- Secondary oocyte (arrested at Metaphase II)
What does the LH surge induce in GCs and DF cells?
- GC: PR expression in all species
- DF: luteinisation (both GCs & TCs)
What occurs to the levels of E2 and P during the LH surge?
- E2 falls
- P stimulated (& 17alpha-OHP)
What are the changes that occur in the ovary after the LH surge?
- GC: PR expression in all species
- DF: luteinisation (both GCs & TCs)
- E2 falls
- P stimulated (& 17alpha-OHP)
- Blood flow to follicle increases
- New vessels appear in avascular GC
- Increased prostaglandins & proteolytic enzymes e.g. collagenase & plasmin (in response to LH & P)
- Appearance of apex or stigma on ovary wall
What do the prostaglandins and proteolytic enzymes do?
- Digest collagen in follicle wall
Give 2 examples of prostaglandins and proteolytic enzymes?
- Collagenase
- Plasmin