Menstrual Cycle II Flashcards
There is selection and exponential growth of the dominant follicle. How does the switch to positive feedback occur?
LH surge lasts for 36-48 hours & triggers ovulation (timing varies from species-species)
» LH surge relatively precise
predictor of timing of
ovulation
LH is rapidly cleared from serum in contrast to..
CG which is cleared
slowly & binds with great affinity to LHCGR
What causes a surge in LH?
Oestradiol release…?
Where are LHr found on the follicle at this stage?
Theca and Granulosa.
What makes the hCG?
Blastocysts
What happens in order for ovulation to take place?
( a) Preovulatory follicle prior to the LH surge
b) Preovulatory follicle following LH stimulation immediately prior to ovulation. )
Loss of OSE & breakdown of underlying basal lamina and GC & TC at apex to allow for rupture.
GC basal lamina is disrupted allowing extension of
blood vessels into GC layer and for infiltration of
theca cells & leukocytes into GC compartment. COC
detaches from surrounding GC to expand.
What factors are responsible for holding the oocyte in meiotic arrest?
- High cAMP → keep maturation promoting factor (MPF) inactive
‒ cGMP enters oocytes from cumulus cells via gap junctions to inhibit oocyte cAMP phosphodiesterase PDE3A activity (PDE3A normally degrades cAMP)
‒ H2O2/NO/calcium
‒ other cells/ ovarian environment & integrity of the follicle?
Within 3-12hr of LH surge, we see:
- Detachment of COC from surrounding
mural GC, followed by cumulus cell
expansion – formation of unique
extracellular matrix between cumulus
cells (aka “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 - 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 Cell (2006), Volume 126, Issue 1
What happens during meiosis in the oocyte?
Early oocytes classified as immature i.e. at
germinal vesicle (GV) or metaphase 1
stage
- Meiosis I is completed with half
chromosomes but nearly all cytoplasm
remaining in the secondary oocyte
- Remaining chromosomes move with small
bag of cytoplasm to form discarded polar
body (PB)
Why is there unequal division of the cytoplasm?
All the organelles/mitochondria (maternal mitochondria) come from the the mature secondary oocyte.
Need to conserve for the oocyte all the relevant material
LH/hCG surge leads to
Activation of PDE3A which dcereases cAMP production–>
Chromosomes of secondary oocyte immediately enter 2nd meiotic division, form the 2nd metaphase spindle and arrest. This arrest is maintained by
Cytostatic factor (protein
complex). Egg is ovulated in this arrested
state
When is meiosis 2 completed?
When the egg is fertilised
What does LH surge induce?
LH surge induces expression of progesterone receptor (PR) in GC in all species and results in luteinisation of DF cells (both granulosa and theca)
As there is an LH surge, what happens to E2 and Progesterone?
E2 production falls and P is stimulated (P & 17α-OHP)
What happens to blood flow when there is an LH surge?
Blood flow to the follicle increases & new vessels appear in
avascular GC
What happens to prostaglandins and proteolytic enzymes when there is an LH surge?
Prostaglandins and proteolytic enzymes eg collagenase and
plasmin, are increased in response to LH and progesterone.
What do proteolytic enzymes do?
Digest collagen in follicle wall
what is the stigma?
Stigma= point of the dominant follicle closest
to the ovarian surface where digestion occurs
Cascade of events → release of COC → Ovulation
Increased secretion of chemokine/cytokines from GC & TC
triggers massive infiltration of leukocytes from circulation →
acute inflammatory response
Ovulation occurs in which ovary?
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 essential for ovulation
» Progesterone inhibitor (RU486) suppress ovulation