Events of Ovarian Cycle

What is ovulation?

Phase of a female’s menstrual cycle where a mature egg is released from the ovarian follicles into the oviduct

What is required for ovulation?

An increase of LH is essential for ovulation as it triggers oocyte maturation, follicle rupture and release of oocyte 

Step 1: What happens after regression of the corpus luteum?

FSH and LH secretion increases because progesterone concentration decreases

Step 2: What does LH stimulate early in the cycle?

LH stimulates theca interna cells to secrete androgens that diffuse into granulosa cells

Step 3: What does FSH stimulate the granulosa cells to do?

Convert andorgens —> estrogen (increases)

Step 4: How does FSH affect granulosa cell receptors 

FSH stimulate formation of LH receptors on granulosa cells 

Step 5: How does the follicular antrum form?

Estrogen rich fluid produced by granulosa cells separate them and forms the antrum

Step 6: What triggers the pre ovulatory LH surge?

Gradually increasing estrogen concentration

Step 7: What does the LH surge do to the oocyte?

Promotes maturation by resuming meiosis to the first body stage

Step 8: How does LH surge affect prostaglandins?

It promotes intrafollicular production of prostaglandins A and E (PGA, PGE) which are associated with follicle rupture

Step 9: What forms simultaneously with prostaglandin production?

Multivesicular bodies (MVBs) form as outpockets of the theca externa

Step 10: What is the function of multivesicular bodies during ovulation?

They secrete proteolytic enzymes that digest the ground substance between theca externa fibroblast allowing oocyte to escape (ovulation)

Step 11: How does the LH surge affect FSH receptors?

It reduces the number of FSH receptors on granulosa cells, decreasing conversion of androgen —> estrogen 

Step 12: How does LH change granulosa cells function after surge?

LH binding converts granulosa cells from estrogen secretion (follicular phase) to progesteron secretion (luteal phase)

Step 13: What happens after ovulation?

The cavity of ruptured follicle becomes corpus luteum

Step 14: What does the corpus lutem secrete and how does it affect the pituitary?

It secretes progesterone which decreases FSH and LH output from anterior pituitary

Step 15: What causes the corpus luetum regresses and what occurs because of it?

Causes: 

• Triggered by prostagladin F₂- alpha (PGF₂α) which is produced by uterine endometrium 

• PGF₂α reduces CL bloow flow causing apoptosis of luteal cells 

Therefore it causes:

• Progesterone output decreases

• Increase of FSH and LH 

Step 16: What causes FSH and LH secretion to rise again and restart the cycle?

The decrease in progesterone after corpus luteum regression

After ovulation, when will the egg be available to be fertilised by sperm?

During the luteal phase

What happens when the corpus luteum regresses and progesterone decreases?

• When the previous CL regresses and progesterone decreases, this removes the negative feedback on the pituitary 

• Causes FSH and LH to increase, stimulating follicular growth 

Then which follicles are recruited when the corpus luteum regresses and progesterone decreases?

The largest actively growing follicles that were already present in the ovary at the time are recruited to be matured 

Once one follicle matures into LC, why is the growth impaired and how?

Why growth impaired: To prevent multiple cycles to occur at once

How growth impaired: 

• Once the corpus luteum is formed, it produces high levels of progesterone

• Causes negative feedback on pituitary

• So this reduces GnRH, FSH and LH secretion which is needed for follicle growth

When the growth impairment stop?

When the CL regresses and progesterone falls

What happens to follicles without adequate LH receptors?

They do not ovulate and become atretic