Menstrual cycle

0 4 8 12 16 20 24 28

Onset of menstruation

Menstruation

Day 1 Day 1

0 4 8 12 16 20 24 28

Key events in ovarian cycle

Menstruation

Day 1

Ovulation

Oestradiol

LH

1. Follicular growth

0 4 8 12 16 20 24 28

LH surge

Menstruation

Day 1 Day 1

Days before Days after

Follicular phase

Luteal phase

Ovulation

LH

0 4 8 12 16 20 24 28

Key events in ovarian cycle

Menstruation

Day 1

Ovulation

Oestradiol

1. Follicular growth

LH

2. Ovulation

When does ovulation occur?

The timing of ovulation varies with the length of a woman's menstrual cycle.

In the average 28 day menstrual cycle, the LH surge usually occurs between cycle days 11-13 and ovulation follows about 36-48 hours later, on or close to cycle day 14.

0 4 8 12 16 20 24 28

Key events in ovarian cycle

Menstruation

Day 1

Ovulation

Oestradiol

1. Follicular growth

LH

2. Ovulation

Oestradiol(& Progesterone)

3. Luteal function

0 4 8 12 16 20 24 28

Key events in ovarian cycle

Menstruation

Day 1

Ovulation

Oestradiol

1. Follicular growth

LH

2. Ovulation

Oestradiol(& Progesterone)

3. Luteal function

4. Luteal regression

Zona pellucida

Antrum

OocyteCumulus cells

The follicle is the fundamental element of the ovary

What control follicular growth?

HypothalamusHypothalamus

PituitaryPituitary

OvaryOvary

GnRH

LHFSH

Estradiol

Reproductive tract

Steroid feedback

What control follicular growth?

04 8 12 16 20 24 28

MenstruationAs each follicle grows, it produces

increasing amt of Estradiol

?????Gonadotropin independent

Ovulation

Oestradiol

The follicle is the fundamental element of the ovary

Androgens

LH

OestradiolFSH

Inhibin

Androgens are converted (aromatized) to estradiol by

granulosa cells

HypothalamusHypothalamus

PituitaryPituitary

OvaryOvary

GnRH

LHFSH

Estradiol

Reproductive tract

Increasing –ve

feedback mechanism

+

-

Increasing amount of estradiol

Inhibin suppress

+↓FSH

4 8 12 16 20 24 280

Estradiol

FSH As the follicle grow, FSH level fall due to the –ve feedback mechanism

04 8 12 16 20 24 28

Menstruation

Many follicles at the start of

the cycle

Ovulation

Why is only one selected & become

dominant ?

HypothalamusHypothalamus

PituitaryPituitary

OvaryOvary

GnRH

FSH

Increasing –ve

feedback mechanism

-

Oestradiol

Inhibin suppress

+↓FSH

Small folicle very

dependent on FSH

Large folicle independent

on FSHGrowth factor

estradiol

+ +X

Dominant follicle

4 8 12 16 20 24 280

Estradiol

FSH

Dominant follicle (s)

can survive

Insufficient FSH to keep smaller follicle

going to become atretic

FSH secretion suppressed

0

What causes ovulation ?

4 8 12 16 20 24 28

What causes LH surge ?

Oestradiol

What effects does it have ?

HypothalamusHypothalamus

PituitaryPituitary

OvaryOvary

GnRH

LHFSH

For most of the cycle negative

feedback mechanism operates

-

Estradiol

↓ LH & FSHInhibited by

estradiol

HypothalamusHypothalamus

PituitaryPituitary

OvaryOvary

GnRH

But, with high level of estradiol maintained for long enough

+

EstradiolLH surge

↑GnRH

↑Sensitivity to GnRH

+

How does the LH surge affects the

follicle ?

About 36 h between LH surge & oocyte

release…..

Oocyte

Enzyme induction in the follicle wall

Loosening of the cumulus cells

Oocyte

Transformation of ruptured follicle into corpus luteum

Mature follicle become Corpus luteum

Progesterone + Estradiol

Steroid secretion changes

0

Follicular phase

4 8 12 16 20 24 28

Luteal phaseEstradiol domination Progesterone domination

What maintains the CL ?

Ovulation

Why does the CL degenerates at the end of the

cycle ?

What maintains the CL ?

HypothalamusHypothalamus

PituitaryPituitary

GnRH

CL

LH very low levels

CL very sensitive to LHProgesterone +

E2

~8 h between LH pulses

What maintains the CL ?

HypothalamusHypothalamus

PituitaryPituitary

GnRH

CL

LH (low levels in luteal phase)

CL very sensitive to LH

Progesterone + E2

Steroid negative feedback keeps LH & FSH level relatively low

Reproductive tract etc

+

-

-

HypothalamusHypothalamus

PituitaryPituitary

GnRH

LH

CL degenerates

Progesterone + E2

Towards the end of the cycle, the sensitivity to LH

reduces

The low level of LH are

insufficient to keeps the CL

going

HypothalamusHypothalamus

PituitaryPituitary

GnRH

↑ FSH + LH

Progesterone + E2

As CL degenerates

steroid negative feedback reduces

A new wave of folicles stimulated by rising FSH &

LH

Other changes in the cycle

1. Outer muscles layer the myometrium

2. Inner glandular layer mucusa the endometrium

4 8 12 16 20 24 28

Endometrial depth

Ovulation

Uterine changes in menstrual cycle

Estradiol cause an ↑ in thickness

(the proliferative phase)

More secretion from the gland, hence the term

secretory phase

Opt time for implantation

0

Time course of menstruation

Onset of menstruation is rapid

Probably, 95% women have a total blood loss of less than 60 ml

This blood loss can represent as a significant loss of iron (leading to anemia)- especially in women on marginal diet

Menstruation….Why ?

In preparation of pregnancy, the human uterine stromal cells go through complex changes and the stromal cells terminal differentiate

If implantation and pregnancy do not occur, this tissue is lost and the uterus prepares itself again for another possible pregnancy

0 4 8 12 16 20 24 28

Key events in ovarian cycle

Menstruation

Day 1

Ovulation

Oestradiol

1. Follicular growth

LH

2. Ovulation

Oestradiol(& Progesterone)

3. Luteal function

4. Luteal regression

Women fertile window