Hormone Control – Two Types!

THE MENSTRUAL

CYCLE

The Importance of the Menstrual Cycle

• Aim: to release a mature ovum (egg) approximately once

per month.

• This makes the female FERTILE (able to become

pregnant).

Facts & Tidbits

• Under the control of the endocrine system

• It is necessary for reproduction

• Average length: 28 days, but typically varies, with shorter

and longer cycles

• Menarche: a woman's first menstruation• typically occurs around age 12

• occurrence depends on overall health and diet

• Menopause: end of a woman's reproductive phase, commonly occurs between ages 45 and 55• age of menopause is largely the result of genetics

• Physiological changes during menstrual cycle can be

observed at three different levels:

• Neuro-endocrine level

• ovaries

• uterus

• Menstrual cycle can be divided into 3 phases:

• menstrual phase

• proliferative / follicular (estrogen) phase

• secretory / luteal phase (progesterone) phase

Phase

Average start and end day

(assuming a 28-day

cycle)

Menstrual phase 1-4

Proliferative phase 5-13

Ovulation 13-16

Secretory phase 16-28

Hormones

• Four hormones involved in controlling

menstruationName Made… Function

FSH Pituitary Stimulates one egg cell

to develop (become

follicle)

estrogen Follicle (ovary) Stimulates rebuilding

of the uterus wall

LH Pituitary Stimulates follicle to

burst and release the

ovum

Progesterone Corpus Luteum (ovary) Completes

development of uterus

wall, promotes

glycogen storage

During the first week of the cycle the

pituitary gland is stimulated, and releases

follicle stimulating hormone (FSH)

• FSH stimulates a potential egg cell in

the ovary

• The cells around the ovum also develop.

This is called a follicle.

• Follicle cells secrete estrogen.

• Estrogen stimulates building of uterus

wall (the endometrial lining)

Initial concentrations of estrogen are low.

The low concentration has a negative

feedback effect on the secretion of FSH.

Negative feedback lowers

FSH concentration

• As follicle grows is produces larger

concentrations of estrogen.

• At a certain threshold, its effect

reverses.

• It now has a positive feedback

effect on secretion of FSH from

pituitary.

• Estrogen stimulates pituitary gland

to release luteinising hormone (LH)

Positive feedback from

increasing estrogen

concentration causes

increase in FSH and LH from

pituitary gland.

Follicle gets bigger – releases

more oestrogen

Ovulation

• Peak of LH causes follicle to burst and release ovum

• This is ovulation (on day 14 of the cycle)

• Follicle reforms to become structure called corpus luteum

(‘yellow body’)

• LH stimulates corpus luteum to produce progesterone.

Ovulation Video

Peak of LH causes

ovulation (day 14)

Estrogen falls because

follicle is gone, but corpus

luteum still produces

some.

Ovulation – ovum released

from follicle. Follicle

become corpus luteum

Progesterone

• Progesterone completes uterus wall.

• Promotes increases blood supply and glycogen storage.

• Rising concentrations of progesterone and estrogen have

negative feedback effect on FSH and LH.

• This prevents new follicles forming.

Fall in FSH and LH due to

negative feedback with

estrogen and progesterone

Progesterone produced by

corpus luteum. Inhibits FSH

and LH

Corpus luteum – produces

progesterone. Uterus wall

fully completed.

Fertilization

• Fertilisation must occur within 2 days of ovulation.

• Embryo takes 3 days to reach the uterus and implant.

• If no embryo implants within a week the corpus luteumstarts to break down.• Progesterone and estrogen concentrations fall.

• Uterus wall begins to break down.

• FSH no longer inhibited, so begins to rise.

• Cycle begins again

FSH and LH totally inhibited

Breakdown of corpus luteum

causes fall in progesterone

and estrogen

Corpus luteum begins to

break down if no embryo has

implanted a week after

ovulation

FSH begins to rise since it’s

not inhibited by oestrogen

and progesterone

Low levels of progesterone

and oestrogen because there

is no follicle or corpus luteum

Uterus wall breaks down due

to low levels of estrogen and

progesterone

New egg cell stimulated by

rising FSH levels

Questions

• 1. What is the overall aim of the menstrual cycle?

• 2. Which hormone triggers ovulation?

• 3. Which hormone is responsible for building the uterus lining?

• 4. Which hormone is responsible for maintaining the uterus lining?

• 5. Where is progesterone produced immediately after ovulation?

• 6. Why does menstruation occur?

• 8. Where is estrogen produced?

• 9. Why does the ovum need to ‘mature’ in the follicle before ovulation can

occur?

• 10. Why does the level of estrogen drop after ovulation?