The menstrual cycle is controlled by hormones from the pituitary gland and ovaries. It aims to release an egg once a month to allow for potential fertilization and pregnancy. The cycle involves three phases - the menstrual, proliferative, and secretory phases. Estrogen stimulates growth of the uterine lining during the proliferative phase, while progesterone maintains the lining during the secretory phase in preparation for potential implantation. If implantation does not occur, progesterone and estrogen levels fall, causing the uterine lining to shed and result in menstruation, restarting the cycle.

1. Hormone Control – Two Types!

2. THE MENSTRUAL
CYCLE

4. 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).

5. 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

6. • 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

7. 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

8. Hormones
• Four hormones involved in controlling
menstruation
Name 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

9. 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)

10. Initial concentrations of estrogen are low.
The low concentration has a negative
feedback effect on the secretion of FSH.

11. 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)

12. Positive feedback from
increasing estrogen
concentration causes
increase in FSH and LH from
pituitary gland.
Follicle gets bigger – releases
more oestrogen

13. 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.

14. Ovulation Video

15. 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

16. 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.

17. 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.

18. 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 luteum
starts 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

19. 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

20. 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

23. 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?