1. Hormones, puberty and the
female menstrual cycle
Doc Brown's Biology exam study revision notes
There are various sections to work through,
after 1 they can be read and studied in any order.
of biology notes on human sexual reproduction
(1a) A brief reminder about hormones
endocrine system and
introduction to homeostasis for other notes and links)
Hormones are produced by and excreted from endocrine glands and are transported
by the blood to their target organs
Know and understand that many
process within the
body are coordinated and controlled by chemical substances called hormones.
Hormones, being directly
released into the blood, are quite rapidly carried to all parts of the body
BUT only affect the function of particular cells.
Hormones effectively act as
'chemical messages' to trigger particular biochemical reactions.
Their effects are relatively long-lasting compared to e.g.
the nerve impulses and
responses of reflex arc,
but both the nervous system and hormones also help us to control
conditions inside our bodies.
Know that hormones are used in some forms of contraception
and in fertility treatments.
The pituitary gland produces the
hormones FSH and LH which are important control chemicals in the female
menstrual cycle as are the hormones oestrogen and progesterone.
The oestrogen, progesterone and
testosterone are steroid hormones - along with others that control
glucose levels in the blood, water balance in the body and protein
metabolism - they are all very 'busy' molecules!
Oestrogen is one of the principal female sex
hormones. Oestrogen is needed for puberty, the menstrual cycle,
pregnancy, bone strength and other functions of the body. Oestrogen
levels vary throughout the menstrual cycle and fall after menopause.
Having too much oestrogen can increase your risk of blood clots and
(1b) Puberty and the
female menstrual cycle
Sex hormones are present in the
foetus, but sex is determine by the X and Y chromosome pairings.
So, but if the foetus has a Y
chromosome (from the XY chromosome pairing), then male
characteristics develop, and if it has the XX chromosome pair,
female characteristics develop.
Sexual reproduction cannot happen
without the intervention of several sex hormones.
When your body reaches puberty
it starts to release sex hormones that trigger the formation of
secondary sex characteristics - exemplified by the obvious
physical changes to the male and female bodies!
e.g. in men, facial hair
develops and the penis increases in size.
The principal reproductive male sex hormone is
produced in the testes and also stimulates sperm production
and is important for the development of the male reproductive
e.g. in women, breasts develop
and 'periods' start - part of the menstrual cycle.
The principal reproductive sex hormone is
in the ovaries. Apart from promoting physical changes like
oestrogen is also involved with other hormones in the menstrual
cycle concerning female fertility (described next, starting with the
summary diagram below).
The menstrual cycle is the
reproductive cycle in women and summarised in the diagram
Note that the 4 hormone level graphs
are NOT to scale, BUT,
graph trends and timing of peaks are important, and crucial to,
understanding the menstrual
A simplified description of four
stages in the 28 day menstrual cycle (need to x-reference with
the above graph)
Four hormones control the monthly
(~28 days) menstrual cycle.
Take it that 'level' means the
same as 'concentration'.
The concentration of these hormones fluctuate in the cycle and they
interact with each other to promote or inhibit the release of other
Follicle stimulating hormone (FSH)
which causes a follicle (egg + surrounding cells) to mature in
one of the ovaries.
Luteinising hormone (LH)
stimulates the release of a mature egg from an ovary at day 14.
Oestrogen and progesterone are
involved in the maintenance of the lining of the uterus Womb) i.e.
to ensure it is renewed to receive a fertilised egg.
(More notes on the hormones after
the four basic stages of the menstrual cycle have been described).
By convention, if the woman is not
pregnant, the menstrual cycle begins with a period
Stage 1 - the menstruation
starts - the bleeding starts on
day 1 as the uterus lining breaks down for 4 days promoted by the
decrease in progesterone level - known as having a 'period'
The mucous lining of the uterus wall
that thickens in the menstrual cycle is called the endometrium.
Stage 2 - over days 5 to
uterus lining builds up again ('repaired') to give a thickish spongy layer of tissue
full of blood vessels, potentially ready to receive a
fertilised egg for implantation - this is promoted by the rise in oestrogen
Stage 2 involves the body preparing the uterus
(womb) to receiving a fertilised egg - implantation.
Stage 3 - around day 14 an
developed and released from an ovary follicle in the process called
- this is facilitated by the increase in the FSH and LH hormone -
they peak at ~day 13, and the egg is released at ~day 14, hardly a
Eggs mature in the ovaries
and are released about once every 28 days - ovulation.
Ovulation is the
release of a mature egg from an ovary follicle which typically
happens once during each menstrual cycle ~day 14 in the cycle.
The egg cell lives for up to 24 hours after being released, if
the egg cell is not fertilised, the egg cell dies and the
menstrual cycle progresses to the next phase
Ovarian follicles are small
sacs filled with fluid and an immature egg, that are
found inside a woman’s ovaries. They secrete hormones which
influence stages of the menstrual cycle and when women begin
puberty. Each has the potential to release an egg for
Follicles and their size and status are a vital
part of assessing fertility and fertility treatment (see later
section on treating infertility).
After ovulation the residue
cells from the follicle form a structure called the corpus
Stage 3 is the time frame in which the woman is
most likely to become pregnant.
Stage 4 - the thickened uterus
tissue wall lasts for around 14 days.
The uterus wall is maintained
by the presence of increased levels of progesterone.
If no fertilised egg
settles on the uterus wall by day 28, the spongy uterus lining breaks up
and passed out with bleeding through the vagina.
whole menstrual cycle repeats itself if no fertilised egg implants
in the lining of the uterus.
If a fertilised egg implants into the lining
of the uterus, the menstrual cycle stops and the period is
missed, suggesting a pregnancy has started.
control of the menstrual cycle (repeated the diagram for
clarity on the four stages)
Several sex hormones are involved
in the menstrual cycle of a woman and hormones are involved in promoting the
release of an egg: The monthly release of an
egg from a woman’s ovaries and the changes in the thickness of the lining of
her womb are controlled by hormones secreted by the pituitary gland and by
the ovaries. You now need to know, as
described below, the function of various hormones that control the different
stages of the menstrual cycle.
(a) The ovaries produce the hormone oestrogen, which causes the lining of the uterus to grow
It also triggers the release of LH (luteinising hormone), which
causes the release of an egg, and inhibits further release of
FSH (follicle-stimulating hormone) so that only one egg
is released in each cycle.
Note that the level of
oestrogen rises to a peak through Stage 2, stimulating the
growth and completion of the spongy uterus lining.
The high level of
oestrogen stimulates a surge in the LH concentration.
(b) The follicle stimulating hormone (FSH)
is secreted by the pituitary gland (in the brain) and causes an egg to mature in one of the ovaries
in a structure called a follicle. It also stimulates the ovaries to produce hormones including
Note that the level of FSH rises to a peak ~day13-14 in Stage 3 when the spongy
uterus lining is complete and ready for an egg delivery.
(c) The luteinising hormone (LH),
is also secreted from the pituitary gland, and stimulates the
of an egg from the ovary around day 14 in the cycle (Stage 3 ovulation).
LH causes the
follicle to rupture and a mature egg is released.
LH also indirectly
stimulates progesterone production.
stimulates the remains of the follicle to develop into a
structure called a corpus luteum, which secretes
progesterone which maintains the physical integrity of
the uterus lining.
Note that the level of LH also rises to a peak ~day13-14 in Stage 3 when the
uterus lining is complete and so an egg release is
(d) Progesterone is
produced in the ovaries by the remains of the follicle (the
corpus luteum) after
ovulation (its level peaks in the middle of Stage 4).
Progesterone maintains the lining of the uterus wall in the 2nd
half of the cycle (see diagram) and when its level falls, the
uterus lining breaks down. It also inhibits the release of LH
and FSH (described above).
Note that the level of progesterone rises to a peak in Stage 4 to maintain the
spongy uterus lining in case a fertilised egg settles on it,
as well as inhibiting the secretion of LH and FSH so that
the cycle can be completed.
If pregnancy occurs, the
progesterone level stays high to maintain the uterus lining.
As the progesterone
level falls and there is a low level of oestrogen, the lining of the uterus begins to break down
as the menstrual cycle repeats itself - it is a low
progesterone level that allows the FSH hormone level to
rise and trigger the cycle to start again.
(e) A change in one
hormone concentration can be used to predict the change in
e.g. an increase in
oestrogen when progesterone concentrations are low, would be
followed by an increase in LH concentration.
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