Hormones |
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Topic Notes |
Additional Support Materials i.e. animations, quizzes, pictures, worksheets |
Reproduction and its Hormonal Control
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Menstrual
Cycle Summary (provided by: University of Virginia) How
hormones work
Useful activity in plotting your own graph of the menstrual cycle - this will help your understanding and give you a visual representation of the interactions of hormones. It will hopefully help you to remember the role of each hormone and its interaction with target organs. Reproduction Hormones Worksheet (BiologyMad)
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Controlling Reproduction in Domestic Animals (Synchronising Breeding)
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Reproduction
and its Hormonal Control
[back to top]
Different
mammals have different patterns of reproduction
Eg
mammals, rats and mice can breed all year round, whereas others have
distinct breeding seasons.
This is to ensure that the young are born when food is abundant.
The timing of the breeding season is regulated by a biological
clock, which is probably adjusted by seasonal changes in day length.
small
motile male gametes (sperm) whish is produced in large quantities
larger,
non-motile food storing female gamete (ovum) which is produced in much
smaller numbers
Gametogenesis
is the formation of gametes
sperm
production is called spermatogenesis
where
the eggs are formed (matured) is called oogenesis
In the females of all mammals, there is a cycle known as:
Oestrous
cycle à
ovulation,
when the female is most fertile
Menstruation cycle à the uterus lining in all mammals undergoes a similar pattern of thickening during a reproductive cycle. However if fertilisation does not occur, the uterine lining of primates breaks down and it is discharged with blood through the vagina, whereas the uterine lining of non-menstruating mammals is reabsorbed and there is no excessive bleeding. The discharge of blood is called menstruation.
Lasts
approximately 28 days in Humans.
It
is controlled by the interaction of several hormones.
The action of one hormone is used to stimulate or inhibit the
production of another. Hormones
are chemical messengers, produced and secreted by organs, which travel via
the blood, and exerts some influence upon a target tissue.
Events are divided into three phases: follicular phase, ovulatory phase, and the leutal phase
The ovaries are organs that are responsible for the development of female gametes. At birth around 400 000 cells have reached prophase of the first meiotic division and are called primary oocytes (often called follicles). Each month after puberty, one of these cells completes its development into an ovum.
The follicular phase à is the first part of the menstrual cycle, where one or more follicles start to develop into a mature female gamete. The follicle cells surround the oocyte (developing egg cell), and produce hormones that trigger other responses.
The Ovulatory phase à is when the oocyte is released (follicle cells remain in the ovary) from the ovary and passes down the fallopian tube and towards the uterus.
The
Luteal phase à
most of the follicle cells remain in the ovary after ovulation.
They continue to develop and form a structure called the corpus luteum,
as a result more hormones are produced.
Use
the diagrams below in conjunction with the flow chart.
Add some of the explanations listed below to the diagram (when you have
printed it of course) or else make your own flow chart – This will help to
reinforce the information you need to learn!
Follicular
Phase |
At
the start of the oestrous cycle, the pituitary gland (in the brain)
secretes follicle-stimulating hormone (FSH) â
FSH
triggers development of one or more follicles in the ovary â As
the follicle grows in size, oestrogen is secreted |
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å |
â |
æ |
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Inhibits
further production of FSH |
Stimulates
the pituitary gland to secrete lutenising hormone (LH) |
Stimulate
growth and repair of the uterine lining (endometrium) |
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â |
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Ovulatory
Phase |
As
the follicular stage progresses, the developing follicle increases in size
and becomes a mature follicle |
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â |
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Oestrogen
levels increase rapidly
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â |
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Triggers
further release of LH (high
concentration of LH in the blood) |
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â |
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Ovulation
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â |
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Oocyte
leaves the ovary and passes into the fallopian tube |
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â |
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Female
is fertile |
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Luteal
Phas |
â |
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The
high concentrations of LH that brings about ovulation has an affect on the
follicle cells that remain in the ovary |
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â |
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Follicle
becomes corpus luteum |
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â |
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Corpus
luteum
secretes some oestrogen and a large amount of progesterone |
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å
æ |
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Progesterone
stimulates
mammary glands and uterus in anticipation of pregnancy |
High
concentrations of oestrogen and progesterone inhibit production of FSH
and LH |
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â |
â |
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If
the Oocyte is not fertilised within 36 hours, it dies |
Without
FSH and LH the cells of the corpus luteum gets smaller – and less
progesterone and oestrogen is secreted |
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â |
â |
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At
day 28, a lack of progesterone brings about another menstruation |
With
less oestrogen and progesterone, the FSH is no longer inhibited, and
the cycle can start again |
**If
pregnancy occurs, initially the corpus luteum secretes sufficient
progesterone to maintain the uterine lining and sustain the developing embryo. After this, the placenta takes over, where progesterone (and
some oestrogen) from the placenta maintain the uterine lining and inhibit the
development of further ova (egg), and prepare the breast tissue for lactation
(milk production). At the end of
pregnancy, progesterone levels fall, and high oestrogen levels trigger the onset
of labour.
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The follicle cells (light green), follicular fluid (cyan) and developing oocyte (magenta) have been coloured to clearly demonstrate the changes in the follicle as the egg matures and is finally ovulated. The follicle undergoes changes as it becomes a hormone producing corpus luteum (gold, dark green). |
SUMMARY
OF EFFECTS |
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Hormone |
Site
of Secretion |
Target
Organ |
Function |
Follicle
Stimulating Hormone (FSH) |
Pituitary
gland |
Ovary |
|
Oestrogen |
Ovary |
Endometrium
(lining of the uterus) |
|
Lutenising
Hormone (LH) |
Pituitary
|
Ovary |
|
Progestrone |
Corpus
luteum |
Uterus |
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How sex hormones work
[back to top]
(Additional
information to help consolidate this topic)
The action of one hormone is used to stimulate or inhibit the production of another. Hormones are chemical messengers, produced and secreted by organs, which travel via the blood, and exerts some influence upon a target tissue.
Hormones can be classified into two groups:
Proteins - lutenising hormone (LH) and follicle stimulating hormone (FSH)
Steroids - oestrogen
and progesterone.
The hormones that are proteins will have its molecules that are folded in such a way that they have a tertiary structure that gives them a specific shape. Once a protein hormone (such as LH or FSH) has reached its target organ, the hormone molecule will bind to a receptor site on the plasma membrane. The receptor molecules are proteins and have specific receptor sites that have a complementary shape to that of the hormone. The hormone is unable to pass through the plasma membrane and enter the cytoplasm of the cell. Instead it binds to the receptor and activates specific events in the cell.
Oestrogen
and progesterone are also sex hormones, however they are not proteins, they are steroids.
Steroids work in a different way to protein hormones.
Steroids are lipids and so they are able to pass through the phospholipid
bilayer of the plasma membrane and enter the target cell.
Once inside, they bind to receptors in the cytoplasm.
The receptors act as carriers for the hormone and transfer it into the
nucleus of the cell. The hormone
then switches on the gene responsible for the synthesis of a particular protein.
Controlling
Reproduction in Domestic Animals
(Synchronising
Breeding)
[back to top]
Reproduction in
non-primate mammals is characterised by the oestrous cycle (ovulation).
The
behaviour of many female mammals changes around the time of ovulation (known as
oestrous), Oestrous marks the time when a female will allow mating to
take place – often referred to as ‘coming into season’ or being ‘on
heat’.
Initially farmers will treat their cows with progesterone (Remember that progesterone inhibits FSH, and that it can be used in contraceptive pills) â Farmers will stop treatment with all cows at the same time â The
reduction of progesterone will cause an increase in FSH, as it is no longer
inhibited. This allows the cows to
start their oestrous cycle at the same time i.e. the cows will now be
synchronised â Follicles will all develop at the same time â Farmers will inject the cow (generally one cow, - usually the one with the best features to pass onto the next generation) with a mixture of FSH and LH. Farmers work out the hormone dose, so that the cow can be made to produce a large number of egg cells â Females follicles in her ovaries develop and ovulation occurs â When the female comes into oestrous (ovulation), she is artificially inseminated, with the ‘best bulls’ sperm â Egg cells are fertilised and start to develop into embryos â 6-8 days later, the embryos are removed â The
embryos developing normally are transferred to other cows, which will act as
surrogate mothers. It is important that the
recipient cows and the donor cow are synchronised or the uterus will not be in a
suitable condition for the embryos to implant. |
Advantages of this method:
Produce offspring at similar times
Allows the farmers to prepare fort many births at one time, allowing them to give similar feeding rations in line with their stage of pregnancy (and after birth of the calf)
Allows farmers to save on veterinary fees
Easier management of reproduction in cows
Allows the farmers to improve the ‘quality’ of the oocyte shed at ovulation
The sex of the embryo can easily be determined - male animals are worthless to dairy farmers (unless needed as a 'stud' bull), pedigree females are very valuable
By growing and then splitting the embryos ('cloning') the farmer can freeze the spare embryos. Some years later the can see the performance of the calves and decide which of their identical 'sisters' the farmer wishes to use. These can then be implanted and so guarantee the highest quality of herd - bizarrely producing identical 'twins' some years apart in age and with different birth mothers!
Last updated 20/06/2004