Hormones

Reproduction and its Hormonal Control
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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.

  In mammals the gametes (sex cells) are:

• 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.

Hormonal Control of the Female Menstrual Cycle  [back to top]

• 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!

**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.
  ** Not directly covered in the AS AQA spec. A syllabus, however is useful for understanding of the roles of the hormones

Menstrual Cycle Summary

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)
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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’. 

E.g., Transplanting embryos in cows  

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!

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Last updated 20/06/2004