UK GCSE level age ~14-16, ~US grades 9-10 Biology revision notes re-edit 19/05/2023 [SEARCH]

 Human sexual reproduction: 1. Hormones, puberty and the female menstrual cycle

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INDEX of biology notes on human sexual reproduction


(1a) A brief reminder about hormones

(see also 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 stroke.

(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 testosterone, produced in the testes and also stimulates sperm production and is important for the development of the male reproductive system.

e.g. in women, breasts develop and 'periods' start - part of the menstrual cycle.

The principal reproductive sex hormone is oestrogen, produced in the ovaries. Apart from promoting physical changes like breast development, 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 below.

Note that the 4 hormone level graphs are NOT to scale, BUT,

 the graph trends and timing of peaks are important, and crucial to, understanding the menstrual cycle.

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 hormones.

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' or menstruation.

The mucous lining of the uterus wall that thickens in the menstrual cycle is called the endometrium.

Stage 2 - over days 5 to 14 the 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 level.

Stage 2 involves the body preparing the uterus (womb) to receiving a fertilised egg - implantation.

Stage 3 - around day 14 an egg is developed and released from an ovary follicle in the process called ovulation - 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 coincidence!

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 fertilisation.

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 luteum.

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.

The 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.


Hormonal 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 and thicken. 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 oestrogen,

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 release 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.

LH further 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 stimulated.

(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 another.

e.g. an increase in oestrogen when progesterone concentrations are low, would be followed by an increase in LH concentration.



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