Sex hormones at puberty
Puberty
The stage of development at which the reproductive organs become active and secondary sexual characteristics develop under the control of sex hormones.
Secondary sexual characteristics
The physical features other than the reproductive organs themselves that develop at puberty under the control of sex hormones.
- Puberty begins when the pituitary gland starts to secrete more of the hormones that act on the ovaries and testes.
- Those hormones make the ovaries or testes become active and secrete sex hormones of their own.
- The sex hormones circulate in the blood and act on tissues throughout the body.
- They bring about secondary sexual characteristics, which are the physical changes other than the reproductive organs themselves.
- In males these include a deeper voice, facial and body hair, broader shoulders and more muscle.
- In females these include breast development, wider hips and the start of the menstrual cycle.
- The reproductive organs also mature, so sperm production begins in males and eggs begin to be released in females.
- Because the hormones stay in the blood for years, these changes happen gradually and are permanent.
Testosterone and oestrogen
Testosterone
The main male reproductive hormone, secreted by the testes, which stimulates sperm production and brings about the changes of puberty.
Oestrogen
The main female reproductive hormone, secreted by the ovaries, which causes the uterus lining to thicken and brings about the changes of puberty.
- Testosterone is secreted by the testes and is the main male reproductive hormone.
- It stimulates the testes to produce sperm from puberty onwards.
- It also brings about the male secondary sexual characteristics.
- Oestrogen is secreted by the ovaries and is the main female reproductive hormone.
- It brings about the female secondary sexual characteristics at puberty.
- During each cycle it makes the lining of the uterus grow thicker and richer in blood vessels.
- A thick lining is needed so a fertilised egg can implant and be supplied with food and oxygen.
- Both hormones are secreted from puberty onwards, but oestrogen output rises and falls within each cycle.
Stages of the menstrual cycle
Menstrual cycle
The approximately 28 day cycle in which the lining of the uterus is built up and broken down and an egg is released from an ovary.
Ovulation
The release of a mature egg from an ovary, which happens around day 14 of the menstrual cycle.
- The menstrual cycle lasts about 282828 days, though the length varies from person to person.
- Day 111 is the first day of menstruation, when the uterus lining breaks down and passes out through the vagina.
- Menstruation usually lasts about 444 days.
- From roughly day 444 to day 141414 the lining of the uterus is rebuilt until it is thick and well supplied with blood.
- Over the same period an egg matures inside a follicle in one of the ovaries.
- Ovulation happens at about day 141414, when the mature egg is released from the ovary into the oviduct.
- From day 141414 to day 282828 the thick lining is maintained so it is ready to receive a fertilised egg.
- If the egg is not fertilised the lining breaks down again, and the next cycle starts at day 111.
Timing within a cycle
- A person has a regular cycle of 282828 days and menstruation begins on day 111.
- The uterus lining is rebuilt over roughly the next 101010 days.
- Ovulation is expected around day 141414, which is 282\dfrac{28}{2}228.
- An egg survives about 242424 hours after release, so fertilisation is possible on days 141414 and 151515.
- With no fertilisation the lining breaks down about 141414 days later, at day 282828.
The four hormones of the cycle
FSH (follicle stimulating hormone)
A hormone secreted by the pituitary gland that makes an egg mature inside a follicle in the ovary and stimulates the ovaries to secrete oestrogen.
LH (luteinising hormone)
A hormone secreted by the pituitary gland that triggers ovulation, the release of a mature egg from the ovary.
Progesterone
A hormone secreted by the ovaries after ovulation that maintains the thickened lining of the uterus.
- Four hormones control the cycle, two secreted by the pituitary gland and two by the ovaries.
- FSH is secreted by the pituitary and makes an egg mature inside a follicle in the ovary.
- FSH also stimulates the ovaries to secrete oestrogen.
- LH is secreted by the pituitary and triggers ovulation at about day 141414.
- Oestrogen is secreted by the ovaries and thickens the uterus lining in the days before ovulation.
- Progesterone is secreted by the ovaries after ovulation and maintains the thickened lining.
- When progesterone output falls, the lining can no longer be maintained and menstruation begins.
Describing the cycle
- Give a day number with every stage you describe, because questions almost always supply a 282828 day cycle.
- Name the ovaries as the source of oestrogen and progesterone, and the pituitary gland as the source of FSH and LH.
- Write that the lining is maintained after ovulation rather than that it carries on growing.
- Describe ovulation as the release of an egg from the ovary, not as the egg leaving the body.
- Do not write that menstruation is the release of the egg, because it is the loss of the uterus lining.
- Do not write that FSH releases the egg, because FSH matures the egg and LH triggers its release.
- Do not treat every cycle as exactly 282828 days, because the length varies and the day of ovulation moves with it.
- Name the gland that secretes FSH and the organ that secretes oestrogen.
- What are secondary sexual characteristics, and which hormone brings them about in males?
- What happens to the uterus lining between day 444 and day 141414?
- On roughly which day of a 282828 day cycle does ovulation take place?
- Which hormone maintains the uterus lining after ovulation?
