7.6 - Puberty & the Menstrual Cycle

Puberty and reproductive hormones

• Puberty is the stage during which adolescents become sexually mature and develop secondary sexual characteristics.
• Secondary sexual characteristics are physical changes in the body - e.g. facial hair developing in males and breasts developing in females.
• Puberty is triggered by reproductive hormones.

Testosterone - The main male hormone

• Produced by the testes.
• Stimulates sperm production.
• Causes male secondary sexual characteristics to develop:
  • Facial and body hair
  • A deeper voice
  • Increased muscle development
  • Growth of the penis and testes

Oestrogen - The main female hormone

• Produced by the ovaries.
• Causes female secondary sexual characteristics to develop:
  • Breast development
  • Widening of the hips
  • Growth of underarm and pubic hair
• Also involved in the menstrual cycle.

The female reproductive system and the menstrual cycle

• The female reproductive system includes the ovaries, fallopian tubes, uterus, and uterus lining, as well as the cervix and the vagina.
• The menstrual cycle coordinates two main things:
  1. The release of an egg from an ovary.
  2. The preparation of the uterus lining in case that egg is fertilised.

The parts and what they do

• Ovaries - Contain egg cells. During the cycle, one egg usually matures and is released from one of the ovaries. This release is called ovulation.
• Fallopian tubes - After ovulation, the egg travels from the ovary through one of the fallopian tubes towards the uterus.
• Uterus - Also called the womb. This is where a fertilised egg can implant and develop during pregnancy.
• Uterus lining - Also called the endometrium. It changes during the cycle, building up to prepare for a fertilised egg.

What happens to the lining

• If a fertilised egg implants in the uterus lining, pregnancy can begin.
• If fertilisation does not happen, the lining breaks down and is lost from the body during menstruation.

The stages of the menstrual cycle

• The menstrual cycle is the monthly sequence of events in which the body releases an egg and prepares the uterus in case the egg is fertilised.
• It's usually described as taking about 28 days, though the length can vary between people.
• The cycle is counted from the first day of a period.

The four stages

1. Stage 1 (about day 1-5) - menstruation - The uterus lining breaks down and is lost from the body, causing bleeding. This usually lasts around five days.
2. Stage 2 (about day 5-14) - The uterus lining builds up again into a thick, spongy layer with many blood vessels, ready in case a fertilised egg implants there.
3. Stage 3 (about day 14) - ovulation - An egg is released from one of the ovaries and travels along one of the fallopian tubes towards the uterus.
4. Stage 4 (about day 14-28) - The uterus lining is maintained in its thickened state.

What happens at the end of the cycle

• If the egg is fertilised and implants in the uterus lining, the lining stays in place and pregnancy can begin.
• If no fertilised egg implants, the uterus lining starts to break down again, menstruation begins, and the cycle repeats.

Hormones controlling the menstrual cycle

• The stages of the menstrual cycle are controlled by hormones.
• Two hormones control the state of the uterus lining, and both are produced by the ovaries: oestrogen and progesterone.
• Two further hormones from the pituitary gland control the release of the egg itself: FSH and LH.

Oestrogen and progesterone - The uterus lining

• Oestrogen - Causes the uterus lining to repair and build up after menstruation, making it thicker and preparing it in case a fertilised egg implants.
• Progesterone - Maintains the uterus lining during the second half of the cycle, keeping it thick and ready for a fertilised egg.
• If the egg is not fertilised, progesterone levels fall towards the end of the cycle.
• When progesterone falls, the lining is no longer maintained, so it breaks down and menstruation begins again.

FSH and LH - Releasing the egg

• FSH (follicle-stimulating hormone) - Produced by the pituitary gland, it causes an egg to mature inside one of the ovaries.
• LH (luteinising hormone) - Also from the pituitary gland, it triggers ovulation (the release of the mature egg from the ovary) at about day 14.