Hello IGCSE Biologists! Let's Talk Hormones!

Welcome to the fascinating world of **Sex Hormones**! This chapter is essential for understanding how human reproduction works, especially the dramatic changes that happen during puberty and the carefully controlled cycle that leads to pregnancy.

Don't worry if the names seem complicated—hormones are just tiny, powerful chemical messengers, like messengers running around your body telling specific organs what to do. We'll break down the roles of the main four hormones you need to know: Testosterone, Oestrogen, FSH, and LH.

1. Core Concepts: Testosterone and Oestrogen (The Puberty Activators)

Sex hormones are chemical substances, produced by specific glands (the endocrine glands, like the testes and ovaries), carried by the blood, that alter the activity of target organs.

Testosterone (The Male Sex Hormone)

Testosterone is primarily produced by the testes in males.
Its main role is the development and regulation of secondary sexual characteristics during puberty.

  • Development of Secondary Sexual Characteristics: These are the physical changes that distinguish males from females, other than the reproductive organs themselves.
  • Examples:
    • Deepening of the voice (voice breaks).
    • Growth of facial and body hair.
    • Enlargement of muscles and bones.
    • Increased libido (sex drive).

Oestrogen (The Female Sex Hormone)

Oestrogen (sometimes spelled Estrogen) is primarily produced by the ovaries in females.

  • Development of Secondary Sexual Characteristics:
    • Development of breasts.
    • Widening of the hips.
    • Onset of the menstrual cycle (menstruation).
    • Distribution of body fat (e.g., on hips and thighs).
  • Regulation: Oestrogen is crucial for the ongoing control of the menstrual cycle, specifically repairing and thickening the lining of the uterus.
Quick Review (Core):

Testosterone: Male hormone, causes growth of body hair, muscle development, and deeper voice.
Oestrogen: Female hormone, causes breast development and controls uterus lining thickness.

2. The Menstrual Cycle (Core Requirements)

The menstrual cycle is a regular series of changes that occur in the reproductive system of fertile women, preparing the body for a possible pregnancy. It usually lasts about 28 days.

Changes in the Ovaries and Uterus Lining

We describe the menstrual cycle by looking at two main areas:

A. Changes in the Ovaries (Egg Production)


The ovaries contain immature egg cells (ova) stored in structures called follicles.

  1. Day 1–13 (Follicle Development): One follicle starts to mature and grow, preparing an egg for release.
  2. Day 14 (Ovulation): The mature egg is released from the ovary into the oviduct. This is the point where fertilization can occur.
B. Changes in the Lining of the Uterus (Uterus Wall)

The lining of the uterus (the endometrium) is where a fertilised egg would implant.

  1. Day 1–5 (Menstruation): If no fertilisation occurred in the previous cycle, the thick lining of the uterus breaks down and is shed along with blood. This process is called menstruation or a "period."
  2. Day 5–28 (Repair and Maintenance):
    • The lining is rebuilt and thickens again, becoming rich in blood vessels, ready to receive an embryo (if fertilization happens). (Days 5–14).
    • After ovulation (Day 14 onwards), the lining is maintained in a thick, spongy state by high hormone levels. (Days 14–28).

Key Takeaway: The cycle is a process of preparation (thickening the uterus lining and maturing an egg) followed by either pregnancy or shedding (menstruation).

3. Extended Content: Hormonal Control of the Menstrual Cycle

(This section explains why the changes in the uterus and ovaries happen, connecting them to four main hormones: FSH, LH, Oestrogen, and Progesterone.)

A. The Four Key Hormones and Their Production Sites

These four hormones work in a tightly coordinated sequence, primarily governed by the pituitary gland (in the brain) and the ovaries.

  1. FSH (Follicle Stimulating Hormone): Produced by the pituitary gland.
  2. LH (Luteinizing Hormone): Produced by the pituitary gland.
  3. Oestrogen: Produced by the follicle (in the ovary).
  4. Progesterone: Produced by the corpus luteum (the remnants of the follicle after ovulation).

Memory Aid: FSH and LH come from the pituitary (headquarters), Oestrogen and Progesterone come from the ovary (on the ground).

B. Step-by-Step Control of the Cycle

The cycle is driven by the rise and fall of these four hormones, which trigger the ovary and uterus changes.

  1. Start of the Cycle (Days 1–5):
    The pituitary gland releases FSH.
    • Role of FSH: Stimulates several follicles in the ovary to start maturing.
  2. Follicle Phase (Days 5–13):
    As the follicle matures, it produces and secretes Oestrogen.
    • Role of Oestrogen: Stimulates the repair and thickening of the uterus lining.
    • High oestrogen levels then feed back to the pituitary, causing a sudden surge in LH (and also suppressing FSH).
  3. Ovulation (Day 14):
    The spike (surge) in LH is the trigger.
    • Role of LH: Causes the follicle to rupture and release the mature egg (ovulation).
    • LH then causes the empty follicle remaining in the ovary to develop into the corpus luteum.
  4. Luteal Phase (Days 15–28):
    The corpus luteum immediately begins producing large amounts of Progesterone (and some Oestrogen).
    • Role of Progesterone: Maintains the thick, spongy, bloody lining of the uterus, keeping it ready for implantation.
    • Progesterone also strongly inhibits the release of FSH and LH, preventing any new follicles from developing.
  5. If No Fertilisation Occurs:
    The corpus luteum degenerates (breaks down) around Day 28. Since the corpus luteum is gone, the levels of Progesterone and Oestrogen plummet. Without these hormones, the uterus lining cannot be maintained, and it is shed (menstruation), restarting the cycle.
Analogy: The Hormonal Control Panel

Think of the hormones as dials:
1. FSH (Start Dial): Turns on the oven to bake the egg.
2. Oestrogen (Lining Builder): Builds the nest (uterus lining). When the nest is thick, it sends a signal (LH surge).
3. LH (Release Button): Pushes the egg out (ovulation) and turns the remaining structure into a maintenance team (corpus luteum).
4. Progesterone (Maintenance Crew/Volume Knob): Keeps the nest perfect. If no baby arrives, the crew leaves, the knob turns off, and the nest breaks down.

4. Extended Content: Hormones in Pregnancy

A. Maintaining the Uterus Lining

If fertilization occurs and the embryo successfully implants in the uterus wall, the cycle must stop so the pregnancy can continue.

When implantation occurs, the developing embryo starts releasing a hormone (Human Chorionic Gonadotropin, hCG – you don't need to recall the name, just the function!).

  1. This new hormone acts like an emergency call signal, telling the corpus luteum to NOT degenerate.
  2. Because the corpus luteum remains active, high levels of Progesterone and Oestrogen are maintained.
  3. High Progesterone levels ensure that the uterus lining remains thick and supplied with blood, allowing the embryo to grow.

B. Hormonal Shifts in Later Pregnancy

During the first trimester (three months), the corpus luteum produces progesterone. However:

  • Later in pregnancy, the placenta (the organ connecting the fetus to the mother) takes over the role of hormone production.
  • The placenta produces large quantities of Oestrogen and Progesterone.
  • These high levels keep the uterine lining intact and also inhibit the production of **FSH** and **LH** from the pituitary gland, ensuring that no new menstrual cycles or ovulations occur during pregnancy.
Key Takeaway (Extended): Progesterone is the 'Pro-Gestation' Hormone.

The primary hormonal requirement for maintaining a pregnancy is a sustained high level of progesterone, which keeps the uterus lining thick and stable.