Sex hormones in humans

1. Overview

Sex hormones are chemical messengers produced by the endocrine glands that coordinate the development of the reproductive system and regulate human fertility. These hormones trigger the physical changes seen during puberty and control the monthly menstrual cycle in females, ensuring the body is prepared for potential pregnancy.

Key Definitions

• Hormone: A chemical substance, produced by a gland and carried by the blood, which alters the activity of one or more specific target organs.
• Puberty: The stage of life during which adolescents reach sexual maturity and become capable of reproduction.
• Secondary Sexual Characteristics: Physical features that appear during puberty which distinguish the two sexes but are not directly part of the reproductive system.
• Ovulation: The release of a mature egg (ovum) from an ovary into the oviduct.
• Menstruation: The monthly shedding of the lining of the uterus (endometrium) through the vagina if fertilization has not occurred.

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Core Content

Roles of Testosterone and Oestrogen in Puberty

During puberty, the pituitary gland stimulates the gonads (testes and ovaries) to release sex hormones.

Testosterone (produced in the testes):

• Primary role: Stimulates the production of sperm.
• Secondary characteristics:
  • Growth of facial and body hair.
  • Deepening of the voice (growth of the larynx).
  • Increased muscle mass and bone density.
  • Growth of the penis and testes.

Oestrogen (produced in the ovaries):

• Primary role: Regulates the menstrual cycle and triggers egg maturation.
• Secondary characteristics:
  • Development of breasts.
  • Widening of the hips (to prepare for childbirth).
  • Growth of pubic and underarm hair.
  • Beginning of the menstrual cycle (periods).

The Menstrual Cycle (Changes in Ovaries and Uterus)

The cycle lasts approximately 28 days and involves two main locations of change:

1. In the Uterus (The Lining):

   • Days 1–5: Menstruation occurs; the thickened uterine lining breaks down and is lost from the body.
   • Days 6–13: The lining begins to thicken again (repair phase) to prepare for a possible embryo.
   • Days 14–28: The lining is maintained. If no egg is fertilized, the lining breaks down at Day 28, and the cycle repeats.

2. In the Ovaries (The Egg):

   • Days 1–13: An egg follicle develops and matures inside the ovary.
   • Day 14 (Ovulation): The mature egg is released from the ovary into the oviduct.
   • Days 15–28: The empty follicle turns into a structure called the corpus luteum.

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Extended Content (Extended Curriculum Only)

Sites of Production

• Menstrual Cycle: Oestrogen and Progesterone are primarily produced by the ovaries (specifically the follicle and the corpus luteum).
• Pregnancy: Initially, the corpus luteum produces these hormones. Later, the placenta takes over the production of progesterone and oestrogen to maintain the pregnancy.

Hormonal Control of the Menstrual Cycle

The cycle is controlled by a complex feedback loop involving four hormones:

1. FSH (Follicle Stimulating Hormone):
   • Produced by: Pituitary Gland.
   • Function: Stimulates the growth of a follicle in the ovary and stimulates the ovary to produce oestrogen.
2. Oestrogen:
   • Produced by: Ovaries.
   • Function: Causes the uterine lining to thicken/repair. It inhibits (stops) FSH and stimulates the pituitary to release LH.
3. LH (Luteinising Hormone):
   • Produced by: Pituitary Gland.
   • Function: A "surge" in LH triggers ovulation at Day 14 and stimulates the empty follicle to become the corpus luteum.
4. Progesterone:
   • Produced by: Corpus Luteum (and later the placenta).
   • Function: Maintains the thick lining of the uterus. It inhibits the release of LH and FSH (to prevent further eggs from maturing).

Hormones in Pregnancy

If fertilization occurs, the corpus luteum does not break down.

• Progesterone levels remain high: This keeps the uterine lining intact so the embryo can remain implanted.
• Oestrogen levels remain high: This prevents the pituitary gland from releasing FSH, ensuring no new eggs are matured during pregnancy.

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Key Equations

• There are no mathematical equations for this topic. However, students should be able to calculate the date of ovulation if given the start date of a cycle (usually Day 14).

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Common Mistakes to Avoid

• ❌ Wrong: Oestrogen causes the lining to be maintained.
• ✓ Right: Oestrogen repairs/thickens the lining; Progesterone maintains it.
• ❌ Wrong: FSH and LH are produced in the ovaries.
• ✓ Right: FSH and LH are produced in the Pituitary Gland (in the brain).
• ❌ Wrong: Testosterone is only found in males and oestrogen only in females.
• ✓ Right: Both sexes produce both hormones, but they are present in much higher concentrations in the respective biological sex.

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Exam Tips

• Interpreting Graphs: You will often be asked to identify hormones on a graph showing their levels over 28 days. Remember: FSH peaks early, Oestrogen peaks just before Day 14, LH peaks sharply at Day 14, and Progesterone peaks during the second half of the cycle (the luteal phase).
• Command Words: If asked to "Describe," state what happens (e.g., "the lining thickens"). If asked to "Explain," state how/why it happens (e.g., "Oestrogen is released which causes the lining to thicken").
• Link Structure to Function: When discussing the placenta in the Supplement section, mention its role in hormone production is vital because it ensures the lining is not shed, which would result in a miscarriage.
• Real-world Context: These hormones are often the focus of questions regarding the "contraceptive pill" (which uses progesterone/oestrogen to inhibit FSH and prevent ovulation).