You will find out:

> what the menstrual cycle does

> how hormones control the menstrual cycle

Even dinosaurs produced eggs

Female dinosaurs laid eggs. The largest known fossilised dinosaur eggs are from Hypselosaurus. They were laid about 100 million years ago. They are 30 cm long and weigh almost 7 kg. One would make a very big breakfast!

Figure 1: Dinosaur egg fossils. Suggest a reason why these dinosaur eggs were big.

Human reproduction

Female humans also produce eggs. The female reproductive system has:

> ovaries that make the eggs

> fallopian tubes to carry the eggs to the uterus, where the baby develops

> a vagina, through which the baby is born.

The male reproductive system has:

> testes to make sperm

> a scrotum to hold the testes outside the body – this temperature will be better for good sperm development

> sperm ducts to carry sperm to the penis.

The ovary and testes are called endocrine glands, as they produce hormones. Ovaries produce oestrogen and progesterone. Testes produce testosterone.

Figure 2: The female reproductive system.

Figure 3: The male reproductive system.

The menstrual cycle

Nisha is a teenager. She has reached puberty and her ovaries are producing eggs. She has a cycle of stages every 28 days. This is called the menstrual cycle. Some of her friends have a slightly longer or shorter cycle.

Between days 13 and 15 of Nisha's cycle, her ovary releases an egg. This is called ovulation. Her uterus lining becomes thicker, with more blood vessels. This will help a fertilised egg to embed in the lining.

The egg may not be fertilised or may not embed in the uterus lining. The uterus lining then breaks down, releasing the broken down cells. This is called menstruation.

Figure 4: The menstrual cycle. For about how long does one cycle last?

Questions

1. Where are:

  1. eggs made?
  2. sperm made?

2. During which days of the menstrual cycle does menstruation take place?

Hormonal control of the menstrual cycle

The menstrual cycle is controlled by hormones. The cycle is triggered by receptors in the hypothalamus in the brain that cause the pituitary gland to produce two hormones:

> follicle stimulating hormone (FSH), which stimulates an egg to mature inside a follicle in an ovary

> luteinising hormone (LH), which controls the release of an egg (ovulation).

As the follicle in the ovary develops, it releases varying amounts of two hormones: oestrogen and progesterone. These hormones control the growth of uterus cells and therefore the thickness of the uterus lining. Oestrogen causes the repair of the uterus wall, and progesterone maintains the uterus wall after ovulation. Body temperature also changes during the menstrual cycle.

If an egg is fertilised:

> the levels of progesterone remain high

> no FSH is produced

> no more eggs develop or are released

> the uterus lining does not break down.

It is possible to control fertility in humans using sex hormones, the contraceptive pill or fertility drugs.

Figure 5: How hormones control the menstrual cycle.

Question

3. Some contraceptive pills contain hormones that stop the release of LH. Explain how this stops an egg from being fertilised.

Hormonal feedback

The production of hormones by the pituitary (FSH and LH) is regulated by other hormones (oestrogen and progesterone) produced by the ovaries. For example,

> FSH is produced by the pituitary gland

> FSH stimulates egg maturation and the release of oestrogen from the ovary

> oestrogen acts as a negative feedback mechanism to control FSH production

> oestrogen also stimulates LH production from the pituitary gland

> LH controls egg release and production of progesterone

> progesterone also acts as a negative feedback mechanism on FSH production.

Figure 6: Negative feedback in hormone control.

menstrual cycle animation
ovulation
sex hormones