Human reproduction: reproductive systems and the menstrual cycle

Learning objectives

By the end of this lesson, you should be able to:

• identify the main structures of the male and female reproductive systems
• state the functions of the reproductive organs
• explain sperm production and the route taken by sperm
• explain the formation and release of ova
• describe the menstrual cycle and the roles of FSH, LH, oestrogen and progesterone
• distinguish ovulation, fertilisation, menstruation and implantation

Human reproduction

Humans reproduce sexually. A male gamete (sperm) fuses with a female gamete (ovum) during fertilisation to form a zygote. Fertilisation normally occurs in the oviduct. The zygote divides repeatedly and, after reaching the uterus, the developing embryo implants in the uterine lining.

The male reproductive system produces sperm and transfers them into the female reproductive tract. The female reproductive system produces ova and provides the conditions for fertilisation, pregnancy and birth.

The male reproductive system

The main structures and their functions are:

The testes are located in the scrotum. Sperm are produced in coiled seminiferous tubules. Testosterone, produced by cells in the testes, supports sperm production and the development and maintenance of male secondary sexual characteristics.

The scrotum helps keep the testes slightly cooler than normal internal body temperature. Excessively high temperature can interfere with sperm production.

The route taken by sperm

Sperm are produced in the seminiferous tubules, mature and are stored in the epididymis, then pass through the sperm duct. They mix with secretions from accessory glands, enter the urethra and leave the body through the penis.

Semen is the mixture of sperm and fluids from accessory glands. It provides a medium that helps transport and support sperm. Semen and sperm are not the same thing: sperm are the male gametes, while semen is the fluid containing them.

The female reproductive system

The ovaries contain follicles in which ova develop. Usually one mature ovum is released during each menstrual cycle. This release is called ovulation.

The oviduct has finger-like projections near its opening that help collect the ovum after ovulation. Cilia and muscular contractions help move the ovum or early embryo along the oviduct.

The uterus has a thick muscular wall. Its inner lining, the endometrium, becomes thicker and more richly supplied with blood during the menstrual cycle. This prepares it for possible implantation.

The menstrual cycle

The menstrual cycle is a series of changes in the ovaries and uterus that prepares the female reproductive system for pregnancy. A commonly used teaching model is a 28-day cycle, but actual cycle length varies among individuals and from one cycle to another.

The cycle is controlled by hormones from the pituitary gland and ovaries.

Main hormones

• Follicle-stimulating hormone (FSH): released by the anterior pituitary; stimulates the development of ovarian follicles.
• Luteinising hormone (LH): released by the anterior pituitary; a surge in LH triggers ovulation and helps form the corpus luteum.
• Oestrogen: produced mainly by developing ovarian follicles; rebuilds and thickens the endometrium and helps regulate FSH and LH release.
• Progesterone: produced mainly by the corpus luteum after ovulation; maintains the uterine lining and helps prevent another ovulation during that cycle.

Stages of a typical cycle

1. Menstruation (approximately days 1–5)

If pregnancy has not occurred, the corpus luteum breaks down and progesterone and oestrogen levels fall. The endometrium is no longer maintained and its superficial layer is shed through the vagina as menstrual flow. Menstruation marks the beginning of the cycle.

2. Follicular phase and rebuilding of the uterine lining

FSH stimulates follicles in the ovary to develop. A developing follicle produces oestrogen. Oestrogen stimulates repair and thickening of the endometrium. As oestrogen rises, it helps regulate pituitary hormone release.

3. Ovulation (often near day 14 in a 28-day model)

A high level of oestrogen from the mature follicle contributes to a surge in LH. The LH surge causes the mature follicle to release an ovum. The ovum enters the oviduct, where fertilisation may occur if sperm are present.

4. Luteal phase (approximately days 15–28)

The remains of the ruptured follicle develop into the corpus luteum. It secretes progesterone and some oestrogen. Progesterone maintains the thick endometrium, making it suitable for implantation.

If fertilisation and implantation do not occur, the corpus luteum degenerates. Progesterone and oestrogen levels fall, the endometrium breaks down and menstruation begins again.

If pregnancy is established, the early embryo produces human chorionic gonadotrophin (hCG), which helps maintain the corpus luteum in early pregnancy. Progesterone therefore remains sufficiently high to support the uterine lining until the placenta takes over much of the hormone production.

Summary of the menstrual hormones

Key reproductive terms

• Ovulation: release of a mature ovum from an ovary.
• Fertilisation: fusion of the nuclei of sperm and ovum, usually in the oviduct.
• Zygote: the diploid cell formed by fertilisation.
• Implantation: attachment and embedding of the early embryo in the endometrium.
• Menstruation: shedding of the uterine lining when pregnancy has not been established.
• Menstrual cycle: recurring ovarian and uterine changes controlled by hormones.

Do not use these terms interchangeably. Ovulation is not fertilisation; fertilisation is not implantation; menstruation is not the release of an ovum.

WASSCE examination focus

• Testes produce sperm and testosterone; ovaries produce ova, oestrogen and progesterone.
• The usual site of fertilisation in humans is the oviduct, not the uterus.
• The endometrium is the inner lining of the uterus; it is not the muscular uterine wall.
• The LH surge triggers ovulation.
• Progesterone maintains the endometrium after ovulation.
• Menstruation follows a fall in ovarian progesterone and oestrogen when pregnancy has not been established.
• A 28-day cycle is a model, not a fixed rule for every woman.

Quick check

1. State two functions of the testes.
2. What is the function of the epididymis?
3. Trace the route taken by sperm from the site of production to the outside of the body.
4. Where does fertilisation usually occur in humans?
5. Define ovulation.
6. Which hormone surge triggers ovulation?
7. State the role of progesterone after ovulation.
8. Why does menstruation occur when pregnancy has not been established?

Answers

1. They produce sperm and secrete testosterone.
2. It allows sperm to mature and stores them.
3. Seminiferous tubules, epididymis, sperm duct, urethra and out through the penis.
4. In the oviduct (fallopian tube).
5. The release of a mature ovum from an ovary.
6. Luteinising hormone (LH).
7. It maintains the endometrium in preparation for implantation.
8. The corpus luteum degenerates, ovarian progesterone and oestrogen levels fall, and the uterine lining is shed.