Lesson Explainer: The Gonads Biology

In this explainer, we will learn how to describe the structure and function of the gonads (sex organs) in the human body.

Did you know that the female and male sex organs are responsible for producing one of the largest and smallest cells in the human body respectively? The male sex organs produce billions of tiny microscopic sperm throughout their lifetime, which are the smallest cells in the male body! The female sex organs produce eggs that are large enough to see with the naked eye!

The egg and sperm are called gametes and are responsible for passing the genetic information of a parent on to their offspring upon fertilization. Producing gametes is not the only role of the sex organs, however.

You may recall that hormones are chemical messengers that are transported throughout the bloodstream. The sex organs, or gonads, are glands that are responsible for producing a wide range of sex hormones. For this reason, the gonads are endocrine glands, as they release these sex hormones directly into the bloodstream. The blood carries these hormones throughout the body to the target cells in which they are needed.

Key Term: Endocrine Gland

An endocrine gland is a group of specialized cells that secrete hormones into the blood.

Key Term: Gonads

Gonads are reproductive glands responsible for producing gametes and sex hormones.

Let’s look at the structure of the gonads in males and females, before looking in more detail at the sex hormones they produce and secrete.

You can see the main structures in the female reproductive system in Figure 1 below.

The female gonads are called the ovaries (singular: ovary). You can see from Figure 1 that females typically have two ovaries, which are each about the size of a large grape. The ovaries are primarily responsible for producing eggs and female sex hormones.

Key Term: Ovaries

The ovaries (singular: ovary) are the female reproductive organs from which egg cells and hormones are released.

Each ovary is attached to one of the two fallopian tubes, sometimes called oviducts. The fallopian tubes lead into the uterus, often called the womb. The uterus, which is located in the pelvic region of the body, sits above a muscular tract called the vagina, which is joined to the uterus by a tissue called the cervix, as you can see in Figure 1.

You can see the main structures in the male reproductive system in Figure 2 below.

The male gonads are called the testes (singular: testis), one of which you can see in Figure 2. Males typically have two testes, which are primarily responsible for producing sperm and male sex hormones. Once a sperm is produced, it is stored in a region called the epididymis until it is released from the penis in a process called ejaculation.

Key Term: Testes

The testes are male gonads that produce sperm and male sex hormones.

Each testis is attached to a tube called the vas deferens. The vas deferens carries sperm from the testes to the urethra which travels through a gland called the prostate. Fluids produced in regions called the prostate, seminal vesicles, and Cowper’s gland are also added to sperm to make semen, which can then enter the penis.

Let’s have a look at the functions of these sex hormones that are released by the gonads.

High quantities of sex hormones begin to be released during puberty. Puberty is a stage in adolescence when humans reach sexual maturity and are capable of reproduction. Puberty is accompanied by a range of physical and hormonal changes in the human body, and it is important to note that though humans may be capable of reproduction, they are unlikely to be emotionally or mentally prepared for it at this stage.

Key Term: Puberty

Puberty is a stage in adolescence when humans reach sexual maturity and are capable of reproduction.

While the main reproductive organs we observed in Figures 1 and 2 are typically present in humans from birth, puberty is when secondary sexual characteristics begin to develop. Secondary sexual characteristics are physically observable traits that develop during puberty that are not directly involved in reproduction. Secondary sexual characteristics can be similar in males and females, for example, growth of pubic hair, but they often physically distinguish males from females. For example, males tend to develop more muscle mass, while females tend to develop breasts.

Key Term: Secondary Sexual Characteristics

Secondary sexual characteristics are physical traits that develop at puberty and that are not directly involved in reproduction.

Example 1: Identifying the Life Stage Initiating the Development of Secondary Sexual Characteristics

At what stage in life do the majority of secondary sexual characteristics most commonly appear?

  1. During infancy
  2. During adulthood
  3. During puberty
  4. During old age

Answer

High quantities of sex hormones begin to be released during puberty. Puberty is a stage in adolescence when humans reach sexual maturity and are capable of reproduction.

While the main reproductive organs are typically present in humans from birth, puberty is when secondary sexual characteristics begin to develop.

Secondary sexual characteristics are physically observable traits that develop during puberty and that are not directly involved in reproduction. Secondary sexual characteristics can be similar in males and females, for example, growth of pubic hair, but they often physically distinguish males from females. For example, males tend to develop more muscle mass, while females tend to develop breasts.

Therefore, the life stage at which the majority of secondary sexual characteristics most commonly appear is during puberty.

Let’s look at the sex hormones that are released in large quantities by the female gonads: the ovaries.

Estrogen is a hormone that is released from a structure called the Graafian follicle found in the ovaries. It has many functions, though its main role is to control the development of female secondary sexual characteristics. In females, these secondary sexual characteristics may involve the development of the breasts, the widening of the hips, and the start of the menstrual cycle.

Another function of estrogen is to control ovulation during the menstrual cycle. This is the process by which an egg is released from the ovary to travel along the fallopian tube where it may, or may not, be fertilized by a sperm cell.

Key Term: Estrogen

Estrogen is a sex hormone, primarily produced by the ovaries, that controls ovulation and the development of secondary sexual characteristics in females.

Progesterone is another hormone that is released from a structure called the corpus luteum found in the ovaries, and also from the placenta during pregnancy. The main role of progesterone is to maintain a thick lining of the uterus. When the lining of the uterus is thick, it is prepared for an embryo to implant in its wall, where it can develop into a fetus. This will only occur if a sperm fertilizes an egg in the fallopian tube. If fertilization does occur, progesterone concentrations will remain high throughout pregnancy to maintain a thick lining of the uterus, providing suitable conditions for the fetus to grow.

Key Term: Progesterone

Progesterone is a sex hormone, primarily produced by the ovaries, that prepares the uterine lining for receiving an embryo by thickening it.

The levels of progesterone and estrogen tend to fluctuate during a female’s menstrual cycle when she begins puberty. The duration of the menstrual cycle can change and differs greatly between different women, but a whole cycle tends to last about 28 days from the start of one period to the start of the next. You can see the changes in the uterine lining over a typical 28-day menstrual cycle in Figure 3 below.

When the levels of progesterone drop, the lining of the uterus sheds in a process called menstruation, marking the start of the menstrual cycle. You can see the uterine lining decreasing in thickness from day 0 to day 6 in Figure 3. As the lining of the uterus has a rich blood supply, when a female menstruates, the uterine lining exits the vagina with blood, sometimes known as a period. Though on the graph this is shown as about 6 days, the length of a period will vary from woman to woman.

The uterine lining will then build up again to prepare for possible implantation, as you can see from around day 6 to day 8 until day 18 in Figure 3. Progesterone maintains this uterine lining to prepare for a potential fertilized egg to implant. This continues until approximately day 28 before the menstrual cycle repeats again.

Example 2: Describing the Function of Progesterone

What is the primary function of progesterone?

  1. To regulate salt and water balance in body fluids
  2. To regulate digestion and metabolism of fats, proteins, and carbohydrates
  3. To promote the development of male secondary sexual characteristics, for example, facial hair growth
  4. To maintain the uterus lining in the second half of the menstrual cycle and during a pregnancy

Answer

Progesterone is a hormone that is released from the ovaries. The ovaries are the gonads responsible for releasing the majority of sex hormones in females.

The main role of progesterone is to maintain a thick lining of the uterus. When the lining of the uterus is thick, it is prepared for an embryo to implant in its wall, where it can develop into a fetus. This will only occur if a sperm fertilizes an egg in the fallopian tube. If fertilization does occur, progesterone concentrations will remain high throughout pregnancy to maintain a thick lining of the uterus.

Regulation of salt and water balance in bodily fluids and regulation of digestion and metabolism are not functions of sex hormones, so these two options do not describe the role of progesterone.

Though progesterone will be released in small volumes in males as well as females, it is not primarily responsible for the development of male secondary sexual characteristics.

Therefore, the primary function of progesterone is to maintain the uterine lining in the second half of the menstrual cycle and during a pregnancy.

Relaxin is a hormone that is released primarily from the corpus luteum in the ovaries and also from the placenta and the uterine lining during pregnancy. The placenta is a temporary endocrine organ that surrounds a growing fetus in the uterus of a pregnant female. Though it is a “female” sex hormone, relaxin is also released in small quantities from the male prostate gland, so it can sometimes be detected in male semen!

Relaxin, as its name suggests, plays an important role in relaxing parts of the female reproductive system at different times. For example, relaxin has been shown to help the walls of the uterus to relax at certain points during the menstrual cycle when an egg is likely to implant, preparing the body for pregnancy. This aids the implantation process.

If implantation occurs, relaxin is usually released in higher concentrations during pregnancy. It has been shown to play a role in the growth of the placenta and is especially important in preparing a woman’s body for labor, which is when she gives birth to a child. Relaxin does this by helping to open the cervix and soften the surrounding tissue. There is evidence that relaxin also relaxes the ligaments in the pelvis to aid the woman in the delivery of the child.

Example 3: Describing the Function of Relaxin

Relaxin is a hormone produced by the ovary. What is the primary role of relaxin in the human body?

  1. To prepare the female body for pregnancy and labor
  2. To initiate the contraction and relaxation of skeletal muscles
  3. To counteract the effects of adrenaline after a fight-or-flight response
  4. To regulate the menstrual cycle

Answer

Relaxin is a hormone that is released from the corpus luteum of the ovary and from the placenta and uterine lining in pregnant women. The ovaries are the gonads responsible for releasing the majority of sex hormones in females.

Relaxin plays an important role in relaxing parts of the female reproductive system at different times. For example, it has been shown to help the walls of the uterus to relax at certain points during the menstrual cycle when an egg is likely to implant, aiding the implantation process and preparing the female body for pregnancy.

If implantation occurs, relaxin is usually released in higher concentrations during pregnancy. Relaxin has been shown to play a role in the growth of the placenta and is especially important in preparing a woman’s body for labor, which is when she gives birth to a child. Relaxin does this by helping to open the cervix and soften the surrounding tissue. There is evidence that relaxin also relaxes the ligaments in the pelvis to aid the woman in the delivery of the child.

The initiation of contraction and relaxation of skeletal muscles and counteracting the effects of adrenaline after a fight-or-flight response are not primary functions of sex hormones, so these two options do not describe the role of relaxin.

Though relaxin is still released when a woman is not pregnant, it is secreted from the ovaries in far smaller quantities. The main sex hormones involved in regulating the female menstrual cycle are estrogen and progesterone.

Therefore, the primary function of relaxin is to prepare the female body for pregnancy and labor.

Let’s look at the sex hormones that are released in large quantities by the male gonads: the testes.

Male sex hormones are sometimes called androgens. The word androgen includes the stem “andro” which means “man.” In spite of this, they are released in both males and females. This is because they are useful for both sexes and can be converted into other sex hormones, such as the hormones estrogen in the female ovaries and testosterone in the male testes. For example, in pregnant females, androgens play a role in preventing premature contractions of the uterus. In males, androgens are involved in increasing the size of skeletal muscle cells to increase their muscle mass.

Key Term: Androgens

Androgens are sex hormones released from the testes and ovaries, and in small volumes from the adrenal glands, to control the development of male sex organs, secondary sexual characteristics, and arousal in both males and females.

The main androgen that is released from the testes is called testosterone. Androsterone is made from the breakdown of testosterone, and out of the two, testosterone is more potent. This means that while both have similar effects, testosterone’s effect will be stronger than that of androsterone.

Key Term: Testosterone

Testosterone is a sex hormone, primarily produced by the testes, that stimulates the growth of sex organs and the development of secondary sexual characteristics in males.

Key Term: Androsterone

Androsterone is a sex hormone, primarily produced by the testes, that is formed by the breakdown of testosterone and has similar but weaker effects in stimulating the growth of sex organs and the development of secondary sexual characteristics in males.

The main functions of androgens are to cause the growth of the male reproductive system and the development of male secondary sexual characteristics during puberty. In males, these secondary sexual characteristics may involve the development of facial and body hair, the voice breaking and becoming deeper, and increased muscle mass.

Example 4: Describing the Function of Androgens

What is the primary function of the androgens in the human body?

  1. To promote the development of male secondary sexual characteristics, for example, facial hair growth
  2. To regulate digestion and metabolism of fats, proteins, and carbohydrates
  3. To regulate the activities of all other endocrine glands in the body
  4. To regulate calcium levels in the blood

Answer

Male sex hormones are sometimes called androgens. The word androgen includes the stem “andro” which means “man.” In spite of this, they are released in both males and females.

The main androgen that is released from the testes is called testosterone, which is broken down into another, weaker sex hormone called androsterone.

The main functions of androgens are to cause the growth of the male reproductive system and the development of male secondary sexual characteristics during puberty. In males, these secondary sexual characteristics may involve the development of facial and body hair, the voice breaking and becoming deeper, and increased muscle mass.

We can tell from this description that our answer is likely to be promoting the development of secondary sexual characteristics. We know that it cannot be one of the other three, as they all describe functions of hormones that are not sex hormones.

Therefore, the primary function of androgens is to promote the development of male secondary sexual characteristics, for example, facial hair growth.

It is interesting to note that although they are called “male” and “female” sex hormones, both sexes produce all the sex hormones, just in different volumes. Males, for example, produce small volumes of estrogen in their testes and adrenal glands that sit on top of the kidneys. Similarly, females produce small quantities of the male sex hormone testosterone in their ovaries and adrenal glands. A fine balance of male and female sex hormones is important for both sexes to go through sexual growth and development.

Let’s recap some of the key points we have covered in this explainer.

Key Points

  • Gonads produce and secrete sex hormones.
  • The gonads in females are the ovaries, which release estrogen, progesterone, and relaxin.
  • Estrogen is involved in the development of female secondary sexual characteristics.
  • Progesterone is involved in preparing the body for pregnancy.
  • Relaxin is involved in labor and childbirth.
  • The gonads in males are the testes, which release the androgens: testosterone and androsterone.
  • Androgens are involved in the growth of the male reproductive glands and the development of male secondary sexual characteristics.

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