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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.