Lesson Video: The Gonads Biology

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


Video Transcript

In this video, we will learn how to describe the structure and function of the gonads, which are otherwise known as the sex organs, in the male and female human bodies. We’ll be learning about their endocrine function and releasing hormones and also how the gonads produce the haploid gametes in males and females, which allow humans to be capable of sexual reproduction.

Did you know that the female and male sex organs, otherwise known as the gonads, are responsible for producing one of the largest and smallest cells in the human body, respectively? These cells are called gametes, and which one you produce depends on whether you’re a biological male or a biological female. The female sex organs produce egg cells, otherwise known as ova, or a singular ovum, which are in theory large enough to be just visible to the naked eye, measuring about 0.1 millimeters in diameter. The male sex organs produce billions of microscopic sperm cells throughout their lifetime, which are the smallest cells in the male body.

These gametes are referred to as haploid as they contain half the genetic information of each of the parents and are responsible for passing this genetic information on to their offspring upon the fertilization of an egg cell by a sperm cell, which produces a diploid zygote, which has a full set of genetic information. Production of gametes is not the only function of the gonads, however. The gonads also function as endocrine glands. You may recall that an endocrine gland is a group of specialized cells that secrete hormones directly into the bloodstream. Hormones are chemical messengers that are released from these cells in an endocrine gland into the bloodstream. The blood carries these hormones throughout the body to the target cells in which they are required.

As the gonads are responsible for producing and secreting a wide range of different sex hormones into the bloodstream to have different effects on various target organs in the body, they can be referred to as endocrine glands. Let’s investigate the structure of the gonads first before we look in more detail of the sex hormones that they produce and secrete, starting with the female reproductive system. The female gonads are called the ovaries or, as singular, ovary. You can see from this diagram that females typically have two ovaries, which are each about the size of a large grape. The ovaries are primarily responsible for producing egg cells and female sex hormones. Each ovary is attached to one of two Fallopian tubes, which are sometimes called oviducts.

The Fallopian tubes lead into the uterus, which is often used interchangeably with the word womb. However, a womb more accurately describes the uterus of a pregnant female. The uterus is located in the pelvic region of the body as you can see in this diagram on the left, so named because of the pelvis, which is a large bone that sits in the same area and protects the organs of the reproductive system. The uterus sits above a muscular tract called the vagina, which is joined to the uterus by a tissue called the cervix.

High quantities of sex hormones will begin to be released when a person reaches 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’s important to note that though humans may be capable of reproduction at this stage, they are unlikely to be emotionally or mentally prepared for it.

Let’s take a look at the sex hormones that begin to be released in large quantities by the female gonads when she reaches puberty. We can see a magnified view of one of these gonads, the ovaries, in this diagram on the right. Estrogen is a hormone that is released from a structure called the Graafian follicle, which is found in the ovaries and from which a mature egg cell will also be released. Estrogen has many functions, though one of its main roles is to control the development of female secondary sexual characteristics. Secondary sexual characteristics, sometimes simply known as secondary sex characteristics, are physically observable traits that develop during puberty but are not directly involved in reproduction.

Secondary sex characteristics can be similar in males and females, for example, growth of pubic hair or armpit hair. But they often physically distinguish the two biological sexes; for example, females tend to develop breasts and often their hips widen slightly. The beginning of the menstrual cycle, which we’ll explore in more detail later on in the video, is also often referred to as a secondary sexual characteristic. The development of these characteristics are mainly regulated by the secretion of sex hormones. But it’s important to note that typically male and typically female sex hormones are actually released in both biological sexes but usually in different quantities.

Another function of estrogen is to control ovulation during the menstrual cycle. This is the process we just observed by which an egg cell is released from the Graafian follicle in the ovary. From the ovary, the egg cell will travel into the Fallopian tube, where it may or may not be fertilized by a sperm cell. Another typically female sex hormone is progesterone. Once the egg cell has been released from the Graafian follicle during ovulation, the Graafian follicle degrades into a structure called the corpus luteum which is responsible for producing and secreting progesterone. Progesterone is also released from a structure called the placenta, which develops during pregnancy. The placenta is a temporary endocrine organ that surrounds a growing fetus in the uterus of a pregnant female.

Let’s erase some of this information so we can look at the role of progesterone a bit more closely. The main role of progesterone is to maintain a thick uterine lining. So let’s see how this works. If an egg cell is successfully fertilized by a sperm cell in the Fallopian tube, the zygote that’s formed tends to divide many times on its journey into the uterus and becomes known as an embryo. Progesterone, which has been released by the corpus luteum in the ovary, maintains a thick uterine lining as you can see in the diagram. This provides the perfect environment for the embryo to implant into the uterine wall, which is sometimes called the endometrium, where the embryo can safely develop into a fetus.

If fertilization does occur, progesterone concentrations remain high throughout pregnancy to maintain a thick lining of the uterus and provide suitable conditions for the fetus to grow into a baby. 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 it differs greatly between different females, but a whole cycle tends to last about 28 days from the start of one period to the next.

Let’s see how the uterine lining changes in thickness over a typical menstrual cycle and how the levels of estrogen and progesterone change by looking at them all on a graph. The typical day on which certain events occur during the menstrual cycle is shown on the 𝑥-axis. And the section in red shows how the thickness of the uterine lining changes over this approximately 28-day cycle.

In this diagram, we can see what it might look like at day zero. Let’s add in a section on this graph above the changes in the uterine lining to show how the levels of the ovarian hormones, progesterone and estrogen, change during the menstrual cycle. We can show the levels of estrogen with an orange line and the level of progesterone with a pink line. From approximately day zero to seven, the uterine lining sheds, and so its thickness decreases via a process called menstruation or sometimes a period, which marks the start of the menstrual cycle. As the lining of the uterus has a rich blood supply, when a female menstruates, the uterine lining exits the vagina with blood. Though in the graph it is shown to be about seven days in duration, the length of the period, or menstruation, will vary from female to female.

It’s accompanied by low levels of progesterone, which remain low until approximately day 14. At this point, ovulation occurs, which you might remember means the Graafian follicle has just released a mature egg cell. The levels of estrogen are also lower at the start of the menstrual cycle, but they increase steadily up until the ovulation date. This is because, as you might recall, the Graafian follicle is responsible for releasing estrogen. As the Graafian follicle degrades shortly after ovulation, it can no longer release estrogen, so estrogen levels drop suddenly.

Following ovulation, the uterine lining begins to build up again and the Graafian follicle degrades into the corpus luteum, which, as you might recall, secretes progesterone. So progesterone levels will increase after ovulation, and these high levels of progesterone will maintain a nice and thick uterine lining prepared for if an embryo implants. The levels of estrogen will also increase somewhat during this time period. And the levels of estrogen and progesterone remain high until approximately day 28 when the levels of both hormones drop and the menstrual cycle begins again. Relaxin is a hormone that, like progesterone, is released primarily from the corpus luteum in the ovaries and also from the placenta and the uterine lining during pregnancy.

Though it is a female sex hormone, relaxin is also released in small quantities from the male prostate gland, so 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 relax at certain points during the menstrual cycle when an egg is likely to implant, preparing the body for pregnancy by aiding the implantation process. If implantation does occur, 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 to prepare a female’s body for childbirth by helping to open the cervix, soften the surrounding tissue, and relaxing the pelvic ligaments to aid the delivery of the child.

Let’s take a look at the main structures in the male reproductive system next. The male gonads are called the testes, or a singular testis, one of which you can see in this diagram showing a side view of the male reproductive system. Males typically have two testes which are primarily responsible for producing sperm cells and male sex hormones. Let’s take a quick look at the route that these sperm cells take through the male reproductive system after they’ve been produced by the testes. One sperm has been produced and it is stored in a region called the epididymis, which is just outside the testes. This green arrow shows the direction of sperms travel. Sperm is stored in the epididymis until ejaculation is anticipated, for example, during sexual arousal.

Each testis is attached to a tube called the vas deferens. The vas deferens carry sperm cells from the epididymis to another tube called the urethra. On this journey, the sperm cells travel through a gland called the prostate gland, which has been labeled here in blue and sits just behind the bladder. The other blue structures are also glands such as the seminal vesicles and the Cowper’s gland. The pink arrows show how fluids which are produced in each of these glands are added to the sperm cells as they travel past them through the urethra. The fluids added to sperm by the prostate gland, seminal vesicles, and Cowper’s gland all help to make up semen. The semen which contains these sperm cells then continues to travel along the urethra and into the penis from which it can be ejaculated from the male’s body.

Let’s take a look at the endocrine functions of the testes next and their role in secreting male sex hormones. Male sex hormones are sometimes called androgens. The word androgen contains the suffix andro-, which means man. In spite of this, androgens are released in both males and females. The main androgen which is released from the testes is called testosterone. The role of testosterone is to stimulate the growth and development of the male sex organs, which is sometimes called their primary sexual characteristics. And testosterone also develops the secondary sexual characteristics.

Androsterone is another androgen secreted by the testes and is formed by the breakdown of testosterone. Androsterone has similar effects to testosterone, but they’re slightly weaker effects. For this reason, testosterone is sometimes referred to as more potent than androsterone. As though they both have similar effects, testosterone’s effects will be stronger. Let’s take a look at the secondary sexual characteristics that can develop as a result of testosterone and androsterone being secreted in high quantities when a male reaches puberty. Males tend to develop more and thicker facial and body hair during puberty than females do as a result of these high androgen concentrations. Males also tend to develop more skeletal muscle mass than females do and their voices break and become considerably deeper.

It’s interesting to come back to the fact that though they are called male and female sex hormones, both of the biological sexes will be releasing all of these hormones just in different quantities. For example, males still produce small volumes of estrogen in their testes and from their adrenal glands which sit on top of their kidneys. And as we’ve mentioned, females will also produce some androgens in their ovaries and adrenal glands. A fine balance of male and female sex hormones are important for both sexes to go through sexual growth and development. Let’s see how much we’ve learned about the gonads by having a go at a practice question.

At what stage in life do the majority of secondary sexual characteristics most commonly appear? During infancy, during adulthood, during puberty, or during old age.

Let’s approach this question by first defining the key term secondary sexual characteristic so we can figure out when they will appear in humans. Secondary sexual characteristics are physically observable traits that develop during a period in adolescence called puberty and are not directly involved in reproduction. Secondary sexual characteristics can be similar in both biological males and females, for example, growth of pubic hair, but they often physically distinguish males from females. For example, usually only females develop breasts that would be capable of producing milk after childbirth to feed a newborn baby.

The release of high quantities of sex hormones triggers the development of these secondary sexual characteristics. And different concentrations of typically male and typically female sex hormones will determine the different characteristics that appear in the two biological sexes. The release of high quantities of these sex hormones begins to occur during a stage called puberty. Puberty is a stage in adolescence when humans reach sexual maturity and become capable of sexual reproduction even if they might not be emotionally or mentally ready for it yet. Therefore, the life stage at which the majority of secondary sexual characteristics most commonly appear is during puberty.

Now it’s time for us to review some of the key points that we’ve covered in this video. The gonads are responsible for producing sex hormones and gametes, which are otherwise known as sex cells. The female gonads are called the ovaries, and the female gametes they release are called egg cells. The ovaries also release sex hormones such as estrogen, progesterone, and relaxin. While estrogen is mainly responsible for controlling the development of female primary and secondary sexual characteristics, progesterone helps to prepare the female body for pregnancy by maintaining a nice and thick uterine lining. And finally, relaxin helps to prepare the female body for implantation of a fertilized egg, pregnancy, and childbirth.

The gonads in males are called the testes, and the gametes they release are called sperm cells. The testes also release male sex hormones which are called androgens and include testosterone and the weaker version androsterone. Androgens are responsible for controlling the growth of the male reproductive glands or their primary sexual characteristics and also the development of their secondary sexual characteristics.

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