Lesson Explainer: Male Genital System Biology • Third Year of Secondary School

In this explainer, we will learn how to identify parts of the male genital system and describe their functions.

Did you know that a healthy male’s testes can produce 1‎ ‎500 sperm cells every second? This means hundreds of millions of sperm cells are made every single day! Just 1 mL of healthy semen, the fluid that contains sperm, can have as many as 300 million sperm cells! Sperm cells are the male sex cells, also called reproductive cells or gametes, which function to fertilize a female’s egg cell in sexual reproduction.

There are many possible reasons why males need to make so many sperm cells. One of the main challenges that sperm face is surviving in the female reproductive system long enough to eventually fertilize an egg cell.

Let’s start by describing the structure of the male genital system before we investigate how it functions to produce sperm and how these plentiful, specialized cells are well adapted to overcome the many obstacles they encounter on their journey to successfully fertilize an egg cell.

The image in Figure 1 below shows a side-view cross section of some of the main structures in the male genital system, which is sometimes known as the male reproductive system.

As you can see in Figure 1, the male gonads, the testes, are linked by the epididymis to a tube called the vas deferens through which sperm travel to the urethra. Fluids are added to the sperm from the prostate gland and the seminal vesicles and, once in the urethra, from the Cowper’s glands. Semen exits the male reproductive system through the penis through the urethra during ejaculation.

Let’s look at all of these different organs and their functions in some more detail!

Males typically have two testes (singular: testis) that are their primary sex organs. The testes are also called the male gonads. Each testis is the shape of an ovoid or a small chicken egg and is around 3–5 cm long and 3–4 cm wide at their thickest point, though this will vary between individuals. It is even normal for the testis size to vary between the two testes of an individual themselves. The testes tend to grow from birth until puberty. As testosterone levels begin to drop when a male passes sexual maturity, the testes can gradually become smaller.

The testes are responsible for producing sperm. Mature sperm cells, also called spermatozoa, are haploid cells, which means they have half the number of chromosomes that a normal body cell has. Sperm cells are produced through a process called spermatogenesis, which occurs in structures within the testes called the seminiferous tubules. You can see the location of the seminiferous tubules inside one testis in Figure 2 below.

Spermatogenesis involves a type of cell division called meiosis. It converts a primary germ cell that is diploid and contains a full set of 46 chromosomes into four mature sperm cells that are haploid and each contain only 23 chromosomes, half the number of a normal body cell.

Sperm cells are haploid because their purpose is to fertilize a female’s egg cell, which is also haploid, to form a diploid zygote with a full set of chromosomes, which eventually develops into an embryo and then a fetus. The sperm cells’ development is supported and nourished by fluids secreted by cells called Sertoli cells in the seminiferous tubules.

The testes are the organs in the male body that produce the most significant quantities of the male sex hormones, sometimes called androgens. The prefix “andro” means “man,” but it is important to know that these hormones play important functions in both males and females.

The main androgen is called testosterone. Androgens are responsible for the growth of various parts of the male reproductive system including the prostate gland and seminal vesicles. Androgens also control the development of male secondary sexual characteristics at puberty, such as the deepening of the voice and the growth of facial and body hair. In the testes, testosterone is secreted from specific interstitial cells called Leydig cells that lie between the seminiferous tubules.

The testes are held outside the body in a sac of skin called the scrotum. The external location of the scrotum in humans keeps the testes at a temperature 2– lower than the internal body temperature. This is beneficial, as sperm production is more efficient at lower temperatures.

In the male human embryo, if the testes do not descend into the scrotum during their development in the last months of pregnancy, it is likely that this individual will be infertile, as they will be unable to produce sperm effectively. Sometimes, this can be treated medically, but other times, procedures like this are not possible.

Interestingly, some other mammals, like elephants, dolphins and seals, do not have their testes held outside their bodies in a scrotum. The reason why and how this trait evolved is still debated among scientists.

Example 1: Identifying the Male Reproductive Organ that Secretes Testosterone

The figure shows a side view of the organs of the male reproductive system.

Which label represents the organ that secretes testosterone?

Answer

The male reproductive system is primarily responsible for sexual reproduction. To carry out this process, sperm cells are produced in the testes and are transported to the penis for ejaculation, which usually occurs during copulation.

Human males tend to have two testes, which hang outside the body between the legs in a sac of skin called the scrotum.

The testes are also responsible for producing and secreting the male sex hormones, one of which is testosterone, into the bloodstream. The male sex hormones aid the production of sperm, the growth of the male sex organs, and the development of secondary sexual characteristics in a biological male when he reaches puberty. These characteristics might involve the deepening of the voice and increased facial and body hair.

Now that we know where testosterone is produced and secreted, the testes, let’s label the structures on the diagram so that we can identify which letter represents it.

Therefore, the label that represents the organ that secretes testosterone is the testis, labeled as E.

The penis is a very important organ in the process of sexual reproduction, and it is also responsible for expelling urine from the male body. It is a spongy tissue that grows in size upon sexual arousal due to an increase in blood flow. It is called erectile tissue, as it can change from flaccid to erect.

The penis contains a duct called the urethra. The urethra is a tube that originates at the base of the bladder and in biological males, travels through the penis forming an opening at the tip of the penis. This opening provides a route for both sperm from the testes and urine from the bladder to exit the male body. Biological females also have a urethra forming a separate opening in front of the vagina, though in biological females, it only carries urine out of the body and not sperm.

For successful fertilization to occur, sperm must be transported from the testes to the penis, from which they can be ejaculated. Ejaculation often occurs during a process called copulation, in which a male’s penis is inserted into a female’s vagina. Copulation is sometimes referred to as sexual intercourse, or simply sex. Semen, which is a fluid containing between 40 million and 1.2 billion sperm cells in a healthy male, is ejaculated out of the male’s body through the penis via the urethra.

When a female ovulates, an egg cell is released from one of her ovaries into a fallopian tube. If sperm cells successfully enter a female’s vagina, they will attempt to swim up through her reproductive system to reach her fallopian tubes. Once in a fallopian tube, a sperm cell might, if it is very lucky, be able to fertilize the egg cell.

But how do sperm cells reach the penis? Let’s summarize the pathway sperm take from their production until they leave the body, which you can see in Figure 3.

Sperm are produced in the seminiferous tubules in the testes. From there, they are transported to the epididymis where they are stored until ejaculation is anticipated. At this point, sperm are transferred along the vas deferens, through the prostate where fluids are added to sperm from the prostate gland and seminiferous tubules to form semen. Sperm then travel into the urethra where the Cowper’s glands add more fluid to the semen. The urethra transports sperm, contained within the liquid semen, along the penis and out of the body in ejaculation.

Let’s look at these processes in a little more detail.

Following the production of sperm cells by meiosis in the seminiferous tubules, they are transported to a long, coiled tube just outside each testis called the epididymis. The epididymis is responsible for storing sperm cells.

When ejaculation is anticipated, sperm are transported from the epididymis to one of the two vasa deferentia. The vas deferens is a thick-walled duct that transports sperm to the urethra, where they can exit the male body through the penis.

There are three different fluid secretions that are added to the sperm cells to make up the total ejaculated volume of semen. These three fluid secretions come from accessory glands called the prostate, seminal vesicles, and Cowper’s glands. They are called accessory glands, as the sperm do not travel directly through them.

These fluids help protect sperm cells from the environment within the female reproductive system and aid sperm motility by providing a liquid medium through which sperm are able to swim. The fluids are all described as exocrine secretions because they are released from exocrine glands through ducts and secreted onto a body surface.

The urethra in both males and females is slightly acidic due to residual urine, which contains uric acid. All of these exocrine secretions are alkaline so they neutralize this acidic environment in the male urethra as they exit the body.

The female vagina is also a fairly acidic environment with a pH ranging between 3.8 and 4.5. In human sexual reproduction, sperm usually need to enter the vagina first in order to fertilize an egg cell. The alkaline nature of semen helps to neutralize the vagina’s acidic environment, too, helping more of the sperm cells survive for longer in the female reproductive system and increasing the chances of successful fertilization.

Let’s look at these individual exocrine secretions in more detail. You can see where each secretion is released from and to in Figure 4 below.

Seminal vesicles are two small glands between 2–4 cm long and 1–2 cm wide located behind the bladder. The seminal vesicles secrete an alkaline fluid that is rich in a sugar called fructose into sperm as they pass through the prostate gland, adding a liquid portion to semen. The fructose provides sperm cells with a source of sugars for cellular respiration. This is important because sperm are particularly active cells due to their high motility, so they require a vast amount of energy to be released through cellular respiration.

Fluid from the seminal vesicles contributes the largest proportion (50–80) of the volume of semen that will be ejaculated from the male’s body.

The prostate gland is located just below the male bladder. As the urethra begins at the base of the bladder, it travels through the prostate gland where it meets the sperm arriving from the vas deferens. The prostate gland secretes an alkaline fluid called prostate fluid.

The Cowper’s glands, sometimes called the bulbourethral glands, are two small pea-sized glands that are located either side of the urethra below the prostate gland. The Cowper’s glands secrete a thick, clear pre-ejaculate mucus into semen when it enters the urethra from the prostate. The mucus begins to be secreted from the penis during sexual arousal, and it is helpful in lubricating and neutralizing the urethra to prepare for the transport of sperm cells out of the body that occurs during ejaculation.

Example 2: Describing the Route of Spermatozoa from Production to Ejaculation

What is the route taken by spermatozoa from their site of production to their exit from the body?

1. Epididymis seminiferous tubules vas deferens ureter
2. Seminiferous tubules epididymis prostate gland ureter
3. Seminiferous tubules epididymis vas deferens urethra
4. Vas deferens epididymis prostate gland urethra
5. Epididymis prostate gland vas deferens ureter

Answer

Mature sperm cells, or spermatozoa, are produced in the testes in structures called the seminiferous tubules. From here, they need to make their way through the male reproductive system to reach the tip of the penis from which they are released from the male body through a process called ejaculation. Ejaculation is usually an essential part of copulation, which is often a process involved in sexual reproduction.

This question is asking us how the spermatozoa reach this exit point from their site of production, so let’s look at the various structures the sperm cells travel through to reach the tip of the penis, which you can see represented by the yellow line in the image below.

From the seminiferous tubules in the testes, the spermatozoa are transported to the epididymis where they are stored. When ejaculation is anticipated, for example during sexual arousal, the spermatozoa are transported from the epididymis to a duct called the vas deferens. The vas deferens eventually connects to another duct called the urethra. The urethra then transports spermatozoa along the penis, which has an opening at its end through which the spermatozoa can exit the body.

Therefore, the route taken by the spermatozoa is option C, seminiferous tubules epididymis vas deferens urethra.

Let’s see how these mature sperm cells, or spermatozoa, are structured to function effectively in sexual reproduction.

The diagram in Figure 5 below shows the basic structures in a mature sperm cell.

A typical mature sperm cell consists of four main sections, a head connected to a midpiece by a neck, and a tail. Let’s look at each of these sections in more detail.

The head of the sperm cell contains the nucleus that carries the genetic material in the form of DNA. This nucleus is haploid, which means it contains half the number of the chromosomes of a normal body cell: just 23 chromosomes instead of the usual 46. This is because the sperm cell undergoes two divisions in its formation through the process of meiosis.

The haploid nature of a sperm cell’s nucleus means that each sperm will only contain one sex chromosome: either X or Y. Female egg cells also contain only one sex chromosome as they too are haploid, though the sex chromosome in an egg cell will always be an X chromosome. If a sperm cell with an X chromosome fertilizes an egg cell, the offspring produced will have two X chromosomes and will be a biological female. If a sperm cell with a Y chromosome fertilizes an egg cell, the offspring produced will have one X chromosome and one Y chromosome and will be a biological male.

The tip of the head of the sperm cell is covered by a structure called the acrosome. The acrosome contains hydrolytic enzymes, commonly called hyaluronidases, that will be secreted if the sperm cell comes in contact with an egg cell. The enzymes break down hyaluronic acid, which allows a sperm cell to digest through the outer membrane of an egg cell so that the sperm cell can penetrate it for fertilization.

The neck of the sperm cell contains two centrioles. Centrioles are organelles that are responsible for organizing the spindle fibers formed during cell division. These centrioles will eventually play an important role in the division of the egg cell if it is successfully fertilized by a sperm cell.

The midpiece of the sperm cell contains many mitochondria. Mitochondria are organelles that are the site of cellular respiration, which is performed to release energy. Sperm cells require vast amounts of energy to swim through the female reproductive system. They must travel distances around 1‎ ‎000 times their own length, through challenging currents and obstacles. Note that, the seminal vesicles provide sperm with a fluid rich in fructose sugars. The energy released by the plentiful mitochondria in sperm cells when they metabolize these sugars allows some sperm to make this lengthy journey successfully.

The movement of a sperm cell is possible due to the presence of a long tail, which allows organisms to be motile within their environment. The tail of a sperm cell ends in a section called the caudal piece.

Example 4: Identifying the Location of the Acrosome in a Human Spermatozoon

The figure shows the structure of a human spermatozoon.

Which letter indicates the acrosome?

Answer

A mature sperm cell, otherwise known as a spermatozoon, consists of four main sections: a head, a neck, a midpiece, and a tail.

The head of the sperm cell contains its haploid nucleus. It is also coated at its tip with a section called an acrosome, which contains hydrolytic enzymes that are able to digest through the outer coating of an egg cell.

The neck of a sperm cell contains two centrioles, which play an important role in completing cell division in the egg cell if it is successfully fertilized.

The midpiece of the sperm cell contains many mitochondria, which release the energy needed for the sperm cell’s movement through cellular respiration.

The tail, otherwise known as the undulipodium, of a sperm cell allows it to move the great distances needed to navigate the female reproductive system in search of an egg cell.

Let’s label these structures on the diagram so that we can identify where the acrosome is located.

Therefore, the letter that indicates the acrosome is C.

The key characteristics of human sperm cells are their ability to move, their small size, and the vast numbers in which they are produced. These features make the sperm cell well adapted to its function as a reproductive cell, which is to reach and successfully fertilize an egg cell to help create a new human life.

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