Video Transcript
In this video, we will learn how to identify parts of the male genital system and describe their functions. We will look at the journey that sperm takes through each part of the genital system step by step. And we’ll also look at the accessory organs that help sperm by producing the fluids that make up semen. Finally, we’re going to investigate the structure of a typical sperm cell and the adaptations that make it well suited for its function in sexual reproduction.
Did you know that a healthy male’s testes can produce up to approximately 1500 sperm cells every single second? This means that hundreds of millions of sperm cells are made every single day. Just one milliliter of healthy semen, which is the fluid that contains sperm, can have as many as 300 million sperm cells. Sperm cells are the male sex cells or reproductive cells, otherwise known as gametes, which function to fertilize the 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. So you need a lot of sperm to carry out this function.
Let’s start by describing the male reproductive system, which is sometimes known as 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. This diagram shows a side-view cross section that we can use as a summary of some of the main structures of the male genital system.
You might already be familiar with a few of these structures, for example, the testis, which is more commonly known as the testicle, and the penis. But in reality, the male reproductive system is much more complex than just these two organs. Let’s take a look at all of the different organs and their functions in more detail, starting with the testis where the journey of the sperm cells begin.
Male humans typically have two testes, which is the plural of a singular testis. The testes are also known as the male gonads. Each testis, one of which you can see in this diagram, is about the shape of a small chicken egg. They’re between approximately three to five centimeters long and around three to four centimeters wide at their thickest point, though clearly this will vary between different males. The testes tend to grow from birth until a male reaches puberty. After a male passes sexual maturity, his testes can gradually become smaller. This is because of drops in testosterone levels, which are a male sex hormone released from the cells of the testes.
Let’s keep a checklist of the structural features of each of these organs as we cover them. It will also be useful to keep a list of their functions. The testes are responsible for producing the male gamete, which are known as sperm cells. Sperm cells are otherwise known as spermatozoa, or a singular spermatozoon. Sperm cells are haploid cells, which means that they have half the number of chromosomes that a normal body cell would. This is because spermatogenesis, which is the process by which sperm cells are made in the testes, includes a type of cell division called meiosis. Meiosis includes two cellular divisions which halves the number of chromosomes in the cells produced. Spermatogenesis occurs in long tubelike structures within the testes called seminiferous tubules, which you can see inside this testis here.
The testes are the organs in the male body that produce the most significant quantities of male sex hormones, which is sometimes called androgens. The prefix andro- means man. But it’s important to know that these hormones play important functions in both males and in females. The main androgen is called testosterone. Testosterone and the other androgens are mainly responsible for stimulating growth of various parts of the male reproductive system such as the prostate gland, which we’ll explore in more detail later on in the video. They also stimulate the development of secondary sexual characteristics, which are sometimes called secondary sex characteristics. These characteristics develop at puberty and are not directly involved in reproduction. They might include deepening of the voice or growth of facial and body hair.
Testosterone is secreted from cells that sit between the seminiferous tubules within the testes, which are called Leydig cells and have been labeled here in pink. The two male testes are held inside a sack of skin called the scrotum. As you can see in this diagram showing a front view of the scrotum in testes, the scrotum is typically divided into two main sections, one of which contains each of the two testes. In male humans, the scrotum is held outside the body, which keeps the testes approximately two to three degrees Celsius lower than the rest of the internal body temperature, especially in the abdomen. This is beneficial as it maintains a lower temperature, which make sperm production more efficient.
The scrotum also provides a degree of protection to the testes, but they are still very sensitive organs. If the testes don’t descend into the scrotum when the male is an embryo, infertility can arise. This is because they’re being kept at body temperature rather than the two to three degrees lower that is suitable for sperm production. 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’s evolved is still debated among scientists.
Now that we know where the sperm cells are produced, let’s learn more about the details of how they’re transported around the male reproductive system until they leave the body by investigating all of these other organs involved. We can use green arrows to show the direction of transport of sperm cells. After being produced in the testes, the sperm cells are transported to a region called the epididymis. You can also see the epididymis on this magnified diagram here. The epididymis is a long coiled tube located just outside each testis. The epididymis is responsible for storing sperm cells until ejaculation is anticipated, for example, during sexual arousal.
Ejaculation is the process by which sperm cells leave the male’s body through the penis. But we will get to that stage later on in the video. When ejaculation is anticipated, sperm cells are transported from the epididymis to the vas deferens. The vas deferens is sometimes called the ductus deferens, as it is a thick-walled duct. There are two vas deferens which form very long tubes attached to each epididymis. The role of the vas deferens is to transport sperm cells from the epididymis to another tube called the urethra. From the urethra, sperm will eventually leave the male’s body.
Interestingly, both males and females have urethras, although their functions differ slightly. While the male urethra is responsible partly for transporting sperm out of the male’s body via the penis, as you can see, it’s also connected to the bladder. This is because in both males and females, the urethra is responsible for transporting urine, which is stored in the bladder, out of the body. This process is called urination, and it’s important to note that it’s entirely separate from the urethra’s role in sexual reproduction.
There are three different fluid secretions that are added to the sperm cells to make up the total ejaculated volume of liquid semen. Semen is a fluid that contains between 40 million and 1.2 billion sperm cells in a healthy average male. The fluids that make up semen are all produced by accessory glands in the male reproductive system, which can be identified on the summary diagram in blue. They’re called accessory glands as sperm does not travel directly through them. These glands are called exocrine glands as they secrete substances into the urethra, which is a duct. They include the prostate gland, the seminal vesicles, and the Cowper’s glands.
The exocrine fluids that these glands produce aid sperm motility by providing a liquid medium through which sperm cells can swim. You would find it pretty hard to swim too if you had no liquid to travel through. The urethra is slightly acidic, as it will contain some uric acid from residual urine. The female vagina, which you can see labeled in this diagram at the bottom of the screen here, is also a fairly acidic environment with a pH ranging between 3.8 and 4.5 in most females. In sexual reproduction, sperm usually needs to travel into the vagina in order to fertilize an egg cell. Note that this sperm cell is shown much larger than they actually are.
All of the exocrine secretions that make up semen are alkaline, so they neutralize these acidic environments, in the male urethra as it exits the body and in the female reproductive tract. This helps more of the sperm cells to survive for longer in the female reproductive system and increases the chances of a successful fertilization. Let’s look at these individual secretions in more detail and show where they enter the male reproductive tract using pink arrows. 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 sperm arriving from the vas deferens. The prostate gland secretes an alkaline fluid called prostate fluid, which provides a liquid portion to sperm cells to make up semen.
Males typically have two seminal vesicles, which are located just behind the bladder. And here in this diagram, you can just see one of them. The seminal vesicle secretes an alkaline fluid which is rich in a sugar called fructose. This is also secreted as the sperm cells travel through the prostate gland. Fructose provides sperm cells with a source of sugars for cellular respiration. This is important, as sperm are particularly active cells due to their high motility, so they require a vast amount of energy to be released. The Cowper’s glands are two small, P-shaped glands located either side of the urethra, just below the prostate gland. They are sometimes called the bulbourethral glands, which is easy to remember as they are bulbar structures either side of the urethra.
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’s helpful in lubricating and neutralizing the urethra to prepare for the transport of sperm cells out of the body. As we now know, the penis contains a duct called the urethra, which forms an opening just 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. The process by which semen moves out of a male’s body through the penis via the urethra is called ejaculation.
Ejaculation often occurs during a process called population, or sometimes sex, in which a male’s penis is inserted into a female’s vagina. In order for a successful fertilization and therefore a pregnancy to occur as a result of this copulation however, a few things must happen first. When a female ovulates, an egg cell is released from one of her ovaries and travels 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 the Fallopian tube, a sperm cell might, if it is very lucky, be able to fertilize the egg cell.
The penis is well adapted for its function in sexual reproduction as it’s a spongy tissue that grows in size upon sexual arousal. It is called erectile tissue as it can change from flaccid, as it appears in the diagram here, to erect during sexual arousal due to an increase in blood flow. Another useful adaptation of the penis is that it contains the urethra, which allows sperm cells to travel out of the body. As we’ve just reviewed a pretty complex process and introduced a lot of new terms, let’s summarize this that we’ve just explored by looking at the male reproductive system from a front view instead so we can see this process occurring from a different angle.
First, sperm is produced in the testes. Sperm is then transported to the epididymis where it’s stored until ejaculation is anticipated, at which point the sperm will travel along the vas deferens. The sperm cells are then transported into the urethra. As they travel along the urethra, several alkaline fluids are added to the sperm cells, firstly from the seminal vesicles, then from the prostate gland, and lastly from the Cowper’s glands, to make up semen. Sperm then travel through the penis via the urethra until it’s ejaculated. Ejaculation means that these sperm cells might be able to fertilize an egg cell.
Let’s take a look at the structure and function of sperm cells next. The function of a sperm cell is to reach and successfully fertilize a female’s egg cell in order to help create a new human life. But how are these sperm cells structured to help them function effectively in sexual reproduction? The key characteristics of human sperm cells are their ability to move their microscopically small size and the vast numbers in which they’re produced. A typical mature sperm cell consists of four main sections: a head, which is connected to a neck, a midpiece, and a long tail. Let’s look at each of these sections in more detail.
The head of a sperm cell contains its nucleus, which carries its genetic material in the form of DNA. This nucleus is described as haploid as it contains half of the number of chromosomes of a normal body cell, 23, instead of the usual 46. This is because, as you might recall, the sperm cell undergoes two divisions in its formation through the process of meiosis. Sperm cells are haploid because their purpose is to fertilize the female’s egg cell, which is also haploid, to produce a diploid zygote with a full set of 46 chromosomes. The haploid nature of a sperm cell’s nucleus also means that each sperm will only contain one sex chromosome, either X or Y. As female egg cells are also haploid, they will also only contain one sex chromosome, though the sex chromosome in egg cell will always be X.
If a sperm cell with an X chromosome fertilizes an egg cell, the zygote produced would have two X chromosomes and would therefore be biologically female. If a sperm cell with a Y chromosome fertilizes an egg cell however, the zygote produced would have one X chromosome and one Y chromosome and would be biologically male. The tip of the head of the sperm cell is covered with a cap-like structure called an acrosome. The acrosome contains hydrolytic enzymes which are commonly called hyaluronidases. These are secreted if the sperm cell comes into contact with an egg cell. These enzymes allow the sperm cell to digest through the outer membrane of the egg cell, so the sperm is able to penetrate it for fertilization.
The neck of the sperm cell contains two centrioles. Centrioles are the organelles that are responsible for organizing the spindle fibers formed during cell division. These centrioles might eventually play an important role in division of the egg cell if it is successfully fertilized by a sperm cell. The midpiece of a sperm cell contains many mitochondria, which’ve been shown here in pink. Mitochondria are the organelles in which cellular respiration takes place and therefore energy is released.
Sperm cells require vast amounts of energy to swim through the female reproductive system, traveling distances around 1000 times their own length through challenging currents and obstacles. Remember that the seminal vesicles provided sperm with a fluid rich in fructose sugars. This fructose could be metabolized in the sperm cells, and the products of this metabolism can be used in the plentiful mitochondria in cellular respiration to release energy. This might allow some sperm to complete the hazardous journey successfully. The movement of sperm cells is possible due to the presence of a long tail, which is sometimes called an undulipodium. This is what allows sperm cells to be mobile within their environment.
Now it’s time for us to review the key points that we’ve covered in this video. The male genital system, otherwise known as the male reproductive system, is responsible for carrying out part of the process of sexual reproduction. The male testes are responsible for producing sperm cells and the male sex hormones, mainly testosterone. Sperm is transported from the seminiferous tubules in the testes to the epididymis for storage, then into the vas deferens, which leads into the urethra so the sperm can be transported out of the penis. Fluids are then added to the sperm to make semen by three different accessory glands.
We also learned how sperm cell is adapted for its function by having a haploid nucleus and acrosomes containing enzymes. And they also have many mitochondria to release energy and a tail to use this energy for movement.
