Lesson Video: Sperm Production Biology

Video Transcript

In this video, we will learn how to describe the structures that can be found within the human male testes and their functions. We will learn specifically how sperm production occurs within the seminiferous tubules of the testes to produce the male gametes, sperm, capable of fertilizing an egg cell.

Sperm cells are the male gametes, which are otherwise known as their reproductive cells or their sex cells. The purpose of a sperm cell is to fertilize the female gamete, otherwise known as the egg cell in a process known as sexual reproduction. The male testis is the site of sperm cell production. Male humans typically have two of them, which are referred to as plural testes.

Did you know that producing these sperm cells requires a constant temperature about two to three degrees lower than the rest of the abdomen? This is proposed by some scientists to explain why the scrotum, which is the sac of skin that contains these two testes, is located hanging just outside the abdomen. The cooler temperatures may also be helpful in preventing mutations in sperm cells. Spermatogenesis is the process by which a mature sperm cell is produced and developed In these testes. The prefix sperm refers to the sperm cells themselves that are being produced, and the suffix genesis comes from the Greek word meaning production, generation, or creation. The testes are also responsible for producing the most male sex hormones, such as testosterone, of any organ in the human body. These male sex hormones are sometimes called androgens. And they’re responsible for stimulating the development of male sex organs and their secondary sexual characteristics.

Secondary sexual characteristics are physical traits that develop when a person reaches sexual maturity during a period called puberty. In males, some examples might include deepening of the voice and development of thicker facial and body hair. Let’s take a closer look at the overall structure of a testis. Seminiferous tubules are structures that make up the bulk of the testes. These seminiferous tubules are long coiled tubes that are located throughout the testes and at the site of spermatogenesis. Once mature sperm cells have been produced in the seminiferous tubules, they’re transported to this region which is called the epididymis. The epididymis is responsible for storing these mature sperm cells until they are needed, at which point they will be transported into the vas deferens. The vas deferens can then transport these sperm cells to the urethra, at the end of which sperm is ejaculated out of the male’s body. Sometimes, this ejaculation might result in sperm entering a female’s vagina where they might be able to swim towards an egg cell to fertilize it.

Let’s take a look at the structure of a seminiferous tubule and the area surrounding it in more detail. This diagram shows the transverse section of a seminiferous tubule and, in blue, some of the cells surrounding it. Transverse is a word that means across. And a transverse section is the view you get when you slice across something like this portion of a seminiferous tubules at a right angle to its vertical axis. Each seminiferous tubule contains a channel of space in its center called the lumen. The lumen is used to transport mature sperm cells like this one to the epididymis for storage. You can see that there are a number of cells making up the epithelium or the lining of the seminiferous tubules surrounding this in a lumen. One type of cell that this epithelium contains are called Sertoli cells.

Sertoli cells provide structural support and secrete a fluid which functions to nourish and support these sperm cells during their development. These secretions also provide the mature sperm cells with a fluid that they can swim through. Sertoli cells also help to regulate spermatogenesis and might even play a role in the immune defense to help the sperm cells survive for longer. The rest of the cells in the seminiferous tubule epithelium tells a story about sperm production. If we observed a transverse section of a seminiferous tubule under a microscope, we’d be able to see cells at each different stage of spermatogenesis. The cells produced in the initial stages of spermatogenesis are located in the outer region of a seminiferous tubule. And as the stages progress, the cells move inwards towards the lumen. Spermatogenesis overall converts diploid primary germ cells into haploid mature sperm cells.

Primary germ cells, which are sometimes called primordial germ cells, are undifferentiated diploid stem cells. In most organisms, they are present when the individuals adjust embryos in both the male testes and in the female ovaries. As this diagram is showing us the transverse section of a seminiferous tubule in a mature male, we therefore cannot see any primary germ cells here. Primary germ cells divide and differentiate to form haploid mature gametes such as sperm cells. The word “diploid” is used to refer to most body cells that have a full set of chromosomes. In humans, this number is 46 chromosomes, which is sometimes referred to as two n. In vertebrates like humans, the term haploid is only used to refer to gametes. Gametes have half the number of chromosomes of a normal body cell, only 23 in humans, and is sometimes referred to simply as n. Both sperm cells and the female gamete egg cells are both haploid cells, as when they fuse together in fertilization, they will form a zygote with a full set of 46 chromosomes.

Like primary germ cells in a sexually mature male, the outermost cells in the seminiferous tubules, called spermatogonia, are also diploid. This is because they were formed through a type of cell division called mitosis. Moving closer towards the lumen, we can see some haploid spermatids. These cells are haploid, as they are formed through another type of cell division called meiosis, which halves the number of chromosomes in a cell. These haploid spermatids can then differentiate and specialize into haploid mature sperm cells. These mature sperm cells can then be released into the lumen of the seminiferous tubule to be transported to the epididymis for storage.

Interstitial cells are any cells that lie in the spaces between the functional cells of a tissue. So which cells that we can see in the diagram do you think might be the interstitial cells of the testes? Pause the video and have a think. If you worked out that it was these blue cells surrounding the seminiferous tubules, well done! They’re called the Leydig cells. And they’re the interstitial cells specific to the testes. As the cells that make up the seminiferous tubules are the functional cells of the testes, those that surround it must be the interstitial cells. Can you remember what the other main role of the testes is? The other main function of the testes is to secrete androgens like testosterone, and that’s exactly what the Leydig cells do. As you can see, they’re located in between seminiferous tubules.

Let’s look at how hormones can control the process of spermatogenesis by stimulating the cells that are involved and regulating the process as a whole. Production of mature sperm cells begins when a male reaches sexual maturity at the onset of a time called puberty and will then typically occur continually throughout his life. Puberty causes many changes in the male body that are driven primarily by hormones. Some of these hormones are released from a gland in the brain called the anterior pituitary gland. One such hormone that is released in greater quantities at puberty from the anterior pituitary gland is luteinizing hormone or LH.

The blood then transports LH all around the body, including to its target tissues in the testes, where LH combine to receptors on its target cells, the Leydig cells. You might recall that the Leydig cells are the interstitial cells of the testes responsible for secreting testosterone. When LH binds to receptors on these Leydig cells, it stimulates them to carry out this function and release testosterone. Both testosterone and luteinizing hormone are essential hormones to the process of spermatogenesis because when either hormone is absent, spermatogenesis stops.

Meanwhile, in the anterior pituitary gland, another hormone called follicle stimulating hormone, or FSH for short, is being released into the bloodstream. FSH has a role in gamete production in both males and in females. So it’s going to travel to target tissues in the testes too. In the male reproductive system, the target cells for FSH are the Sertoli cells. Upon binding to receptors on these Sertoli cells, FSH can stimulate spermatogenesis. FSH stimulates these Sertoli cells to produce testosterone receptors, which is sometimes called androgen-binding proteins. These receptors are presented on the surface of the Sertoli cells, so the testosterone, which has been produced by the Leydig cells, combine to them. Both testosterone and FSH help to regulate spermatogenesis. They do this through their effects on Sertoli cells by helping the sperm cells to mature and function effectively.

For example, testosterone and FSH stimulate the secretion of nourishing nutrient filled fluids and the production of regulatory molecules from the Sertoli cells to aid spermatogenesis. Testosterone also helps the Sertoli cells support the adjacent developing sperm cells until they are mature enough to detach from the seminiferous tubule epithelium. A lack of testosterone has been found to result in sperm cells either detaching when they are immature or not detaching at all. Testosterone might even be responsible for completing the process of meiosis, among its various other functions in spermatogenesis.

Let’s take a look at the stages of spermatogenesis in more detail and the different cells that each stage produces. There are four main stages to spermatogenesis: the multiplication phase, the growth phase, the maturation phase, and the metamorphosis phase. First, let’s look at the multiplication phase. When the male is still in his embryonic form, most of the cells in the seminiferous tubules are primary germ cells, which as we learned earlier are diploid cells. We also know that the male primary germ cells undergo mitosis. The process of mitosis and proliferation produces a large number of diploid prospermatogonia. The prefix pro- means before. This can help us remember that prospermatogonia are produced before birth. Following birth, these spermatogonia are going to develop into the cells we already learned about earlier, diploid spermatogonia.

A single one of these cells is called a spermatogonium. But let’s see what these spermatogonia might look like in the seminiferous tubules. Spermatogonia will be located in the outer region of the seminiferous tubule, as they will be the first to develop from the primary germ cells. As we go through the other stages in this process, we can add the other types of cell to this diagram.

Next, let’s look at the second stage of spermatogenesis which is sometimes called the growth phase. During this stage, some spermatogonia divide by mitosis, and they differentiate into cells called primary spermatocytes. As you can see, primary spermatocytes will be located a little bit closer to the lumen of the seminiferous tubules. And they generally lose contact with the seminiferous tubules basement membrane.

Can you work out whether these cells will be diploid or haploid? Pause the video and have a think. If you said diploid, good job! Mitosis has occurred, so diploid primary spermatocytes are produced. The third stage of spermatogenesis is the maturation phase. We’ve already learned that when a male reaches puberty, there is usually an increase in his testosterone levels. This triggers the primary spermatocytes which have duplicated their genetic information during the growth phase to undergo meiosis. You might recall that meiosis involves two stages of cellular division, meiosis one and meiosis two.

In meiosis one, the primary spermatocytes becomes secondary spermatocytes. Now, will these cells be diploid or haploid? Pause the video and have a think. Secondary spermatocytes are haploid cells. So these cells have half the number of chromosomes of primary spermatocytes. In meiosis two, the secondary spermatocytes divide again to form haploid spermatids. The final stage of spermatogenesis is the metamorphosis phase. During the metamorphosis phase, the round haploid spermatids develop tails and other specializations to become mature haploid sperm cells, otherwise known as spermatozoa or a singular spermatozoon. These mature sperm cells are now capable of fertilizing a female’s egg cell.

Aside from the formation of a tail, another specialization that the sperm cells undergo is the formation of a structure called an acrosome on the sperm’s head, from which hydrolytic enzymes can be released that might eventually help them to penetrate the egg cell. These mature sperm cells are now ready to go and can be released into the lumen of the seminiferous tubule to move towards the epididymis for their final development and storage.

To summarize, spermatogenesis consists of a multiplication phase where diploid primary germ cells divide many times by mitosis to form many diploid prospermatogonia which then develop into diploid spermatogonia after birth. The growth phase then follows, where spermatogonia divide by mitosis and differentiates to develop into diploid primary spermatocytes. In the maturation phase, the primary spermatocytes enter meiosis one which halves their chromosome number to form haploid secondary spermatocytes. These cells then enter meiosis two to form haploid spermatids. The spermatids then undergo metamorphosis and develop into mature haploid sperm cells.

Now it’s time for us to review the key points that we’ve covered in this video. The human male testes contain structures called seminiferous tubules, where spermatogenesis or sperm production occurs. The seminiferous tubules also contain Sertoli cells that support the developing sperm cells and secrete a fluid that nourishes them. The testes also contain specific interstitial cells called Leydig cells that secrete testosterone.

There are four stages to spermatogenesis: multiplication, growth, maturation, and metamorphosis. Multiplication involves primary germ cells dividing by mitosis to produce many spermatogonia following the male’s birth. Growth includes these spermatogonia dividing by mitosis again and differentiating into primary spermatocytes. Maturation includes primary spermatocytes dividing by meiosis, first producing haploid secondary spermatocytes and then haploid spermatids. Metamorphosis converts these spermatids into mature sperm cells that can be released into the lumen of the seminiferous tubule.