Lesson Explainer: Sperm Production Biology • Third Year of Secondary School

In this explainer, we will learn how to describe the structure of the transverse section of the testes and that of the seminiferous tubules and the process of sperm production.

Did you know that the process of sperm production requires a constant temperature that is about 2– lower than the body temperature? According to scientists, this explains why the scrotum, a sac of skin that contains the testes, where the process of sperm production occurs, is located outside the human body. Such cooler temperatures may also be helpful in preventing mutations in sperm cells.

Generally, human males have two testes. Testes are the site of sperm production, a process that is often called spermatogenesis. The prefix spermato- refers to the sperm cells that are formed, and the suffix -genesis comes from a Greek word meaning “the creation of something.” Sperm cells are the male gametes, and their function is to fertilize the female egg cell during the process of sexual reproduction.

The testes are also responsible for producing most of the male sex hormones, such as testosterone. These hormones, sometimes called androgens, help in the development of the male sex organs and secondary sexual characteristics, such as deepening of the voice and development of facial hair, at puberty.

Let’s have a look at the overall structure of a testis, which you can see in Figure 1 below, before zooming in on the microscopic structures that are involved in spermatogenesis.

Spermatogenesis occurs in the seminiferous tubules, structures that make up the bulk of the testes, which you can see in Figure 1.

Each seminiferous tubule is a tube-like structure with a channel and a center called the lumen. The lumen is used to transport mature sperm cells to the epididymis. The epididymis, whose location is also shown in Figure 1, is responsible for storing sperm cells until they are required. At this point, the sperm cells are transferred from the epididymis to the vas deferens. The vas deferens then transports the sperm cells to the urethra, at the end of which sperm are ejaculated out of the male’s body. Sperm cells are often ejaculated into a female’s vagina, where they can swim toward the egg cell to fertilize it.

The micrograph below in Figure 2 shows a magnified transverse section (TS) of a testis, displaying a seminiferous tubule in the center. You can see the edges of four other seminiferous tubules around the perimeter of the micrograph. A transverse section describes a way of viewing part of a specimen or body part. The word transverse means “across.” A transverse section is the view that you get when you slice across something at a right angle to its vertical axis.

You can see the lumen as a white space in the center of the seminiferous tubule in the micrograph. You can also see that there are a number of different cells making up the epithelium (lining) of the seminiferous tubule. We will discuss these cells in more detail later. The distinct, dark, purplish-blue cells belong to the epithelium of the seminiferous tubule.

The cells that secrete testosterone are an example of interstitial cells found in the testes, sometimes more specifically called Leydig cells. You can see them in the micrograph above, stained with a lighter purplish-pink colour, surrounding the seminiferous tubule.

Let’s look at a TS of the seminiferous tubule in more detail, which you can see in Figure 3 below.

In addition to the interstitial cells that can be seen in Figure 2, Figure 3 shows more details of the cells making up the epithelium of the seminiferous tubules.

One type of cell that the seminiferous tubule contains is the Sertoli cell. Sertoli cells secrete a fluid that functions to nourish and support sperm cells during their development and provide mature sperm cells with a fluid they can swim through. Sertoli cells may also play a role in the immune defense to help the sperm cells survive for longer.

Example 1: Identifying the Testosterone-Secreting Cell in the Seminiferous Tubules of the Testes

The figure shows a drawing of a cross section of the seminiferous tubules in the testes.

Identify the cell that secretes testosterone.

Answer

The seminiferous tubules make up the bulk of the testes in male humans and are the location of sperm production. This process is also known as spermatogenesis, and it functions to convert diploid primary germ cells into haploid mature sperm cells. The seminiferous tubules also contain Sertoli cells, which secrete fluids to nourish and support the developing sperm cells.

Interstitial cells can be located in various regions of the body. For example, some interstitial cells are interspersed between the seminiferous tubules in the testes. A specific example of these interstitial cells in the testes is Leydig cells. Leydig cells are responsible for producing and secreting the hormone testosterone. Testosterone plays an important role in the completion of meiosis in spermatogenesis and various other processes that aid it, such as fluid secretion from Sertoli cells.

By looking at the diagram, we can see that the large structure at the center is a seminiferous tubule, which consists of many cells at various stages of development, with Sertoli cells between them. Interstitial cells are not located within the seminiferous tubules themselves, so these must be the clusters of round cells between the central seminiferous tubule and the adjacent ones.

Therefore, the cell that secretes testosterone is an interstitial cell, labelled E.

Seminiferous tubules tell the story of sperm production. If you observe a TS of a seminiferous tubule under a microscope, you will be able to see cells at each different stage of spermatogenesis. The cells produced at the first stage of spermatogenesis are located on the outer region of the seminiferous tubule, and the stages progress as you move inwards toward the lumen of the tubule.

Spermatogenesis overall converts diploid primary germ cells into haploid mature sperm cells.

You may notice from Figure 3 that the outer cells in the epithelium of the seminiferous tubule, the spermatogonia, are diploid (2n), like most other body cells. The cells closer to the lumen, the spermatids and mature sperm cells, are haploid (n). This is because spermatogenesis involves the process of meiosis, a type of cell division that halves the genetic material of cells.

The word diploid means that the cells contain the full set of chromosomes in their nucleus, often denoted with a “2n.” In humans, diploid cells have 23 pairs of chromosomes in their nucleus, amounting to 46 chromosomes in total. The word haploid means that the cells contain half of the full set of chromosomes (23 in total) in their nucleus. All gametes, sperm cells and egg cells, are haploid, and this is often denoted with an “n.”

Mature sperm cells, sometimes called spermatozoa (singular: spermatozoon), are released from the Sertoli cells into the lumen of the seminiferous tubule in a process called spermiation. Once in the lumen, spermatozoa can be transported to the epididymis for storage.

Example 2: Identifying Haploid Cells in the Seminiferous Tubules of the Testes

The figure shows a drawing of a cross section of the seminiferous tubules in the testes.

Identify the cells that are haploid.

1. A and C
2. B and C
3. B, C, and D
4. C only
5. C and E

Answer

The majority of cells in the human body are diploid. A human cell is diploid if it contains a full set of 23 pairs of chromosomes or a total of 46 chromosomes located within its nucleus.

Gametes, otherwise known as sex cells or reproductive cells, are haploid. The word haploid means that gametes (egg cells and sperm cells) contain half the number of chromosomes of a normal diploid body cell. This is because when fertilization occurs, the egg cell and sperm cell fuse together. Fertilization forms a diploid zygote with a full set of 46 chromosomes: 23 from the biological father and 23 from the biological mother.

Spermatogenesis is the process of sperm production, which forms haploid male gametes. The primary germ cells that enter the process of spermatogenesis are diploid and initially divide via mitosis to produce spermatogonia, which are also diploid. The stages of spermatogenesis are reflected in the seminiferous tubule, which contains more immature spermatogonia toward its outer region.

Spermatogonia then divide to form a large number of haploid spermatids, which specialize into mature haploid sperm cells. Both of these types of cells are haploid, as meiosis halves the number of chromosomes in the cells it produces. Sperm cells have a tail and are released into the lumen of the seminiferous tubule to be transported to the epididymis for storage.

Let’s look at the diagram so that we can work out which cells are haploid.

Most of the cells in the image are diploid, as most cells in the human body have a full set of 46 chromosomes. We know that the only haploid cells are those near the lumen of the seminiferous tubule, as these cells are approaching the end of the process of spermatogenesis and have undergone meiosis.

The haploid spermatids are labelled B, and the mature sperm cells that have been released into the lumen of the seminiferous tubule are labelled C.

Therefore, the cells in the seminiferous tubule that are haploid are the spermatids and mature sperm cells (spermatozoa), labelled B and C respectively.

Spermatogenesis continues as a male reaches sexual maturity at the onset of puberty. This process thereafter typically occurs continuously throughout his life. Puberty causes many changes in the male body that are driven by hormones. Let’s look at some of the hormones involved in stimulating the cells involved in spermatogenesis and regulating the process as a whole. You can see the effects of these hormones outlined in Figure 4 below.

At puberty, luteinizing hormone (LH) is secreted from the anterior pituitary gland in the brain. This hormone binds to receptors on the interstitial cells of the testes (Leydig cells), which stimulates them to start producing and secreting testosterone. Both testosterone and LH are essential hormones to the process of spermatogenesis because when either hormone is absent, spermatogenesis stops.

Follicle-stimulating hormone (FSH) is also released from the anterior pituitary gland and plays a role in gamete formation in both males and females. In the male reproductive system, Sertoli cells have receptors for FSH. FSH stimulates spermatogenesis and promotes the production of androgen-binding proteins, which act as testosterone receptors on the surface of Sertoli cells. This is also important as testosterone can bind to these receptors and affect these cells, which also helps maintain a high concentration of testosterone in the testes. FSH and testosterone both regulate spermatogenesis through their effects on Sertoli cells by helping the cells mature and function effectively.

For example, testosterone and FSH stimulate the secretion of nourishing nutrient-filled fluids and the production of regulatory molecules from 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’s epithelium. A lack of testosterone has been found to result in sperm cells either detaching while they are still immature or not detaching at all. Testosterone may also be responsible for completing the process of meiosis, among its various other functions in spermatogenesis.

To summarize, LH stimulates the production of testosterone in interstitial cells (Leydig cells). FSH stimulates the production of testosterone receptors (androgen-binding proteins) by Sertoli cells. This allows testosterone to bind to these Sertoli cell receptors and activate effects such as fluid secretion from Sertoli cells, as well as meiosis in spermatogenesis.

Let’s take a look at the stages of spermatogenesis and the different cells they produce.

There are four main stages in spermatogenesis: (1) multiplication, (2) growth, (3) maturation, and (4) metamorphosis. You can see these four stages displayed in Figure 5 below.

First, let’s look at the multiplication phase.

When the male is still an embryo, the epithelium of his seminiferous tubules mostly consists of primary germ cells, sometimes called primordial germ cells. These primary germ cells (2n) undergo mitosis and proliferation to produce a large number of prospermatogonia, which develop into spermatogonia (2n) after birth. This stage is called the multiplication phase. You can see spermatogonia (singular: spermatogonium) in the outer layer of the epithelium of the seminiferous tubule in Figure 5 because they are the first to develop.

Next, let’s look at the growth phase.

The second stage of spermatogenesis is sometimes called the growth phase. During this stage, some types of spermatogonia divide via mitosis and differentiate into primary spermatocytes (2n). These cells can be found a little closer to the lumen of the seminiferous tubule, and they lose contact with the seminiferous tubules’ basement membrane as you can see in Figure 5.

The third stage of spermatogenesis is the maturation phase.

At puberty, the levels of testosterone increase. This triggers the primary spermatocytes (2n) to undergo meiosis during the maturation phase. You may recall that meiosis consists of two stages of division: meiosis I and meiosis II. In meiosis I, the primary spermatocytes become secondary spermatocytes (n). Secondary spermatocytes are haploid cells, so these cells have half the number of chromosomes of primary spermatocytes. In meiosis II, the secondary spermatocytes divide again to form haploid spermatids (n).

The final stage of spermatogenesis is the metamorphosis phase.

During the metamorphosis stage, the round spermatids (n) develop tails and other specializations to become mature sperm cells (n) that can fertilize a female’s egg cell. One such specialization is the production of enzymes, such as hyaluronidase, in the sperm cells’ heads, which eventually help them penetrate the egg cell. These sperm cells are then released into the lumen of the seminiferous tubule to move toward the epididymis for their final development and storage.

To summarize, spermatogenesis consists of a multiplication phase, at which primary (primordial) germ cells (2n) divide many times via mitosis to form many prospermatogonia, which then develop into spermatogonia (2n) after birth. This is then followed by the growth phase, at which these spermatogonia divide via mitosis and differentiate to develop into primary spermatocytes (2n). During the maturation phase, primary spermatocytes enter meiosis I, which halves their chromosome number to form secondary spermatocytes (n). These cells then enter meiosis II to form spermatids (n). Spermatids then undergo metamorphosis and develop into mature sperm cells (n).

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