Lesson 1, Topic 1
In Progress

Testes

July 2, 2023

Structure and location

The paired testes are the gonads in males. They are located in the pouchlike scrotum, which is suspended outside of the body cavity behind the penis as you can see in Figure 23-1. This exposed location provides an environment about 1° C to 3° C cooler than normal core body temperature, an important requirement for the normal production and survival of sperm.

Each testis is a small, oval gland about 3.8 cm (1.5 inches) long and 2.5 cm (1 inch) wide. The testis is shaped like an egg that has been flattened slightly from side to side. Note in Figure 23-2 that each testis is surrounded by a tough, whitish membrane called the tunica albuginea. This membrane covers the testis and then enters the gland to form the many septa that divide it into sections or lobules.

FIGURE 23-2​Tubules of the testis and epididymis. ​The ducts and tubules are exaggerated in size. In the photograph, the testis is the egg-shaped mass in the center; note that the comma-shaped epididymis, seen on the left, is continuous with the vas (ductus) deferens.

As you can see in Figure 23-2, each lobule consists of a narrow but long and coiled seminiferous tubule. These coiled structures form the bulk of the testicular tissue mass. Small endocrine cells lying near the septa that separate the lobules can be seen in Figure 23-3. These are the interstitial cells of the testes that secrete the male sex hormone testosterone.

FIGURE 23-3​Testis tissue. ​Several seminiferous tubules surrounded by septa containing interstitial cells are shown.

Each seminiferous tubule is a long duct with a central lumen or passageway (see Figure 23-3). Sperm develop in the walls of the tubule and are then released into the lumen and begin their journey to the exterior of the body.

Testis functions

Spermatogenesis

Sperm production is also called spermatogenesis. From puberty on, the seminiferous tubules continuously form spermatozoa, or sperm. Although the number of sperm produced each day diminish with increasing age, most men continue to produce significant numbers throughout life.

The testes prepare for sperm production before puberty by increasing the numbers of sperm precursor (stem) cells called spermatogonia. These cells are located near the outer edge of each seminiferous tubule (Figure 23-4, A). Before puberty, spermatogonia increase in number by the process of mitotic cell division, which was described in Chapter 3. Recall that mitosis results in the division of a “parent” cell into two “daughter” cells, each identical to the parent and each containing a complete copy of the genetic material represented in the normal number of 46 chromosomes.

FIGURE 23-4​Spermatogenesis. ​A, Cross section of seminiferous tubule shows layers of cells undergoing the process of spermatogenesis. B, Diagram of spermatogenesis, including the role of meiosis in producing daughter sperm cells with half the number of nuclear chromosomes found in typical body cells.

The hypothalamus is a small but functionally important structure located near the base of the brain. One of its many functions, in both males and females, is to secrete gonadotropin-releasing hormone (GnRH), which then stimulates the anterior pituitary to secrete the gonadotropins follicle-stimulating hormone (FSH) and luteinizing hormone (LH). A gonadotropin is a hormone that has a stimulating effect on the gonads—the testes and ovaries.

You may want to review these roles of the hypothalamus and pituitary gland in Chapter 10 (p. 261) and Chapter 12 (p. 326). Also, peek ahead to Figure 23-16, where you will see the hypothalamus and pituitary depicted at the top of the diagram.

When a boy enters puberty, circulating levels of FSH cause a spermatogonium to undergo a unique series of cell divisions to produce sperm cells. When the spermatogonium undergoes cell division and mitosis under the influence of FSH, it produces two daughter cells. One of these cells remains as a spermatogonium and the other forms another type of cell called a primary spermatocyte. These primary spermatocytes then undergo another type of cell division characterized by meiosis, which ultimately results in sperm formation.

Note in Figure 23-4, B, that in meiosis two cell divisions occur (not one as in mitosis) and that four daughter cells (not two as in mitosis) are formed. The daughter cells are called spermatids. Unlike the two daughter cells that result from mitosis, the four spermatids each have only half the genetic material in its nucleus and half of the nuclear chromosomes (23) of other body cells. These spermatids then develop into spermatozoa.

Look again at the diagram of meiosis in Figure 23-4, B. It shows that each primary spermatocyte ultimately produces four sperm cells. Note that, in the portion of a seminiferous tubule shown in Figure 23-4, spermatogonia are found at the outer surface of the tubule, primary and secondary spermatocytes lie deeper in the tubule wall, and mature but immotile sperm are seen about to enter the lumen of the tube and begin their journey through the reproductive ducts to the exterior of the body.

Sperm

Sperm are among the smallest and most unusual cells in the body (Figure 23-5, A). The term sperm comes from Latin spermatozoan meaning “seed animal.” This is because, somewhat like a seed, each sperm cell is part of the reproductive process. 621And each sperm cell has a tail and moves independently somewhat like a microscopic animal.

FIGURE 23-5​Human sperm. ​A, Micrograph shows the heads and long, slender tails of many spermatozoa. B, Illustration shows the components of a mature sperm cell and an enlargement of a sperm head and midpiece.

All of the characteristics that a baby will inherit from its father at fertilization are contained in the nuclear chromosomes found in each sperm head. However, this genetic information from the father will unite with chromosomes contained in the mother’s ovum only if successful fertilization occurs.

The forceful ejection of fluid containing sperm, or ejaculation, into the female vagina during sexual intercourse is only one step in the long journey that these sex cells must make before they can meet and fertilize an ovum. To accomplish their task, these tiny packages of genetic information are equipped with tails for motility and enzymes to penetrate the outer membrane of the ovum when contact occurs with it.

The structure of a mature sperm is diagrammed in Figure 23-5, B. Note the sperm head containing the nucleus with its genetic material from the father. The sperm head is covered by the acrosome—a caplike structure containing enzymes that enable the sperm to break down the covering of the ovum and permit entry if contact occurs.

In addition to the head with its covering acrosome, each sperm has a midpiece and an elongated tail. Mitochondria in the midpiece release adenosine triphosphate (ATP) to provide energy for the tail movements required to propel the sperm and allow them to “swim” for relatively long distances through the female reproductive ducts. The tail is actually a flagellum, previously described in Chapter 3—see Figure 3-4 and Figure 3-5 (p. 49).

Production of testosterone

In addition to spermatogenesis, the other function of the testes is to secrete the male hormone testosterone. This function is carried on by the interstitial cells of the testes, not by their seminiferous tubules. The gonadotropin LH stimulates interstitial cells to develop and produce testosterone.

Testosterone serves the following general functions:

  1. Testosterone masculinizes. The various characteristics that we think of as “male” develop because of testosterone’s influence. For instance, when a young boy’s voice changes, it is testosterone that brings this about.
  2. Testosterone promotes and maintains the development of the male accessory organs (prostate gland, seminal vesicles, and so on).
  3. Testosterone has a stimulating effect on protein anabolism—it is an anabolic steroid hormone. Testosterone thus is responsible for the greater average muscular development and strength of the male.

A good way to remember testosterone’s functions is to think of it as “the masculinizing hormone” and the “anabolic hormone.” Go back and review the box Enhancing Muscle Strength in Chapter 9 (p. 227), which discusses the abuse of anabolic steroids by some athletes.

QUICK CHECK

  1. List the accessory organs of reproduction in men.
  2. In what specific structures of the gonad are the sperm produced?
  3. What is a gonadotropin?
  4. Summarize the general functions of testosterone.