Testes Anatomy, Function, and Associated Conditions

The testes, or testicles, are two egg-shaped sex organs that play an important role in the male reproductive system. The testes are where sperm cells are produced and are also responsible for the production of the sex hormone testosterone.

The testes are housed in a pouch of skin beneath the penis called the scrotum. Their location outside the abdomen ensures that the testes remain at the ideal temperature for sperm cells to develop. Although the testes can produce sperm and testosterone throughout a person's lifetime, they are vulnerable to injury can medical conditions that can cause infertility.

Anatomy of the Testes

Most males are born with two testes (testicles). These are soft, egg-shaped organs located inside the scrotum. One testicle is referred to as a testis.

Each testicle is surrounded by three membranes: the inner tunica vasculosa (which delivers blood to the testes), the middle tunica albuginea (which stabilizes the testes and adjacent structures), and the outer tunica vaginalis (which helps lubricate the testes inside the scrotum).

Within the scrotum, the testes are suspended from the abdomen by the spermatic cord. This is a tight grouping of blood vessels, nerves, and ducts that support the health and function of the testes.

The testes are made up of several lobes that contain a network of narrow tubes called seminiferous tubules. These are where sperm are produced in a process known as spermatogenesis. Situated between seminiferous tubules are Leydig cells that secrete testosterone crucial to the development of sperm cells.

As the sperm cells start to mature, they move through these tubules until they reach a wider conduit called the rete testes.

The immature cells are then passed to a tightly coiled tube on the outside of each testicle called the epididymis. This is where sperm cells are stored and complete maturation.

In adult males, the testes are around 2 to 3 centimeters wide and roughly 3 to 5 centimeters long. The testes increase in size through adulthood and then decrease in size later in life due to the natural decline in testosterone.

Function of the Testes

The testes have two primary functions: the production of testosterone and the creation of mature sperm cells.

Production of Testosterone

Testosterone is the hormone produced mainly by the testes in males and the ovaries in females. It is the main male sex hormone (androgen) whose function varies at different stages in life:

• During fetal development, a sex-related gene on the Y chromosome initiates the development of the testes, which immediately start producing testosterone. The amount secreted further triggers the development of the internal male organs (like the prostate and urethra) and the external male organs (like the penis and scrotum).
• During puberty, testosterone is responsible for the sudden growth spurt and the development of male secondary characteristics like facial and body hair, an Adam's apple, a deeper voice, and increased muscle mass. It is also responsible for libido (sex drive).
• During adulthood, testosterone continues to bolster sex drive as well as general feelings of well-being. Testosterone is also needed to maintain healthy bones and to stimulate the production of red blood cells via a process known as erythropoiesis.

Spermatogenesis

The second function of the testes is to produce sperm. Spermatogenesis is a continuous, life-long process that occurs in three steps:

1. A germ cell (meaning a "blank" reproduction cell that can either become a sperm cell or an egg) starts rapidly dividing in the walls of the seminiferous tubule, eventually becoming an immature cluster of cells known as spermatogonia.
2. As cell division continues, the undifferentiated cells will start to move from the wall of the seminiferous tubule and form a ball-shaped spermatid.
3. Under the influence of testosterone, the cells of the spermatid will start to differentiate into the head, neck, body, and tail of a fully mature sperm cell (spermatozoa).

Unlike females, who only produce a limited number of eggs over a lifetime, males can produce millions of sperm each day. It then takes several months for the sperm cells to mature enough to be functional. The maturation starts in the testes but mostly takes place inside the epididymis.

Without ample testosterone, sperm cells cannot mature as they should, resulting in infertility.

Congenital Conditions

There are a number of congenital conditions that can affect either the location or appearance of a testicle. Congenital conditions are those that occur during fetal development.

Cryptorchidism

Cryptorchidism is a condition where one or both testicles have not moved into the scrotum prior to birth. Also known as an undescended testicle, it is one of the most common congenital conditions affecting male babies.

Cryptorchidism in infants is not considered a medical emergency, and the testicle will often descend on its own without treatment.

If the testicle has not descended within the first few months, a surgical procedure known as orchiopexy may be used to repair the abnormality. This is because cryptorchidism can expose the testicle to higher body temperatures within the abdomen, increasing the risk of infertility and testicular cancer.

Retractile testis is a similar condition in which a testicle moves back and forth from the scrotum to the abdomen. As long as the testes spend most of their time in the scrotum, it is not considered as problematic as undescended testes and may not require treatment.

Polyorchidism

In addition to undescended testicles, there are times when a male baby may be born with no testicles, one testicle, or more than two testicles.

When a person is born with more than two testicles, the condition is referred to as polyorchidism. People with polyorchidism most commonly have three testicles but can have as many as five. It is a very rare condition with fewer than 200 reported cases.

Transverse Testicular Ectopia

Transverse testicular ectopia, or crossed testicular ectopia, is an equally rare condition. It occurs when both testicles descend to the same side of the scrotum.

Transverse testicular ectopia usually occurs alongside other congenital conditions such as hypospadias (in which the opening of the urethra is not at the tip of the penis).

Congenital Hydrocele

Congenital hydrocele is the accumulation of watery fluid around one or both testes, causing the scrotum and groin area to swell. While the swelling may look scary, it is usually not a problem and tends to resolve by the time a child is two years old.

Hydrocele occurs when a testicle descends into the scrotum along with some of the lining of the abdomen. But, rather than shriveling up and closing like it's supposed to, the lining may have a tiny opening that allows body fluids to leak into the scrotum.

In most boys, the opening will eventually close and be completely sealed off by age two. Whatever fluid is left in the testicle will eventually be reabsorbed by the body.

However, if the swelling is especially severe, minor surgery can be needed to drain the scrotum. Left untreated, the extreme swelling can cause damage to the testes and adjacent structures, affecting future fertility. Hydrocele that continues beyond the age of two may also need treatment.

Associated Medical Conditions

Beyond congenital disorders affecting the testes, there are also medical conditions that can affect the testes later in life.

Orchitis

Orchitis refers to inflammation of the testes. In sexually active young males, it is most commonly due to sexually transmitted infections like chlamydia and gonorrhea. Pain and swelling are the central features. In older males and children, orchitis may be caused by a bacterial or viral infection.

Among older children and adults with mumps, orchitis occurs in up to 40% of cases, typically bilaterally (meaning both sides). Of those affected, 30% will have impaired fertility or infertility.

Varicocele

Varicocele is another common condition affecting the testes. It involves an abnormal enlargement of veins within the scrotum (similar to varicose veins) that can starve the testicles of the blood needed to produce healthy sperm.

In addition to scrotal discomfort and pain, varicoceles can decrease testosterone production, leading to reduced fertility. Studies suggest that up to 15% of males and more than a third of males with infertility are diagnosed with varicocele.

Testicular Torsion

Testicular torsion occurs when the testicle rotates inside the scrotum, cutting off the blood supply. Symptoms include pain and swelling of the testicle, typically sudden and severe. Nausea, vomiting, and an abnormal elevation of the testicle are also common.

Testicular torsion can happen to males of any age but is most common in 12- to 18-year-olds. It can happen after strenuous exercise, while sleeping, or after a blow or injury to the scrotum.

Males with a "bell clapper deformity" (meaning a testicle that is unanchored by ligaments to the back of the scrotum) are especially vulnerable to testicular torsion.

Testicular Cancer

Testicular cancer affects thousands of males each year in the United States. Fortunately, the disease is highly curable, and death rates are low. Unlike other cancers, it is more likely to affect younger males than older males.

Symptoms of testicular cancer include a painless lump on a testicle, a dull ache in the groin or abdomen, a feeling of heaviness in the scrotum, and back pain. Risk factors include having an undescended testicle and a family history of testicular cancer.

According to the National Cancer Institute, an estimated 9,750 men in the United States were diagnosed with testicular cancer in 2023 with roughly 500 deaths attributed to the disease.

Testicular cancer accounts for less than 1% of all cancers in the U.S. with a relative five-year survival rate of 95% (meaning that 95 out of 100 men will live for at least five years after their diagnosis).

Diagnosis

The diagnosis of testicle problems usually involves a physical examination and imaging studies to look for abnormalities within or surrounding the testicles.

The physical exam involves palpation (light touching) to detect lumps, swelling, or other abnormalities on or around the testes. The healthcare provider may also manipulate your leg, pelvis, or torso to check for pain or the abnormal movement of a testicle.

Ultrasound is the most commonly used tool to examine the testes. This non-invasive test uses sound waves to look inside the scrotum for any abnormalities. It can also check if the blood flow is normal or for signs of testicular torsion, testicular cancer, and varicocele.

Magnetic resonance imaging (MRI) uses powerful magnetic and radio waves to create highly detailed images of soft tissues. An MRI is the test of choice for diagnosing cryptorchidism. It can also help differentiate benign (noncancerous) lumps from malignant (cancerous) ones.

Summary

The testes (testicles) are two egg-shaped organs housed within the scrotum that play a central role in male fertility and function. They are tasked with producing sperm as well as the sex hormone testosterone.

There are several congenital conditions that can affect the testes, including cryptorchidism (undescended testicle) and hydrocele. The testicles are also vulnerable to other medical conditions such as orchitis, varicocele, testicular torsion, and testicular cancer.