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Testicles

Testicles, also called testis, (Latin: testis), are pair organs, the male reproductive gland, and the endocrine gland. The testicles produce sperm, male reproductive cell, and synthesize androgens, sex hormones, mainly testosterone, and a minimal amount of estrogen.

Testicles anatomy

The testicles are analogous to the ovaries in females. Both testicles are located next to each other in the scrotum, a saccular extension of the anterior abdominal wall; the left testicle is slightly inferior compared to the right one. During the development, the testicles are retroperitoneally located on the posterior wall of the abdominal wall. 

Before birth, the testicles descend to the scrotum. Afterward, the testicles are still connected to the abdomen by spermatic cords and, with the testicular ligament's help, are attached to the scrotum. The scrotal septum, a fibrous tissue layer, divides the scrotum into two compartments and acts as a wall between the testicles. 

The testicle is oval-shaped. Its length is 4-4.5 cm, thickness 2.5-3.5 cm. The weight of one testicle in an adult male is 20-30 grams. The testicle has two poles, two surfaces, and two margins.

Testicle poles:

  • Superior pole 
  • Inferior pole – has the scrotal ligament that serves to fix the testicle to the bottom of the scrotum.

Testicle surfaces:

  • Medial surface - flat
  • Lateral surface - more curved.

Testicle margins:

  • Anterior margin - facing towards, free
  • Posterior margin - posterior, connected to the epididymis.

Epididymis

The epididymis is attached to the posterior margin of the testicle. The epididymis has three parts: the head of the epididymis, the body of the epididymis, the tail of the epididymis. 

The head of the epididymis is the biggest part of the epididymis. Without any specific border, it changes into the body of the epididymis that is the middle part of the epididymis. The epididymis ends with its tail, from which the ductus deferens start. Between the testicle and lateral epididymis surface is the sinus of the epididymis

Scrotum

The scrotum is a cutaneous sac that protects the testicles. The scrotum has two layers: the skin and the dartos fascia. The dartos fascia has muscle fibers that contract due to cold, which can be seen as wrinkling of the scrotal skin and results in the testicles getting closer to the body. The scrotum contains the layers of the testicles. The scrotum houses the testicles and the epididymis, the beginning part of the ductus deferens, blood vessels, nerves, and the layers of the testicles and the epididymis.  

The layers of the testicles

Both the testicle and the epididymis are covered by a dense fibrous connective tissue capsule known as tunica albuginea. Above the tunica albuginea, the testicle and the epididymis have 5 (6th being the skin) layers formed by the exaggeration of the peritoneum and the elements of the abdominal wall.

The tunica vaginalis (the first layer) encloses the testicles partially. The tunica vaginalis is derived from the embryonic vaginal process. It has visceral and parietal lamina. The visceral lamina covers the surface of the testicles, the head of the epididymis, and the inferior part of ductus deferens growing together with tunica albuginea. It also covers the sinus of the epididymis. 

At the posterior epididymis margin, the visceral lamina changes into the parietal lamina. The parietal lamina is larger, and superiorly covers the distal portion of the spermatic cord. It then continues over the visceral lamina and covers the duct of the epididymis before blending with the visceral lamina. Between both laminas is the cavity of the tunica vaginalis filled with a small amount of serous fluid. The serous fluid prevents friction and allows the testicles to move in the scrotum. 

Internal spermatic fascia (the second layer) coalescences with the parietal lamina of the tunica vaginalis. 

Cremaster muscle (the third layer) is made from the muscle fibers of the transverse abdominal muscle and the abdominal internal oblique muscle. In humans, the cremaster muscle is poorly developed. Fascia cremasterica covers the cremaster muscle. Fascia is formed by the aponeurosis fibers of the abdominal external oblique muscle.

External spermatic fascia (the fourth layer) is a thin plate grown together with the previous fascia. 

Dartos fascia (the fifth layer) is a part of the scrotum layers that muscle fibers within it. Fascia is followed by skin (the sixth layer). 

Testicles microanatomy

The seminiferous tubules are surrounded by the stromal cells from the connective tissue. Stromal cells have Leydig cells within them. The tubules have seminiferous epithelium that has Sertoli cells and spermatogenic cells. The spermatogenic cells multiply continuously and, through spermatogenesis, differentiate into mature sperm. The testicles also have a blood-testis barrier between different stages of matured cells. The testicles have an extensive duct system and their way of temperature regulation

Leydig cells

Leydig cells, also known as interstitial cells, are found between the seminiferous tubules together with collagen fibers, nerve fibers, lymphatic vessels, and capillaries. Leydig cells synthesize testosterone under the influence of the luteinizing hormone. The luteinizing hormone works under orders from the gonadotropin-releasing hormone from the hypothalamus. Leydig cells also synthesize small amounts of estrogen

Leydig cells have an extensive network of the smooth endoplasmic reticulum; tubules connect some Golgi apparatus. Cytoplasmic inclusions called Reinke crystals are characteristics of Leydig cells and androgen-releasing tumor cells. Crystals' functions are unknown. Apart from crystals and endoplasmic reticulum, Leydig cells contain lipofuscin vesicles, lipochromic pigments, and lipid droplets. 

Sertoli cells 

Sertoli cells are the place for the development of sperm, located in the seminiferous tubules. Testosterone, secreted by the Leydig cells, together with follicle-stimulating hormone, acts on the Sertoli cells to stimulate spermatogenesis. Sertoli cells are large cells that have invaginated cellular nucleus, voluminous nucleoles, cisternae of rough endoplasmic reticulum, and smooth endoplasmic reticulum. 

In Sertoli cells cytoplasm also houses lysosomes, crystals, and residual bodies. Sertoli cells have tight connections between them, and they are put between a layer of spermatogonia and mature types of spermatogenic cells. 

Peritubular myoid cells

Peritubular myoid cells are smooth muscle cells that surround the seminiferous tubules. These cells are responsible for the contractions of the seminiferous tubule. This contractile function allows moving the spermatozoa and fluid to the rete testes. The peritubular myoid cells have an essential part in the maintenance of the spermatogonial stem cells, so the supply of spermatogonia never runs out. 

Blood-testis barrier 

The blood-testis barrier is a barrier formed by Sertoli cells between the immature and mature spermatogenic cells. The blood-testis barrier is made by the basal lamina and layers of fibrocytes and myofibroblasts. This barrier forbids arterial blood to reach more mature spermatogenic cells. Due to the barrier, the immune cells have never met these cells, so the immune system does not know about them. If the barrier gets broken, the immune cells would not recognize them as domestic, so the immune cells would start to attack and destroy them. 

Duct system 

Next to the testicle's posterior margin, the connective tissue from the tunica albuginea forms the mediastinum testis, a tissue network that penetrates the testicle's parenchyma. The mediastinum testis gives rise to the septa of the testis that divides the testicles parenchyma into lobules. Each testicle has 200-300 lobules. In each lobule are 2-3 convoluted seminiferous tubules in which spermatogenesis occurs. Moving towards the mediastinum testis, convoluted seminiferous tubules straighten and become straight seminiferous tubules

After entering the mediastinum testis, the straight seminiferous tubules converge, forming rete testis. 12-18 efferent ductules leave the rete testis and enter the head of the epididymis. At the posterior epididymis margin, the efferent duct bends down, collects the efferent ductules within it, and forms one epididymal duct. The epididymal ducts length is 5-8 meters. By bending, it fills the body and the tail of the epididymis and ends by turning into the ductus deferens

The ductus deferens continues from the epididymal duct and passes through the spermatic cord on its way to the pelvis. The ductus deferens transports sperm from the epididymis to the ejaculatory duct. The ductus deferens is joined by the duct of the seminal vesicle and forms the ejaculatory duct in front of the rectum and alongside the base of the bladder. The ejaculatory duct penetrates through the prostate and connects with the prostatic urethra.

Temperature regulation

The spermatogenesis is affected by temperature. A temperature little less than core body temperature enhances spermatogenesis. If the temperature is higher or lower than 33°C, spermatogenesis is less efficient. The temperature is maintained to be around 35°C. The cremasteric muscle is part of the spermatic cord. If the environment is cold, the muscle contracts, the cord gets shorter, and the testicles are moved closer to the body to maintain warmth. If the surrounding is warm, the muscle relaxes, and the testicles are lowered from the body to allow them to cool. 

Vasculature and innervation of the testicles

Blood supply 

The testicles receive their supply from the testicular arteries, direct abdominal aorta branches, below the renal arteries. The testicular arteries reach the scrotum and the testicles through the inguinal canal. Together with other external genitalia, the scrotum is supplied by the internal pudendal artery, a branch from the internal iliac artery. 

Additionally, the testicles have a collateral blood supply that is important if the testicular arteries, for some reason, get obstructed. In case of obstruction, these arteries will keep the testes functioning and allow their survival. The collateral blood supply comes from the cremasteric artery, a branch of the inferior epigastric artery, and the artery to ductus deferens, a branch of the inferior vesical artery.

Venous drainage 

Venous drainage happens through the spermatic veins that together form the pampiniform plexus. The testicular arteries are surrounded by the pampiniform plexus. This formation is responsible for temperature regulation - the veins, forming the plexus, cool the arteries, so the arteries' blood temperature drops. 

The veins are leaving the pampiniform plexus travel through the inguinal canal in front of the ductus deferens. The veins connect into three to four veins that continue to the abdomen right before exiting the inguinal canal. After that, these veins merge to create one testicular vein on each side. 

The right testicular vein enters the inferior vena cava, while the left testicular vein drains into the left renal vein. Both veins enter their target veins by specific angles - the right testicular vein enters at an acute angle but the left vein at a right angle. It is essential to know these angles because the testicular veins can get obstructed, which can cause blood flow blockage in the testes. 

Lymphatic drainage 

The lymph from the testicles is drained to the pre-aortic and lateral aortic lymph nodes. The scrotum's lymph is drained to the inguinal lymph nodes

Innervation 

The testicular plexus, nearby the testicular artery, gives parasympathetic and sympathetic fibers. So the testicles are innervated by the autonomic nerve plexus that reaches the testicles through the spermatic cord. 

The lumbar splanchnic nerves and the superior and the inferior hypogastric plexuses with their sympathetic branches innervate the ductus deferens, seminal glands, and ejaculatory duct. The pelvic splanchnic nerves supply the testicles with parasympathetic innervation. 

Functions of the testicles

The cells in the testicles are responsible for the synthesis of androgens and affecting spermatogenesis. The testicles are a place where spermatogenesis occurs, resulting in the development of mature sperm.

Spermatogenesis

Spermatogenesis is a process during which the germ cells develop into mature sperm; it happens in the seminiferous tubules. Spermatogenesis has three stages:

  1. Spermatocytogenesis – the proliferation of the spermatogonia
  2. Spermatidogenesis – reduction of the number of chromosomes
  3. Spermiogenesis - development of the mature sperm.

During spermatocytogenesis, a spermatogonium is dividing mitotically and produces two diploid cells called primary spermatocytes. The primary spermatocyte during subsequent division produces secondary spermatocytes that will eventually divide into spermatids. The division from a spermatogonium to a spermatid is incomplete

The cells keep connections to one and another with the help of the cytoplasm. To stop the running out of the spermatogonia supply, not all spermatogonia divide into spermatocytes. To avoid this, spermatogonial stem cells divide mitotically to produce copies of themselves.

Spermatidogenesis is a process during which secondary spermatocytes develop into spermatids.

Spermiogenesis is the last stage of spermatogenesis, during which spermatids change into mature spermatozoa. The spermatids contain the nucleus, Golgi apparatus, centriole, mitochondria, and tail. Testosterone is responsible for the maturation of spermatozoa. Testosterone removes unnecessary cytoplasm and organelles. The maturation happens in close contact with Sertoli cells. The mature spermatozoa are released from the Sertoli cells into the lumen of the seminiferous tubule.

A healthy man produces approximately 200 million sperm every day, from puberty to old age. The spermatogenesis until a mature sperm is developed takes 70 days, following 12-21 days of transportation to the ejaculatory duct. Each spermatogonium gives rise to 64 sperm. Each ejaculate has around 200 million sperm.

Testosterone

The testicles synthesize the male primary sex hormone - testosterone. Testosterone is responsible for the initiation of spermatogenesis and the development of secondary sexual characteristics. The role of testosterone in males can be divided based on the life period:

  • Intrauterine period - while still in the uterus, the testosterone already starts its work. Testosterone is responsible for developing male phenotype, the development of male reproductive organs, including the descending of the testicles.
  • During puberty - testosterone affects the development of male secondary sex characteristics, hypertrophy of larynx that causes deepening of the voice, the development of seminal vesicles and prostate, and initiation of sperm production, an increase of muscle mass and skin thickness, growth of pubic and axillary hairs, and fusion of epiphyseal plates in long bones. 
  • During adulthood - testosterone has metabolic and reproductive effects. Metabolic effects - increased metabolic rate, increased red blood cell number, muscle mass maintenance, and bone density. Reproductive effects - maintenance of spermatogenesis, secondary sex characteristics, libido, and feedback inhibition of hypothalamic gonadotropin-releasing hormone secretion. 

Estrogens

Secreted by Leydig cells, estrogen has an effect on the brain for the sex drive and behavior. Estrogen also helps the negative feedback effects on the hypothalamo-pituitary-testicular axis. During puberty, estrogen takes part in epiphyseal closure and maintaining bone density. 

Most common testicular diseases

The testicles can be affected by many conditions. The most common disease are cryptorchidism, torsion, testicular cancer, epididymitis, and hypogonadism.

Cryptorchidism

Cryptorchidism occurs when the testicle has not descended and moved into its proper position in the scrotum before birth. Cryptorchidism is uncommon among adult males but common among premature babies. The cause is unknown, but there known risk factors:

  • low birth weight
  • premature birth
  • mother using alcohol during pregnancy
  • intrauterine growth restriction due to different condition, e.g., Down syndrome
  • mother smoking during pregnancy.

The main sign is not seeing or feeling the testicle in the scrotum. This is usually seen during the examination after birth. 

If the testicle is not located correctly, it could lead to testicular cancer, fertility problems, testicular torsion, trauma, and inguinal hernia. The earlier the treatment is done, the less risk for developing complications. 

Most of the time, the testicle goes into its proper position on its own during the first months. In case that is not happening, surgery needs to be done. The surgery should be done until the boy is 18 months old. 

Testicular torsion

The testicles are fixated with the help of the spermatic cord. The blood vessels supplying the testicle go through the spermatic cord. If the spermatic cord gets twisted, it stops the blood supply to the testicle. The testicular torsion mainly affects teens or men in their twenties, but it can occur at other ages as well. The torsion's primary cause is the way the testicles sit in the scrotum; something male is born with and cannot affect. Even though sometimes an injury or exercise can cause torsion. 

The symptoms are severe, sudden pain in the testis area, enlargement of the testicle, tenderness, nausea, and vomiting.

Testicular torsion is an immediate medical problem that acquires the doctor's attention. The only way to treat torsion is with surgery. The faster the surgery is done, the better outcome. If the surgery is delayed, it can lead to atrophy due to no blood supply and removing the testicle. 

Testicular cancer

Testicular cancer is a rare cancer type but common in males between the ages of 15-35. The exact cause is not known, but as for every cancer, it has risk factors:

  • cryptorchidism
  • abnormal testis developmet
  • family history
  • age - 15-35
  • race - more common in white men.

Symptoms for testicular cancer are:

  • a lump or enlargement
  • heaviness
  • pain in the abdomen or groin
  • a sudden collection of fluid in the scrotum
  • pain in the testis
  • back pain
  • lower leg swelling

The treatment can be surgery, radiotherapy, or chemotherapy. Testicular cancer is highly treatable

Epididymitis

Epididymitis is an inflammation of the epididymis. The cause most commonly is sexual transmission infection due to gonorrhea or chlamydia. The other bacteria from the urinary tract can spread to the epididymis. Sometimes urine backflow into the epididymis can occur and cause inflammation. Also, groin injury can cause epididymitis. 

Symptoms resemble classical inflammation signs:

  • a swollen, red, or warm scrotum
  • ain in the testis
  • painful urination
  • discharge from the penis
  • discomfort or pain in the lower abdomen
  • blood in the semen
  • fever

If bacteria cause epididymitis, then treatment always is antibiotics. Sometimes surgery may be needed if an abscess has formed. Men should have bed rest, apply cold packs, avoid lifting weights. 

Hypogonadism

Hypogonadism happens when the testicles do not produce enough testosterone. Hypogonadism can have two types:

  • Primary hypogonadism - problems in the testicles. Caused by Klinefelter's syndrome, cryptorchidism, mumps orchitis, testicular injuries, chemotherapy, or radiotherapy.
  • Secondary hypogonadism - the testicles are functioning normally, but the problem occurs in the hypothalamus or pituitary gland that affects the hormone signaling.

If hypogonadism happens before puberty, puberty does not occur at all. If hypogonadism happens in adulthood, symptoms are:

  • reduced libido
  • depression
  • erection problems
  • infertility
  • decrease of muscle mass
  • decreased beard growth 
  • symptoms similar to women menopause - hot flashes, mood swings, irritability.

The primary treatment is testosterone replacement therapy. Sometimes human chorionic gonadotropin can be prescribed to stimulate the luteinizing hormone receptors to promote testosterone synthesis. 

Summary on the testicles

How do you know if you have testicular problems?

The signs for testicular problems can be pain in the lower abdomen, groin, or in the testicle, enlargement of the testicle, tenderness, a lump, heaviness, a sudden collection of fluid in the scrotum, back pain, a swollen, red, or warm scrotum, painful urination, discharge from the penis

What is the normal size of a man's testicles?

The testicle length is 4-4.5 cm, thickness 2.5-3.5 cm. The weight of one testicle in an adult male is 20-30 grams. 

What are testicles functions?

The testicles produce sperm, male reproductive cell and synthesize androgens, sex hormones, mainly testosterone, and a minimal amount of estrogen. Testosterone is responsible for the initiation of spermatogenesis and the development of secondary sexual characteristics. 

What is usually the first sign of testicular cancer?

The first signs usually are a small lump or hardness, enlarged testicle, pain, and tenderness. 

Can testicular torsion fix itself?

Testicular torsion cannot fix itself. Testicular torsion is an immediate medical emergency. If medical help is delayed, it can lead to testicular atrophy. 

How bad is testicular torsion pain?

Testicular torsion pain is always described as sudden and extremely severe.