Testicle: Wikis


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Diagram of male (human) testicles
Latin testis
Gray's subject #258 1236
Artery Testicular artery
Vein Testicular vein, Pampiniform plexus
Nerve Spermatic plexus
Lymph Lumbar lymph nodes
Dorlands/Elsevier Testicle

The testicle (from Latin testiculus, diminutive of testis, meaning "witness" [of virility],[1] plural testes) is the male generative gland in animals.



The etymology of the word is based on Roman law. The Latin word "testis", witness, was used in the firmly established legal principle "Testis unus, testis nullus" (one witness [equals] no witness), meaning that testimony by any one person in court was to be disregarded unless corroborated by the testimony of at least another. This led to the common practice of producing two witnesses, bribed to testify the same way in cases of lawsuits with ulterior motives. Since such "witnesses" always came in pairs, the meaning was accordingly extended, often in the diminutive (testiculus, testiculi).[citation needed]

Another theory says that testis is influenced by a loan translation, from Greek parastatēs "defender (in law), supporter" that is "two glands side by side".[2]

Anatomy and physiology


Like the ovaries to which they are homologous, testes are components of both the reproductive system (being gonads) and the endocrine system (being endocrine glands). The respective functions of the testes are:

Both functions of the testicle, sperm-forming and endocrine, are under control of gonadotropic hormones produced by the anterior pituitary:

External appearance

Almost all healthy male vertebrates have two testes. They are typically of similar size, although in sharks, that on the right side is usually larger, and in many bird and mammal species, the left may be the larger. The primitive jawless fish have only a single testis, located in the midline of the body, although even this forms from the fusion of paired structures in the embryo.[3]

In mammals, the testes are often contained within an extension of the abdomen called the scrotum. In mammals with external testes it is most common for one testicle to hang lower than the other. While the size of the testicle varies, it is estimated that 21.9% of men have their higher testicle being their left, while 27.3% of men have reported to have equally positioned testicles.[4] This is due to differences in the vascular anatomical structure on the right and left sides.

In healthy European adult human males, average testicular volume is 18 cm³ per testis, with normal size ranging from 12 cm³ to 30 cm³[5]. The average testicle size after puberty measures up to around 2 inches long, 0.8 inches in breadth, and 1.2 inches in height (5 x 2 x 3 cm). Measurement in the living adult is done in two basic ways:

  • comparing the testicle with ellipsoids of known sizes (orchidometer).
  • measuring the length, depth and width with a ruler, a pair of calipers or ultrasound imaging.

The volume is then calculated using the formula for the volume of an ellipsoid: 4/3 π × (length/2) × (width/2) × (depth/2).

Internal structure

Transverse section through the left side of the scrotum and the left testis.
Cross section of rabbit testis, magnified 40 times.

Duct system

Under a tough membraneous shell, the tunica albuginea, the testis of amniotes, and some teleost fish, contains very fine coiled tubes called seminiferous tubules. The tubules are lined with a layer of cells (germ cells) that from puberty into old age, develop into sperm cells (also known as spermatozoa or male gametes). The developing sperm travel through the seminiferous tubules to the rete testis located in the mediastinum testis, to the efferent ducts, and then to the epididymis where newly-created sperm cells mature (see spermatogenesis). The sperm move into the vas deferens, and are eventually expelled through the urethra and out of the urethral orifice through muscular contractions.

Amphibians and most fish do not possess seminiferous tubules. Instead, the sperm are produced in spherical structures called sperm ampullae. These are seasonal structures, releasing their contents during the breeding season, and then being resorbed by the body. Before the next breeding season, new sperm ampullae begin to form and ripen. The ampullae are otherwise essentially identical to the seminiferous tubules in higher vertebrates, including the same range of cell types.[3]

Primary Cell Types

Within the seminiferous tubules
  • Sertoli cells - the true epithelium of the seminiferous epithelium, critical for the support of germ cell development into spermatozoa. Sertoli cells secrete inhibin[7].
Between tubules (interstitial cells)
  • Also present are:

Blood supply and lymphatic drainage

Blood supply and lymphatic drainage of the testes and scrotum are distinct:


Many anatomical features of the adult testis reflect its developmental origin in the abdomen. The layers of tissue enclosing each testicle are derived from the layers of the anterior abdominal wall. Notably, the cremasteric muscle arises from the internal oblique muscle.

The blood-testis barrier

Large molecules cannot pass from the blood into the lumen of a seminiferous tubule due to the presence of tight junctions between adjacent Sertoli cells. The spermatogonia are in the basal compartment (deep to the level of the tight junctions) and the more mature forms such as primary and secondary spermatocytes and spermatids are in the adluminal compartment.

The function of the blood-testis barrier (red highlight in diagram above) may be to prevent an auto-immune reaction. Mature sperm (and their antigens) arise long after immune tolerance is established in infancy. Therefore, since sperm are antigenically different from self tissue, a male animal can react immunologically to his own sperm. In fact, he is capable of making antibodies against them.

Injection of sperm antigens causes inflammation of the testis (autoimmune orchitis) and reduced fertility. Thus, the blood-testis barrier may reduce the likelihood that sperm proteins will induce an immune response, reducing fertility and so progeny.

Temperature regulation

The testes work best at temperatures slightly less than core body temperature. The spermatogenesis is less efficient at lower and higher temperatures. This is presumably why the testes are located outside the body. There are a number of mechanisms to maintain the testes at the optimum temperature.

Cremasteric muscle

The cremasteric muscle is part of the spermatic cord. When this muscle contracts, the cord is shortened and the testicle is moved closer up toward the body, which provides slightly more warmth to maintain optimal testicular temperature. When cooling is required, the cremasteric muscle relaxes and the testicle is lowered away from the warm body and is able to cool. This phenomenon is known as the cremasteric reflex. It also occurs in response to stress (the testicles rise up toward the body in an effort to protect them in a fight). There are persistent reports that relaxation indicates approach of orgasm.[citation needed] There is a noticeable tendency to also retract during orgasm.

The testicles can also be lifted voluntarily using the pubococcygeus muscle, which partially activates related muscles. This can sometimes be triggered by tightening or sucking in the stomach or abdomen.

This movement will also happen spontaneously in some men when they unintentionally contract their abdominal or PC muscles.


There are two phases in which the testes grow substantially; namely in embryonic and pubertal age.


During mammalian development, the gonads are at first capable of becoming either ovaries or testes.[8] In humans, starting at about week 4 the gonadal rudiments are present within the intermediate mesoderm adjacent to the developing kidneys. At about week 6, sex cords develop within the forming testes. These are made up of early Sertoli cells that surround and nurture the germ cells that migrate into the gonads shortly before sex determination begins. In males, the sex-specific gene SRY that is found on the Y-chromosome initiates sex determination by downstream regulation of sex-determining factors, (such as GATA4, SOX9 and AMH), which leads to development of the male phenotype, including directing development of the early bipotential gonad down the male path of development.


The testes grow in response to the start of spermatogenesis. Size depends on lytic function, sperm production (amount of spermatogenisis present in testis), interstitial fluid, and Sertoli cell fluid production. After puberty, the volume of the testes can be increased by over 500% as compared to the pre-pubertal size.[citation needed] Testicles are fully descended before one reaches puberty.


External testes

The basal condition for mammals is to have internal testes. Only the Boreoeutherian land mammals, the large group of mammals that includes humans, have externalized testes. Their testes function best at temperatures lower than their core body temperature. Their testes are located outside of the body, suspended by the spermatic cord within the scrotum. The testes of the non-boreotherian mammals such as the monotremes, armadillos, sloths, elephants remain within the abdomen.[9] There are also some Boreoeutherian mammals with internal testes, such as the rhinoceros.

Marine boreotherian mammals such as whales and dolphins, also have internal testes, but it has recently been shown (e.g., for dolphins) that they use elaborate vascular networks to provide the necessary temperature lowering for optimum function. As external testes would increase drag, many boreotherian aquatic mammals have internal testes which are kept cool by special circulatory systems that cool the arterial blood going to the testes by placing the arteries near veins bringing cooled venous blood from the skin.

There are several hypotheses why most boreotherian mammals have external testes which operate best at a temperature that is slightly less than the core body temperature, e.g. that it is stuck with enzymes evolved in a colder temperature due to external testes evolving for different reasons, that the lower temperature of the testes simply is more efficient for sperm production.

1) More efficient. The classic hypothesis is that cooler temperature of the testes allows for more efficient fertile spermatogenesis. In other words, there are no possible enzymes operating at normal core body temperature that are as efficient as the ones evolved, at least none appearing in our evolution so far.

The early mammals had lower body temperatures and thus their testes worked efficiently within their body. However it is argued that boreotherian mammals have higher body temperatures than the other mammals and had to develop external testes to keep them cool. It is argued that those mammals with internal testes, such as the monotremes, armadillos, sloths, elephants, and rhinoceroses, have a lower core body temperatures than those mammals with external testes.

However, the question remains why birds despite having very high core body temperatures have internal testes and did not evolve external testes.[10] It was once theorized that birds used their air sacs to cool the testes internally, but later studies revealed that birds' testes are able to function at core body temperature.[10].

Some mammals which have seasonal breeding cycles keep their testes internal until the breeding season at which point their testes descend and increase in size and become external[11].

2) Irreversible adaptation to sperm competition. It has been suggested that the ancestor of the boreoeutherian mammals was a small mammal that required very large testes (perhaps rather like those of a hamster) for sperm competition and thus had to place its testes outside the body.[12] This led to enzymes involved in spermatogenesis, spermatogenic DNA polymerase beta and recombinase activities evolving a unique temperature optimum, slightly less than core body temperature. When the boreoeutherian mammals then diversified into forms that were larger and/or did not require intense sperm competition they were stuck with enzymes that operated best at cooler temperatures and had to keep their testes outside the body. This position is made less parsimonious by the fact that the kangaroo, a non-boreoeutherian mammal, has external testicles. The ancestors of kangaroos might, separately from boreotherian mammals, have also been subject to heavy sperm competition and thus developed external testes, however, kangaroo external testes are suggestive of a possible adaptive function for external testes in large animals.

3) Protection from abdominal cavity pressure changes. One argument for the evolution of external testes is that it protects the testes from abdominal cavity pressure changes caused by jumping and galloping.[13]

Testicular size

Testicular size as a proportion of body weight varies widely. In the mammalian kingdom, there is a tendency for testicular size to correspond with multiple mates (e.g., harems, polygamy). Production of testicular output sperm and spermatic fluid is also larger in polygamous animals, possibly a spermatogenic competition for survival. The testes of the right whale are likely to be the largest of any animal, each weighing around 500 kg (1,100 lb).[14] Testis weight also varies in seasonal breeders like deer and horses. The change is related to changes in testosterone production.

Health issues

Physical injury

  • The testicles are well-known to be very sensitive to impact and injury. The pain involved travels up from each testicle into the abdominal cavity, via the spermatic plexus, which is the primary nerve of each testicle.
  • Testicular torsion, a medical emergency, which can lead to necrosis of the testis in as little as 30 minutes[citation needed]
  • Testicular rupture, a medical emergency, caused by blunt force impact, sharp edge, or piercing impact to one or both testicles, which can lead to necrosis of the testis in as little as 30 minutes

Diseases and conditions that affect the testes

Some prominent conditions and differential diagnoses include:

  • Varicocele, swollen vein(s) from the testes, usually affecting the left side,[15] the testis usually being normal
  • Hydrocele testis, swelling around testes caused by accumulation of clear liquid within a membranous sac, the testis usually being normal
  • Certain inherited conditions involving mutations in key developmental genes also impair testicular descent, resulting in abdominal or inguinal testes which remain nonfunctional and may become cancerous. Other genetic conditions can result in the loss of the Wolffian ducts and allow for the persistence of Müllerian ducts.
  • Bell Clapper Deformity is a deformity in which the testicle is not attached to the scrotal walls, and can rotate freely on the spermatic cord within the tunica vaginalis. This deformity has been linked to Testicular torsion.

Effects of exogenous hormones

To some extent, it is possible to change testicular size. Short of direct injury or subjecting them to adverse conditions, e.g., higher temperature than they are normally accustomed to, they can be shrunk by competing against their intrinsic hormonal function through the use of externally administered steroidal hormones. Steroids taken for muscle enhancement (especially anabolic steroids) often have the undesired side effect of testicular shrinkage.

Similarly, stimulation of testicular functions via gonadotropic-like hormones may enlarge their size. Testes may shrink or atrophy during hormone replacement therapy or through chemical castration.

In all cases, the loss in testes volume corresponds with a loss of spermatogenesis.

As food

Testicles from some animals comprise a dish, sometimes called Rocky Mountain oysters.[16]


See also


  1. ^ Blackwell Synergy
  2. ^ The American Heritage Dictionary of the English Language, Fourth Edition
  3. ^ a b Romer, Alfred Sherwood; Parsons, Thomas S. (1977). The Vertebrate Body. Philadelphia, PA: Holt-Saunders International. pp. 385-386. ISBN 0-03-910284-X. 
  4. ^ Scrotal Asymmetry: Right-Left and the scrotum in male sculpture"[1]" By I. C. Manus
  5. ^ Andrology: Male Reproductive Health and Dysfunction"[2]" By E. Nieschlag, Hermann M. Behre, H. van. Ahlen
  6. ^ Histology, A Text and Atlas by Michael H. Ross and Wojciech Pawlina, Lippincott Williams & Wilkins, 5th ed, 2006
  7. ^ Skinner M, McLachlan R, Bremner W (1989). "Stimulation of Sertoli cell inhibin secretion by the testicular paracrine factor PModS". Mol Cell Endocrinol 66 (2): 239–49. doi:10.1016/0303-7207(89)90036-1. PMID 2515083. 
  8. ^ Online textbook: "Developmental Biology" 6th ed. By Scott F. Gilbert (2000) published by Sinauer Associates, Inc. of Sunderland (MA).
  9. ^ http://www.uwyo.edu/wjm/Repro/spermat.htm
  10. ^ a b http://www.biolreprod.org/cgi/reprint/56/6/1570.pdf BIOLOGY OF REPRODUCTION 56, 1570-1575 (1997)- Determination of Testis Temperature Rhythms and Effects of Constant Light on Testicular Function in the Domestic Fowl (Gallus domesticus)
  11. ^ Ask a Biologist Q&A / Human sexual physiology - good design?
  12. ^ 'The Human Body as an Evolutionary Patchwork' by Alan Walker, Princeton.edu - RichardDawkins.net
  13. ^ Newscientist.com - bumpy-lifestyle-led-to-external-testes
  14. ^ Crane, J.; Scott, R. (2002). "Eubalaena glacialis". Animal Diversity Web. http://animaldiversity.ummz.umich.edu/site/accounts/information/Eubalaena_glacialis.html. Retrieved 2009-05-01. 
  15. ^ Varicocele
  16. ^ Reviving a very delicate delicacy

Simple English

[[File:|thumb|300px|Human male anatomy]] Testicles are parts that are found on the bodies of male creatures. Male mammals, including men, have two testicles, supported in a sac of skin below the penis called the scrotum. Along with the penis, testicles are called reproductory organs or "sex organs". Only males have testicles; females have ovaries.

Testicles are a type of organ called glands. Like other glands, testicles make chemical substances, called hormones that keep the body working. Testicles also make sperm which can join with ova to make new life. (Read more about this in "What do testicles do?")

Most glands, like women's ovaries, are inside the body, but testicles are outside the main part of the body. This is because the testicles work better if they are cooler than the inside of the body. In cold weather, testicles pull up nearer to the body to keep warm.


Description of testicles

The testicles are firm, oval-shaped glands. Most testicles match in size, but some testicles are much larger or smaller than others. Normal sized testicles are between 14 cm³ to 35 cm³.

Nearly all men have one testicle that hangs lower than the other one. Scientists think this may be so that they do not hit against each other. It can be either the left or the right that hangs lower, but in 85% of men, it is the left one. The left testicle usually descends first and this may affect which one hangs lower.

The testicles hang in the scrotum by the spermatic cord. They are outside the body because they need to be cooler than the temperature inside the body, in order to make sperm. If a man's testicles get cold, they are automatically pulled closer to the body. The spermatic cord is tightened by the cremasteric muscle. When this muscle contracts (pulls tighter), the cord gets shorter, which pulls the testicles closer to the body. If the testicles are too warm, the cremasteric muscle relaxes (gets longer.) This lowers the testicles to keep them cooler. This is how the testicles are kept at the right temperature. This is called the cremasteric reflex. (A reflex is something your body automatically does without you thinking about it.)

There are other uses for the cremasteric reflex apart from making the testicles the right temperature. Stress can also make the cremasteric reflex work. If a male human or mammal gets into a fight, then its testicles are in danger. They automatically pull up close to the body. The cremasteric reflex also happens during sexual intercourse.

Function of testicles

Testicles have two important jobs in the body. They are part of two of the body's systems, the endocrine system which keeps the body working, and the reproductive system which makes new life.

Making hormones

Testicles are a type of organ called glands. (This makes them part of the body's endocrine system.) The human body has many types of glands. The job of glands within a body is to make chemical substances, and put them out into the body's system. A body has lots of different systems that keep it in good working order all the time, and that also provide for a body's special needs. Some of the body's special needs happen when a person gets angry, frightened or sick, or if the person wants to have sexual intercourse. At these times a body uses more of some types of chemical substance, so the glands that make them work harder.

Testicles make several types of chemical substances. They are not simple chemicals, but are very complicated and very important to life. Making these substances is controlled by the pituitary gland which is a small gland in the brain.

One substance made by the testicles is a type of substance known as a hormone. It is the hormone testosterone. Testosterone is important in a male person's body, because it makes him grow into a man and feel like a man during puberty. (Women's bodies make some testosterone too, but they make more female hormone called estrogen.)

[[File:|thumb|200px|An illustration of a testicle. The sperm are made in the oval part of the testicle. They pass through lots of small tubes at the top and then downwards through the epididymus. When the sperm are ready, they pass through the long tube at the right and into the urethra. The picture also shows the artery that blood travels through.]]

Making sperm

As well as being chemical-producing glands, testicles are gonads. (This makes them part of the body's reproductive system.) Apart from hormones, the other important substances made by the testicles are spermatozoa, which are generally just called sperm. (The word "sperm" is plural, so you do not add a "s" and say "sperms".) The sperm are tiny living cells which can join with another cell, called an "ovum" (or egg) inside a female, to start a new human life. Scientists call making sperm spermatogenesis. Boys begin making sperm when they start growing into men, at a time of life called puberty.

Women do not have testicles. The glands in their bodies that do the same sort of jobs as testicles are called ovaries. Women have two ovaries, but unlike testicles, ovaries are inside the body, on either side of the uterus. They release ova (or eggs) and female hormones. Ovaries and testicles are sometimes called "gonads".

Details of how testicles are constructed and how they work

(See the picture, right)
  • The testicles, inside the scrotum, are covered with a tough type of white protective skin called a membrane.
  • The inner part of the testicles have many small tubes called seminiferous tubules which are coiled (or twisted around). Sperm are made inside the seminiferous tubules.
  • The sperm pass through some small tubes called ducts into another part at the back of the testicle, called the epididymis, which is where the sperm cells finish maturing.
  • The sperm cells then travel along a tube called the vas deferens until they reach the urethra.
  • The urethra is the main tube in the penis and leads to the opening at the end called the urethral opening. The urethral opening is also the way that urine leaves the body from the bladder.
  • When a man feels sexual desire, sperm cells go through the ejaculatory duct. They are combined with liquid called seminal fluid from a gland called the prostate. The prostate also has muscles that push the sperm and fluid into the urethra.
  • During sexual intercourse, seminal fluid containing the sperm comes out through the urethral opening when a man ejaculates.
  • If the sperm from the testicles enters the vagina of a woman during sexual intercourse, it may pass into the uterus and join with a mature ovum (or egg) to begin a new baby.

Health issues

Testicle size

Size in animals

Studying animals can help in understanding things about people. In mammals, testicles can be very big or very small in relation to the size of the animal itself. The size has to do with the amount of sperm that the male animal needs to make. Some types of male animal generally only have one sexual partner at a time and are called monogamous. Some types of animals generally live in a herd or flock where one male will have many female sexual partners. These males are called polygamous. More sperm is needed by polygamous males than by monogamous males. Polygamous males generally have larger testicles than monogamous males. The testicles grow larger to make more sperm.

Size in men

Most men's testicles are from 14 cm³ to 35 cm³. This is a measure of volume. Sometimes Doctors need to measure a man's testicles to see if there is a problem. Doctors can measure the volume of testicles in two ways:

  • Orchidometer – This is a set of small objects that look like eggs. They are many different sizes. The doctor picks the one that is the size of the testicle. Doctors know what size the orchidometer is. Then they know the size of the testicle.
  • Measuring the size – with a ruler, calipers, or ultrasound. Then these lengths are used to calculate the volume.

Sometimes the size of a man's testicles changes.

Some reasons why testicles get smaller are:

  • If a man takes testosterone, the testicles do not have to work to make testosterone, so they become smaller.
  • If a man's testicles are kept more warm than usual, they make less sperm. So if they are kept too warm, they get smaller.
  • If testicles are injured(hurt) they can become smaller.

Some reasons why testicles get bigger are:

  • If a man takes hormones from the pituitary gland, (called gonadotropins), it can make testicles bigger. Gonadtropins are the hormones that tell the testicles to make sperm and testosterone.
  • Some diseases cause testicles to swell.

Illness of the testicles

The most important diseases of testicles are:

  • Inflammation of the testicles. This is called orchitis
  • Testicular cancer
  • Fluid around a testicle. This is called a hydrocele
  • Inflammation of the epididymis. This is called epididymitis
  • Spermatic cord torsion. This is also called Testicular torsion. It is when the cord that the testicle hangs from gets twisted.
  • Varicocele - swollen veins to the testes. Varicocele happens more on the left.[1]

Loss and injury

Most boys are born with two testicles in the scrotum. The testicles have formed inside the baby's body, but moved into the scrotum before the birth. Sometimes one or both the testicles are still inside when the baby is born. Sometimes an operation is needed to fix this.

If a testicle is injured, it is extremely painful. Sometimes testicles get crushed. Sometimes it is necessary for a doctor to remove one or both the testicles. This operation is called an orchidectomy. If a man loses a testicle, a doctor can put a testicular prosthesis (false testicle) into the scrotum. It looks and feels like a testicle.

If a male loses both his testicles it called castration. Because the man cannot produce sperm or testosterone, it has a big effect on the man's life. Because he cannot make sperm, he cannot have children. And because he cannot make the male hormone testosterone, he becomes less like a male. He may lose some of his muscles, and put on weight. A man who has lost his testicles may take testosterone medicine so that he does not have these changes.

Until the 1700s in Europe, and more recently in some countries, there was a custom to castrate young male slaves and boys. Young slaves were castrated so that they could be used as guards over the wives of rich men. It meant that the wives would not fall in love with their guards. Sometimes young boys who had beautiful singing voices were castrated before puberty. This meant that their voices would stay high all their lives. These singing men were called castrati.

Castrating animals

It is very common to castrate (remove the testicles from) male domestic animals. Male dogs and cats that are castrated do not fight so much, and do not stray around looking for females to mate with (have sex with).

Male horses are usually castrated so that they become more calm and safer to ride. A castrated male horse is called a gelding. A male horse that is not castrated is a stallion.

Male animals that are used for meat are generally castrated because it makes them fatter and also makes them quieter to keep because they do not fight each other. Most male cattle are castrated.

Other names for testicles

Testicles can also be called "testes" or "gonads".

There are many slang words for testicles, like balls, nuts, bollocks, nads, crown jewels, etc. Slang words are other names that are used for fun or to be vulgar (rude).

Other pages

  • Cryptorchidism (cryptorchismus)
  • Infertility
  • List of homologues of the human reproductive system
  • Orchidometer
  • Spermatogenesis
  • Sterilization (surgical procedure), vasectomy

Endocrine system
Adrenal gland - Corpus luteum - Hypothalamus - Ovaries - Pancreas - Parathyroid gland - Pineal gland - Pituitary gland - Testes - Thyroid gland - Hormone

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