testis
   HOME

TheInfoList



OR:

A testicle or testis (plural testes) is the male reproductive gland or gonad in all
bilateria The Bilateria or bilaterians are animals with bilateral symmetry as an embryo, i.e. having a left and a right side that are mirror images of each other. This also means they have a head and a tail (anterior-posterior axis) as well as a belly an ...
ns, including humans. It is homologous to the female
ovary The ovary is an organ in the female reproductive system that produces an ovum. When released, this travels down the fallopian tube into the uterus, where it may become fertilized by a sperm. There is an ovary () found on each side of the body ...
. The functions of the testes are to produce both
sperm Sperm is the male reproductive cell, or gamete, in anisogamous forms of sexual reproduction (forms in which there is a larger, female reproductive cell and a smaller, male one). Animals produce motile sperm with a tail known as a flagellum ...
and
androgen An androgen (from Greek ''andr-'', the stem of the word meaning "man") is any natural or synthetic steroid hormone that regulates the development and maintenance of male characteristics in vertebrates by binding to androgen receptors. This inc ...
s, primarily
testosterone Testosterone is the primary sex hormone and anabolic steroid in males. In humans, testosterone plays a key role in the development of male reproductive tissues such as testes and prostate, as well as promoting secondary sexual characteristi ...
. Testosterone release is controlled by the anterior pituitary luteinizing hormone, whereas sperm production is controlled both by the anterior pituitary
follicle-stimulating hormone Follicle-stimulating hormone (FSH) is a gonadotropin, a glycoprotein polypeptide hormone. FSH is synthesized and secreted by the gonadotropic cells of the anterior pituitary gland and regulates the development, growth, pubertal maturation, a ...
and gonadal testosterone.


Structure


Appearance

Males have two testicles of similar size contained within the scrotum, which is an extension of the abdominal wall. Scrotal asymmetry, in which one testicle extends farther down into the scrotum than the other, is common. This is because of the differences in the vasculature's anatomy. For 85% of men, the right testis hangs lower than the left one.


Measurement and volume

The volume of the testicle can be estimated by palpating it and comparing it to ellipsoids of known sizes. Another method is to use calipers (an
orchidometer An orchidometer (or orchiometer) is a medical instrument used to measure the volume of the testicles. The orchidometer was introduced in 1966 by Swiss pediatric endocrinologist Andrea Prader of the University of Zurich. It consists of a strin ...
) or a ruler either on the person or on an ultrasound image to obtain the three measurements of the x, y, and z axes (length, depth and width). These measurements can then be used to calculate the volume, using the formula for the volume of an ellipsoid: :Volume = \frac \cdot \pi \cdot \frac \cdot \frac \cdot \frac :\approx length \cdot width \cdot depth \cdot 0.52 However, the most accurate calculation of actual testicular volume is gained from the formula: :\approx length \cdot width \cdot depth \cdot 0.71 An average adult testicle measures up to . The
Tanner scale The Tanner scale (also known as the Tanner stages or Sexual Maturity Rating (SMR)) is a scale of physical development in children, adolescents and adults. The scale defines physical measurements of development based on external primary and se ...
, which is used to assess the maturity of the male genitalia, assigns a maturity stage to the calculated volume ranging from stage I, a volume of less than 1.5 cm3; to stage V, a volume greater than 20 cm3. Normal volume is 15 to 25 cm3; the average is 18 cm3 per testis (range 12–30 cm3). The number of spermatozoa an adult human male produces is directly proportional to testicular volume, as larger testicles contain more seminiferous tubules and
Sertoli cells Sertoli cells are a type of sustentacular "nurse" cell found in human testes which contribute to the process of spermatogenesis (the production of sperm) as a structural component of the seminiferous tubules. They are activated by follicle-stimul ...
as a result. As such, men with larger testicles produce on average more sperm cells in each
ejaculate Ejaculation is the discharge of semen (the ''ejaculate''; normally containing sperm) from the male reproductory tract as a result of an orgasm. It is the final stage and natural objective of male sexual stimulation, and an essential compon ...
, as testicular volume is positively correlated with semen profiles.


Internal structure


Duct system

The testes are covered by a tough fibrous shell called the tunica albuginea. Under the tunica albuginea, the testes contain very fine-coiled tubes called
seminiferous tubule Seminiferous tubules are located within the testes, and are the specific location of meiosis, and the subsequent creation of male gametes, namely spermatozoa. Structure The epithelium of the tubule consists of a type of sustentacular cells ...
s. The tubules are lined with a layer of cells ( germ cells) that develop from
puberty Puberty is the process of physical changes through which a child's body matures into an adult body capable of sexual reproduction. It is initiated by hormonal signals from the brain to the gonads: the ovaries in a girl, the testes in a boy ...
through old age into
sperm Sperm is the male reproductive cell, or gamete, in anisogamous forms of sexual reproduction (forms in which there is a larger, female reproductive cell and a smaller, male one). Animals produce motile sperm with a tail known as a flagellum ...
cells (also known as
spermatozoa A spermatozoon (; also spelled spermatozoön; ; ) is a motile sperm cell, or moving form of the haploid cell that is the male gamete. A spermatozoon joins an ovum to form a zygote. (A zygote is a single cell, with a complete set of chromosome ...
or male
gamete A gamete (; , ultimately ) is a haploid cell that fuses with another haploid cell during fertilization in organisms that reproduce sexually. Gametes are an organism's reproductive cells, also referred to as sex cells. In species that produce ...
s). 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 ( spermatogenesis). The sperm move into the vas deferens, and are eventually expelled through the
urethra The urethra (from Greek οὐρήθρα – ''ourḗthrā'') is a tube that connects the urinary bladder to the urinary meatus for the removal of urine from the body of both females and males. In human females and other primates, the urethra ...
and out of the urethral orifice through muscular contractions.


Primary cell types

Within the seminiferous tubules, the germ cells develop into spermatogonia, spermatocytes, spermatids and spermatozoa through the process of spermatogenesis. The gametes contain DNA for fertilization of an ovum Sertoli cellsthe true epithelium of the seminiferous epithelium, critical for the support of germ cell development into spermatozoa. Sertoli cells secrete inhibin. Peritubular myoid cells surround the seminiferous tubules. Between tubules (interstitial cells) exist Leydig cellscells localized between seminiferous tubules that produce and secrete
testosterone Testosterone is the primary sex hormone and anabolic steroid in males. In humans, testosterone plays a key role in the development of male reproductive tissues such as testes and prostate, as well as promoting secondary sexual characteristi ...
and other
androgen An androgen (from Greek ''andr-'', the stem of the word meaning "man") is any natural or synthetic steroid hormone that regulates the development and maintenance of male characteristics in vertebrates by binding to androgen receptors. This inc ...
s important for
puberty Puberty is the process of physical changes through which a child's body matures into an adult body capable of sexual reproduction. It is initiated by hormonal signals from the brain to the gonads: the ovaries in a girl, the testes in a boy ...
(including secondary sexual characteristics like facial hair),
sexual behavior Human sexual activity, human sexual practice or human sexual behaviour is the manner in which humans experience and express their sexuality. People engage in a variety of sexual acts, ranging from activities done alone (e.g., masturbation ...
, and
libido Libido (; colloquial: sex drive) is a person's overall sexual drive or desire for sexual activity. Libido is influenced by biological, psychological, and social factors. Biologically, the sex hormones and associated neurotransmitters that act u ...
. Sertoli cells support spermatogenesis. Testosterone controls testicular volume. Immature Leydig cells and interstitial macrophages and epithelial cells are also present.


Blood supply and lymphatic drainage

The testis has three sources of arterial blood supply: the testicular artery, the
cremasteric artery The cremasteric artery (external spermatic artery) is a branch of the inferior epigastric artery which accompanies the spermatic cord to supply the cremaster muscle as well as other coverings of the spermatic cord in the male (whereas in the female ...
, and the
artery to the ductus deferens The artery to the ductus deferens (deferential artery) is an artery in males that provides blood to the ductus deferens. Course The artery arises from the superior vesical artery or the inferior vesical artery, which in turn arises from the anteri ...
. Blood supply and lymphatic drainage of the testes and scrotum are distinct: * The paired testicular arteries arise directly from the
abdominal aorta In human anatomy, the abdominal aorta is the largest artery in the abdominal cavity. As part of the aorta, it is a direct continuation of the descending aorta (of the thorax). Structure The abdominal aorta begins at the level of the diaphr ...
and descend through the inguinal canal, while the scrotum and the rest of the external genitalia is supplied by the internal pudendal artery (a branch of the internal iliac artery). * The testis has collateral blood supply from the cremasteric artery (a branch of the
inferior epigastric artery In human anatomy, inferior epigastric artery refers to the artery that arises from the external iliac artery. It anastomoses with the superior epigastric artery. Along its course, it is accompanied by a similarly named vein, the inferior epiga ...
, which is a branch of the external iliac artery), and the artery to the ductus deferens (a branch of the inferior vesical artery, which is a branch of the internal iliac artery). Therefore, if the testicular artery is ligated, e.g., during a Fowler-Stevens
orchiopexy Orchiopexy (or orchidopexy) is a surgery to move and/or permanently fix a testicle into the scrotum. While orchiopexy typically describes the operation to surgically correct an undescended testicle, it is also used to resolve testicular torsion. ...
for a high undescended testis, the testis will usually survive on these other blood supplies. * Lymphatic drainage of the testes follows the testicular arteries back to the
paraaortic lymph nodes The periaortic lymph nodes (also known as lumbar) are a group of lymph nodes that lie in front of the lumbar vertebrae near the aorta. These lymph nodes receive drainage from the gastrointestinal tract and the abdominal organs. The periaortic lym ...
, while lymph from the scrotum drains to the
inguinal lymph nodes Inguinal lymph nodes are lymph nodes in the human groin. Located in the femoral triangle of the inguinal region, they are grouped into superficial and deep lymph nodes. The superficial have three divisions: the superomedial, superolateral, and in ...
.


Layers

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. 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 occupy the basal compartment (deep to the level of the tight junctions) and the more mature forms, such as primary and secondary spermatocytes and spermatids, occupy the adluminal compartment. The function of the blood–testis barrier may be to prevent an auto-immune reaction. Mature sperm (and their antigens) emerge significantly after immune tolerance is set in infancy. Since sperm are antigenically different from self-tissue, a male animal can react immunologically to his own sperm. The male can make antibodies against them. Injection of sperm antigens causes inflammation of the testis (auto-immune orchitis) and reduced fertility. The blood–testis barrier may reduce the likelihood that sperm proteins will induce an immune response, reducing fertility and so progeny.


Temperature regulation and responses

Spermatogenesis is enhanced at temperatures slightly less than core body temperature. The spermatogenesis is less efficient at lower and higher temperatures than 33 °C. Because the testes are located outside the body, the smooth tissue of the scrotum can move them closer or further away from the body. The temperature of the testes is maintained at 34.4 °C, a little below body temperature, as temperatures above 36.7 °C impede spermatogenesis. There are a number of mechanisms to maintain the testes at the optimum temperature. The cremasteric muscle covers the testicles and the spermatic cord. When this muscle contracts, the cord shortens and the testicles move 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 testicles lower away from the warm body and are able to cool. Contraction also occurs in response to physical stress, such as blunt trauma; the testicles withdraw and the scrotum shrinks very close to the body in an effort to protect them. The cremasteric reflex will reflexively raise the testicles. The testicles can also be lifted voluntarily using the pubococcygeus muscle, which partially activates related muscles.


Gene and protein expression

The
human genome The human genome is a complete set of nucleic acid sequences for humans, encoded as DNA within the 23 chromosome pairs in cell nuclei and in a small DNA molecule found within individual mitochondria. These are usually treated separately as the ...
includes approximately 20,000 protein coding genes: 80% of these genes are expressed in adult testes. The testes have the highest fraction of tissue type-specific genes compared to other organs and tissues. About 1000 of them are highly specific for the testes, and about 2,200 show an elevated pattern of expression. A majority of these genes encode for proteins that are expressed in the seminiferous tubules and have functions related to spermatogenesis. Sperm cells express proteins that result in the development of flagella; these same proteins are expressed in the female in cells lining the Fallopian tube and cause the development of
cilia The cilium, plural cilia (), is a membrane-bound organelle found on most types of eukaryotic cell, and certain microorganisms known as ciliates. Cilia are absent in bacteria and archaea. The cilium has the shape of a slender threadlike pro ...
. Sperm cell flagella and Fallopian tube cilia are homologous structures. The testis-specific proteins that show the highest level of expression are
protamine Protamines are small, arginine-rich, nuclear proteins that replace histones late in the haploid phase of spermatogenesis and are believed essential for sperm head condensation and DNA stabilization. They may allow for denser packaging of DNA ...
s.


Development

There are two phases in which the testes grow substantially. These are the embryonic and pubertal phases. During mammalian development, the gonads are at first capable of becoming either ovaries or testes. 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 The Y chromosome is one of two sex chromosomes (allosomes) in therian mammals, including humans, and many other animals. The other is the X chromosome. Y is normally the sex-determining chromosome in many species, since it is the presence or a ...
initiates sex determination by downstream regulation of sex-determining factors (such as GATA4, SOX9 and AMH), which lead to development of the male phenotype, including directing development of the early bipotential gonad toward the male path of development. Testes follow the path of descent, from high in the posterior fetal abdomen to the inguinal ring and beyond to the inguinal canal and into the scrotum. In most cases (97% full-term, 70% preterm), both testes have descended by birth. In most other cases, only one testis fails to descend. This is called cryptorchidism. In most cases of cryptorchidism, the issue will mostly resolve itself within the first half year of life. However, if the testes do not descend far enough into the scrotum, surgical anchoring in the scrotum is required due to risks of infertility and testicular cancer. The testes grow in response to the start of spermatogenesis. Size depends on lytic function, sperm production (amount of spermatogenesis present in testis), interstitial fluid, and Sertoli cell fluid production. The testicles are fully descended before the male reaches puberty.


Clinical significance


Protection and injury

* The testicles are very sensitive to impact and injury. The pain involved travels up from each testicle into the abdominal cavity, via the
spermatic plexus The spermatic plexus (or testicular plexus) is derived from the renal plexus, receiving branches from the aortic plexus. It accompanies the internal spermatic artery The testicular artery (the male gonadal artery, also called the internal sperm ...
, which is the primary nerve of each testicle. This will cause pain in the hip and the back. The pain usually fades within a few minutes. * Testicular torsion is a medical emergency. This is because the longer it takes to access medical intervention with respect to extending ischemia, the higher the chance that the testicle will be lost. There is a 90% chance to save the testicle if de-torsion surgery is performed within six hours of testicular torsion onset. * Testicular rupture is severe trauma affecting the tunica albuginea. * Penetrating injuries to the scrotum may cause
castration Castration is any action, surgical, chemical, or otherwise, by which an individual loses use of the testicles: the male gonad. Surgical castration is bilateral orchiectomy (excision of both testicles), while chemical castration uses pharm ...
, or physical separation or destruction of the testes, possibly along with part or all of the penis, which results in total sterility if the testicles are not reattached quickly. In an effort to avoid severe infection, ample application of saline and
bacitracin Bacitracin is a polypeptide antibiotic. It is a mixture of related cyclic peptides produced by ''Bacillus licheniformis'' bacteria, that was first isolated from the variety "Tracy I" ( ATCC 10716) in 1945. These peptides disrupt Gram-positive b ...
help remove debris and foreign objects from the wound. * Jockstraps support and protect the testicles.


Diseases and conditions

* To improve the chances of catching cases of testicular cancer, other neoplasms, or other health issues early, regular testicular self-examination is recommended. * Varicocele, swollen vein(s) from the testes, usually affecting the left side, the testis usually being normal. *
Hydrocele testis A hydrocele testis is an accumulation of clear fluid within the cavum vaginale, the potential space between the layers of the tunica vaginalis of the testicle. It is the most common form of hydrocele and is often referred to simply as a "hydrocele" ...
is swelling around testes caused by accumulation of clear liquid within a membranous sac, the testis usually being normal. It is the most common cause of scrotal swelling. * Spermatocele is a retention cyst of a tubule of the rete testis or the head of the epididymis distended with barely watery fluid that contains spermatozoa. * Endocrine disorders can also affect the size and function of the testis. * 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 The mesonephric duct (also known as the Wolffian duct, archinephric duct, Leydig's duct or nephric duct) is a paired organ that forms during the embryonic development of humans and other mammals and gives rise to male reproductive organs. Stru ...
and allow for the persistence of
Müllerian ducts Paramesonephric ducts (or Müllerian ducts) are paired ducts of the embryo that run down the lateral sides of the genital ridge and terminate at the sinus tubercle in the primitive urogenital sinus. In the female, they will develop to form the f ...
. Both excess and deficient levels of estrogens can disrupt spermatogenesis and cause infertility. *
Bell-clapper deformity Testicular torsion occurs when the spermatic cord (from which the testicle is suspended) twists, cutting off the blood supply to the testicle. The most common symptom in children is sudden, severe testicular pain. The testicle may be higher than ...
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. Those with Bell-clapper are at a higher risk of testicular torsion. * Orchitis is inflammation of the testicles * Epididymitis is a painful inflammation of the epididymis or epididymides, frequently caused by bacterial infection but sometimes of unknown origin. * Anorchia is the absence of one or both testicles. * Cryptorchidism, or "undescended testicles", is when the testicle does not descend into the scrotum of an infant boy. *Testicular enlargement is an unspecific sign of various testicular diseases, and can be defined as a testicular size of more than 5 cm ( long axis) × 3 cm (short axis). * Blue balls is a condition concerning temporary fluid congestion in the testicles and prostate region, caused by prolonged sexual arousal. Testicular prostheses are available to mimic the appearance and feel of one or both testicles, when absent as from injury or as treatment in association to gender dysphoria. There have also been some instances of their implantation in dogs.


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 Anabolic steroids, also known more properly as anabolic–androgenic steroids (AAS), are steroidal androgens that include natural androgens like testosterone (medication), testosterone as well as synthetic androgens that are structurally related ...
) often have the undesired side effect of testicular shrinkage. Stimulation of testicular functions via gonadotropic-like hormones may enlarge their size. Testes may shrink or atrophy during
hormone replacement therapy Hormone replacement therapy (HRT), also known as menopausal hormone therapy or postmenopausal hormone therapy, is a form of hormone therapy used to treat symptoms associated with female menopause. These symptoms can include hot flashes, vaginal a ...
or through chemical castration. In all cases, the loss in testes volume corresponds with a loss of spermatogenesis.


Society and culture

The testicles of calves, lambs, roosters,
turkeys The turkey is a large bird in the genus ''Meleagris'', native to North America. There are two extant turkey species: the wild turkey (''Meleagris gallopavo'') of eastern and central North America and the ocellated turkey (''Meleagris ocellat ...
, and other animals are eaten in many parts of the world, often under euphemistic culinary names. Testicles are a by-product of the castration of young animals raised for meat, so they might have been a late-spring seasonal specialty. In modern times, they are generally frozen and available year-round. In the
Middle Ages In the history of Europe, the Middle Ages or medieval period lasted approximately from the late 5th to the late 15th centuries, similar to the post-classical period of global history. It began with the fall of the Western Roman Empire ...
, men who wanted a boy sometimes had their left testicle removed. This was because people believed that the right testicle made "boy" sperm and the left made "girl" sperm. As early as 330 BC,
Aristotle Aristotle (; grc-gre, Ἀριστοτέλης ''Aristotélēs'', ; 384–322 BC) was a Greek philosopher and polymath during the Classical period in Ancient Greece. Taught by Plato, he was the founder of the Peripatetic school of ...
prescribed the ligation (tying off) of the left testicle in men wishing to have boys.


Etymology and slang

One theory about the etymology of the word ''testis'' is based on Roman law. The original
Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally a dialect spoken in the lower Tiber area (then known as Latium) around present-day Rome, but through ...
word , "witness", was used in the firmly established legal principle "" (one witness qualsno 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''). Another theory says that ''testis'' is influenced by a loan translation, from Greek "defender (in law), supporter" that is "two glands side by side". There are multiple slang terms for the testes. They may be referred to as "balls". Frequently, " nuts" (sometimes intentionally misspelled as "nutz") are also a slang term for the testes due to the geometric resemblance. One variant of the term includes "Deez Nuts", which was used for a satirical political candidate in 2016. In Spanish, the term is used, which is Spanish for eggs.


Other animals


External appearance

In seasonal breeders, the weight of the testes often increases during the breeding season. The testicles of a dromedary camel are long, deep and in width. The right testicle is often smaller than the left. In sharks, the testicle on the right side is usually larger. In many bird and mammal species, the left may be larger. The primitive
jawless fish Agnatha (, Ancient Greek 'without jaws') is an infraphylum of jawless fish in the phylum Chordata, subphylum Vertebrata, consisting of both present ( cyclostomes) and extinct (conodonts and ostracoderms) species. Among recent animals, cyclostom ...
have only a single testis, located in the midline of the body, although this forms from the fusion of paired structures in the embryo.


Location


Internal

The basal condition for mammals is to have internal testes. The testes of monotremes,
xenarthra Xenarthra (; from Ancient Greek ξένος, xénos, "foreign, alien" + ἄρθρον, árthron, "joint") is a major clade of placental mammals native to the Americas. There are 31 living species: the anteaters, tree sloths, and armadillos. Ext ...
ns, and
elephant Elephants are the largest existing land animals. Three living species are currently recognised: the African bush elephant, the African forest elephant, and the Asian elephant. They are the only surviving members of the family Elephantidae ...
s remain within the abdomen. There are also some marsupials with external testes and Boreoeutherian mammals with internal testes, such as the rhinoceros.Schaffer, N. E., et al.
Ultrasonography of the reproductive anatomy in the Sumatran rhinoceros (Dicerorhinus sumatrensis)
" Journal of Zoo and Wildlife Medicine (1994): 337-348.
Cetaceans such as whales and dolphins also have internal testes. As external testes would increase drag in the water they 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. In odobenids and phocids, the location of the testes is para-abdominal, though otariids have scrotal testes.


External

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 and are suspended by the spermatic cord within the scrotum. There are several hypotheses as to why most boreotherian mammals have external testes that operate best at a temperature that is slightly less than the core body temperature. One view is that it is stuck with enzymes evolved in a colder temperature due to external testes evolving for different reasons. Another view is that the lower temperature of the testes simply is more efficient for sperm production. The classic hypothesis is that cooler temperature of the testes allows for more efficient fertile spermatogenesis. There are no possible enzymes operating at normal core body temperature that are as efficient as the ones evolved. Early mammals had lower body temperatures and thus their testes worked efficiently within their body. However, boreotherian mammals may have higher body temperatures than the other mammals and had to develop external testes to keep them cool. One argument is that 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. Researchers have wondered why birds, despite having very high core body temperatures, have internal testes and did not evolve external testes.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)
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. Some mammals with seasonal breeding cycles keep their testes internal until the breeding season. After that, their testes descend and increase in size and become external. The ancestor of the boreoeutherian mammals may have been a small mammal that required very large testes for sperm competition and thus had to place its testes outside the body. This might have led to enzymes involved in spermatogenesis, spermatogenic DNA polymerase beta and
recombinase Recombinases are genetic recombination enzymes. Site specific recombinases DNA recombinases are widely used in multicellular organisms to manipulate the structure of genomes, and to control gene expression. These enzymes, derived from bacteria ...
activities evolving a unique temperature optimum that is slightly less than core body temperature. When the boreoeutherian mammals diversified into forms that were larger or did not require intense sperm competition, they still produced enzymes that operated best at cooler temperatures and had to keep their testes outside the body. This position is made less parsimonious because the kangaroo, a non-boreoeutherian mammal, has external testicles. Separately from boreotherian mammals, the ancestors of kangaroos might 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. One argument for the evolution of external testes is that it protects the testes from abdominal cavity pressure changes caused by jumping and galloping. Mild, transient scrotal heat stress causes DNA damage, reduced fertility and abnormal embryonic development in mice. DNA strand breaks were found in spermatocytes recovered from testicles subjected to 40 °C or 42 °C for 30 minutes. These findings suggest that the external location of the testicles provides the adaptive benefit of protecting spermatogenic cells from heat-induced DNA damage that could otherwise lead to infertility and germline mutation.


Size

The relative size of the testes is often influenced by mating systems. 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). Among the Hominidae, gorillas have little female promiscuity and sperm competition and the testes are small compared to body weight (0.03%).
Chimpanzee The chimpanzee (''Pan troglodytes''), also known as simply the chimp, is a species of great ape native to the forest and savannah of tropical Africa. It has four confirmed subspecies and a fifth proposed subspecies. When its close relative th ...
s have high promiscuity and large testes compared to body weight (0.3%). Human testicular size falls between these extremes (0.08%). Testis weight also varies in seasonal breeders like red foxes, golden jackals, and
coyote The coyote (''Canis latrans'') is a species of canine native to North America. It is smaller than its close relative, the wolf, and slightly smaller than the closely related eastern wolf and red wolf. It fills much of the same ecological n ...
s.


Internal structure

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 reabsorbed 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.


Gallery

File:Slide4aaaaaa.JPG, Testicle File:Slide2CC.JPG, Testicle Image:Hanging testicles.JPG, Testicle hanging on cremaster muscle. These are two healthy testicles. Heat causes them to descend, allowing cooling. Image:Human_Scrotum.JPG, A healthy scrotum containing normal size testes. The scrotum is in tight condition. The image also shows the texture. Image:Testicle-cat.jpg, Testicle of a cat: 1: Extremitas capitata, 2: Extremitas caudata, 3: Margo epididymalis, 4: Margo liber, 5: Mesorchium, 6: Epididymis, 7: testicular artery and vene, 8: Ductus deferens Image:Illu testis surface.jpg, Testis surface Image:Illu testis cross section.jpg, Testis cross section Image:Gray1148.png, The right testis, exposed by laying open the tunica vaginalis. Image:Rabbitttestis100x2.jpg, Microscopic view of Rabbit testis 100× File:Slide8eee.JPG, Testicle


See also

* Anorchia * Bollocks * Cryptorchidism (cryptorchismus) * Ejaculation * Eunuchs * Gelding * List of homologues of the human reproductive system * Neutering * Perineum * Polyorchidism * Sterilization (medicine), Sterilization (surgical procedure), vasectomy * Testicondy * Testicular nubbin


Notes

*


References


External links

* wikt:Thesaurus:testicles, List of synonyms and slang words for testicles in many languages at wikt:Wiktionary:Thesaurus, Wiktionary's Thesaurus
Testis at the Human Protein Atlas
{{Authority control Testicle, Endocrine system Glands Mammal male reproductive system Men's health Scrotum