The testicle (frae Laitin testiculus, diminutive o testis, meanin "witness" o virility,[1] plural testes) is the male gonad in ainimals. Lik the ovaries tae which they are homologous, testes are components o baith the reproductive seestem an the endocrine seestem. The primary functions o the testes are tae produce sperm (spermatogenesis) an tae produce androgens, primarily testosterone.

Inner wirkins o the testicles.
Diagram o male (human) testicles
ArteryTesticular artery
VeinTesticular vein, Pampiniform plexus
NerveSpermatic plexus
LymphLumbar lymph nodes
Anatomical terminology




Male gonads (testes, left) an female (ovaries, richt)

Males hae twa testicles o seemilar size conteened within the scrotum, that is an extension o the abdominal wall. Scrotal asymmetry is nae unusual: ane testicle extends forder doun intae the scrotum nor the ither due tae differences in the anatomy o the vasculatur.



The vollum o the testicle can be estimatit bi palpating it an comparin it tae ellipsoids o kent sizes. Anither method is to use calipers (an orchidometer) or a ruler aither on the person or on an ultrasoond eemage tae obteen the three meisurments o the x, y, an z axes (lenth, deepth an weenth). Thir measurements can then be uised tae calculate the vollum, uisin the formula for the vollum o an ellipsoid: 4/3 π × (lenth/2) × (weenth/2) × (deepth/2).

The dimensions o the average adult testicle are up tae 2 inches lang, 0.8 inches in breidth, an 1.2 inches in hicht (5 × 2 × 3 cm). The Tanner scale for the maturity o male genitals assigns a maturity stage tae the calculatit vollum rangin frae stage I, a vollum o less nor 1.5 ml; tae stage V, a vollum greater nor 20 ml. Normal vollum is 15 tae 25 ml; the average is 18 cm³ per testis (range 12 cm³ to 30 cm³.[2]

Internal structur

Transverse section throu the left side o the scrotum an the left testis.

Duct seestem


The testes are kivert bi a teuch membranous shell cried the tunica albuginea. Within the testes are verra fine coiled tubes cried seminiferous tubules. The tubules are lined wi a layer o cells (germ cells) that develop frae puberty throu auld age intae sperm cells (kent as spermatozoa or male gametes forby). The developin sperm travel throu the seminiferous tubules tae the rete testis locatit in the mediastinum testis, tae the efferent ducts, an then tae the epididymis whaur newly creatit sperm cells matur (see spermatogenesis). The sperm muive intae the vas deferens, an are eventually expelled throu the urethra an oot o the urethral orifice throu muscular contractions.

Primary cell teeps

Within the seminiferous tubules
Atween tubules (intersteetial cells)

Bluid supplee an lymphatic drainage


Bluid supplee an lymphatic drainage o the testes an scrotum are distinct:



Mony anatomical featurs o the adult testis reflect its developmental oreegin in the abdomen. The layers o tishie enclosin ilk testicle are derived frae the layers o the anterior abdominal waw. Notably, the cremasteric muscle arises frae the internal oblique muscle.

The bluid–testis barrier


Muckle molecules canna pass frae the bluid intae the lumen o a seminiferous tubule due tae the presence o ticht junctions atween adjacent Sertoli cells. The spermatogonia are in the basal compartment (deep tae the level o the ticht junctions) an the mair matur forms such as primar an seicontar spermatocytes an spermatids are in the adluminal compairtment.

The function o the bluid–testis baurier (reid heichlicht in diagram abuin) mey be tae prevent an auto-immune reaction. Matur sperm (an thair antigens) arise lang efter immune tolerance is established in infancy. Tharefore, syne sperm are antigenically different frae sel tishie, a male ainimal can react immunologically tae his awn sperm. In fact, he is capable o makkin antibouks against them.

Injection o sperm antigens causes inflammation o the testis (auto-immune orchitis) an reduced fertility. Sicweys, the bluid–testis baurier mey reduce the likeliheid that sperm proteins will induce an immune response, reducin fertility an sae progeny.

Temperatur regulation


Spermatogenesis is enhanced at temperaturs slichtly less nor core bouk temperatur.[6] The spermatogenesis is less efficient at lawer an heicher temperaturs nor 33 °C.[6] Acause the testes are locatit ootside the bouk, the smuith tishie o the scrotum can muive them closer or forder awey frae the bouk.[6] The temperatur o the testes is mainteened at 35 degrees Celsius (95 degrees Fahrenheit), i.e. twa degrees ablo the bouk temperatur o 37 degrees Celsius (98.6 degrees Fahrenheit). Heicher temperaturs affect spermatogenesis.[7] Thare are a nummer o mechanisms tae mainteen the testes at the optimum temperatur.[6]

Gene an protein expression


The human genome includes approximately 20,000 protein codin genes: 80% o thir genes are expressed in adult testes.[8] The testes have the heichest fraction o tishie teep-speceefic genes compared tae ither organs an tishies:[9] aboot 1000 o them are heichly speceefic for the testes,[8] an aboot 2,200 shaw an elevatit pattern o expression here. A majority o thir genes encode for proteins that are expressed in the seminiferous tubules an hae functions relatit tae spermatogenesis.[9][10] Sperm cells express proteins that result in the development o flagella; thir same proteins are expressed in the female in cells linin the fallopian tube, an cause the development o cilia. In ither wirds, sperm cell flagella an Fallopian tube cilia are homologous structurs. The testis-specific proteins that shaw the heichest level o expression are protamines.



Thare are twa phases in that the testes growe substantially; namely in embryonic an pubertal age.



In mammalian development, the gonads are at first capable o acomin aither ovaries or testes.[11] In humans, stairtin at aboot week 4 the gonadal rudiments are present within the intermediate mesoderm adjacent tae the developin neers. At aboot week 6, sex cords develop within the formin testes. Thir are made up o early Sertoli cells that surroond an nurtur the germ cells that migrate intae the gonads shortly afore sex determination begins. In males, the sex-speceefic gene SRY that is foond on the Y-chromosome ineetiates sex determination bi dounstream regulation o sex-determinin factors, (sic as GATA4, SOX9 an AMH), that leads tae development o the male phenoteep, includin directin development o the early bipotential gonad doun the male path o development.

Testes follae the "path o strynd" frae heich in the posterior fetal abdomen tae the inguinal ring an ayont tae the inguinal canal an intae the scrotum. In maist cases (97% full-term, 70% preterm), baith testes hae stryndit bi birth. In maist ither cases, anerly ane testis fails tae strynd (cryptorchidism) an that will probably express itsel within a year.



The testes growe in response tae the stairt o spermatogenesis. Size depends on lytic function, sperm production (amoont o spermatogenesis present in testis), interstitial fluid, and Sertoli cell fluid production.

Clinical signeeficance


Pertection an injure

  • The testicles are weel kent tae be verra sensitive tae impact an injure. The pyne involved traivels up frae ilk testicle intae the abdominal cavity, via the spermatic plexus, that is the primar nerve o ilk testicle. This will cause pyne in the hip an the back. The pain uisually goes away in a few minutes.
  • Testicular torsion is a medical emergency.
  • Testicular ruptur is a medical emergency caused bi blunt force impact, shairp edge, or piercin impact tae ane or baith testicles.
  • Penetratin injuries tae the scrotum mey cause castration, or pheesical separation or destruction o the testes, possibly alang wi pairt or aw o the penis, that results in tot sterility if the testicles are nae reattached quickly.
  • Some jockstraps are designed tae provide support tae the testicles.[12]

Diseases an condeetions

Testicular disease
Clessification an freemit resoorces
ICD-10E29, N43-N44
ICD-9-CM257, 603-604
  • Testicular cancer an ither neoplasms – Tae impuive the chances o catchin possible cases o testicular cancer or ither heal issues early, regular testicular sel-examination is recommendit.
  • Varicocele, swalt vein(s) frae the testes, uisually affectin the left side,[13] the testis uisually bein normal
  • Hydrocele testis, swallin aroond testes caused bi accumulation o clear liquid within a membranous sac, the testis uisually bein normal
  • Spermatocele, a retention cyst o a tubule o the rete testis or the heid o the epididymis distendit wi barely watyery fluid that conteens spermatozoa
  • Endocrine disorders can n aw affect the size an function o the testis.
  • Certaint inheritit condeetions involvin mutations in key developmental genes impair testicular strynd an aw, resultin in abdominal or inguinal testes that remeen nonfunctional an mey acome cancerous. Ither genetic condeetions can result in the loss o the Wolffian ducts an allou for the persistence o Müllerian ducts. Baith excess an deficient levels o estrogens can disrupt spermatogenesis an cause infertility.[14]
  • Bell-clapper deformity is a deformity in that the testicle is nae attached tae the scrotal waws, an can rotate freely on the spermatic cord within the tunica vaginalis. It is the maist common unnerlyin cause o testicular torsion.
  • Orchitis inflammation o the testicles
  • Epididymitis, a pynefu inflammation o the epididymis or epididymides frequently caused bi bacterial infection but whiles o unkent oreegin.
  • Anorchia, the absence o ane or baith testicles.
  • Cryptorchidism or "unstryndit testicles", when the testicle daes nae strynd intae the scrotum o the infant boy.

Testicular enlairgement is an unspeceefic sign o various testicular diseases, an can be defined as a testicular size o mair nor 5 cm (lang axis) x 3 cm (short axis).[15]

Blue baws is a slang term for a temporar fluid congestion in the testicles an prostate region caused bi prolanged sexual arousal.

Testicular prostheses are available tae mimic the appearance and feel o ane or baith testicles, whan absent as frae injury or as treatment in association to gender dysphoria. Thare hae been some instances o thair implantation in dugs an aw.[16]

Effects o exogenous hormones


Tae some extent, it is possible tae chynge testicular size. Short o direct injury or subjectin them tae adverse condeetions, e.g., heicher temperatur nor thay are normally accustomed tae, thay can be shrunk bi competin against their intrinsic hormonal function throu the uise o externally admeenistered steroidal hormones. Steroids taken for muscle enhancement (especially anabolic steroids) eften hae the undesired side effect o testicular shrinkage.

Seemilarly, stimulation o testicular functions via gonadotropic-lik hormones mey enlarge thair size. Testes mey shrink or atrophy in hormone replacement therapy or throu chemical castration.

In aw cases, the loss in testes vollum corresponds wi a loss o spermatogenesis.

Society an cultur


Testicles o a male cauf or ither fermstockin are cuiked an eaten in a dish whiles cried Rocky Mountain oysters.[17]

As early as 330 BC, Aristotle prescrived the ligation (tyin aff) o the left testicle in men wishin tae hae boys.[18] In the Middle Ages, men that wantit a boy whiles haed thair left testicle remuived. This wis acause fowk believed that the richt testicle made "boy" sperm an the left made "girl" sperm.[19]



Ane theory aboot the etymology o the wird testis is based on Roman law. The oreeginal Laitin wird testis, "witness", wis used in the firmly established legal principle "Testis unus, testis nullus" (ane witness [equals] na witness), meanin that testimony bi ony ane person in coort wis tae be disregairdit unless corroboratit bi the testimony o at least anither. This led tae the common practice o producin twa witnesses, bribed tae testify the same wey in cases o lawsuits wi ulterior motives. Syne siv "witnesses" ayeweys came in pairs, the meanin wis accordinly extendit, eften in the diminutive (testiculus, testiculi).

Anither theory says that testis is influenced bi a lend translation, frae Greek parastatēs "defender (in law), supporter" that is "twa glands side bi side".[20]

Ither ainimals


Freemit appearance


In shairks, the testicle on the richt side is uisually mair muckle, an in mony bird an mammal species, the left mey be the larger. The primitive jawless fish hae anerly a single testis, locatit in the midline o the bouk, awtho even this forms frae the fusion o paired structurs in the embryo.[21]

In saisonal breeders, the wecht o the testes eften increases in the breedin saison.[22] The testicles o a dromedary camel are 7–10 cm (2.8–3.9 in) lang, 4.5 cm (1.8 in) deep an 5 cm (2.0 in) in weenth. The richt testicle is eften smawer nor the left.[23] The testicles o a male reid tod atteen thair greatest wecht in December–Februar.[24] Spermatogenesis in male gowden jackals occurs 10–12 days afore the females enter estrus an, in this time, males' testicles treeple in wecht.[25]





The basal condeetion for mammals is tae hae internal testes.[26] Thare are some marsupials wi freemit testes[27] an Boreoeutherian mammals wi internal testes, sic as the rhinoceros.[28] Cetaceans sic as whauls an dowphins hae internal testes an aw.[29] As freemit testes wad increase drag in the watter thay hae internal testes that are keepit cuil bi special circulatory seestems that cuil the arterial bluid gaein tae the testes bi placin the arteries near veins bringin cuiled venous bluid frae the skin.[30][31]



Boreoeutherian laund mammals, the muckle group o mammals that includes humans, hae freemitised testes.[32] Thair testes function best at temperaturs lawer nor thair core bouk temperatur. Thair testes are locatit ootside o the bouk, suspendit bi the spermatic cord within the scrotum.

Thare are several hypotheses why maist boreotherian mammals hae fremmit testes that operate best at a temperatue that is slichtly less nor the core bouk temperatur, e.g. that it is stuck wi enzymes evolved in a caulder temperatur due tae freemit testes evolvin for different raisons, that the lawer temperatur o the testes semply is mair efficient for sperm production.

1) Mair efficient. The clessic hypothesis is that cuiler temperatur o the testes allous for mair efficient fertile spermatogenesis. In ither wirds, thare are na possible enzymes operatin at normal core bouk temperatur that are as efficient as the anes evolved, at least nane appearin in our evolution so faur.

Houiver, the quaisten remeens why birds despite haein verra heich core bouk temperaturs hae internal testes an did nae evolve freemit testes.[33] It wis ance theorised that birds uised their air sacs tae cuil the testes internally, but later studies revealed that birds' testes are able tae function at core bouk temperatur.[33]

Some mammals that hae saisonal breedin cycles keep thair testes internal till the breedin saison at that pynt thair testes strynd an increase in size an acome freemit.[34]

2) Irreversible adaptation tae sperm competeetion. It haes been suggestit that the auncestor o the boreoeutherian mammals was a smaw mammal that required verra muckle testes (aiblins raither lik thae o a hamster) for sperm competeetion an sicweys haed tae place its testes ootside the byle.[35] This led tae enzymes involved in spermatogenesis, spermatogenic DNA polymerase beta an recombinase activities evolvin a unique temperatur optimum, slichtly less nor core bouk temperatur. Whan the boreoeutherian mammals then diversifee'd intae forms that war muckler an/or did nae require intense sperm competeetion thay still produced enzymes that operatit best at cuiler temperaturs an haed tae keep thair testes ootside the bouk. This poseetion is made less parsimonious bi the fact that the kangaroo, a non-boreoeutherian mammal, has freemit testicles. The auncestors o kangaroos micht, separately frae boreotherian mammals, hae been subject to hivy sperm competeetion an sicweys developit freemit testes, houiver, kangaroo freemit testes are suggestive o a possible adaptive function for freemit testes in muckle ainimals.

3) Protection frae abdominal cavity pressure changes. Ane argiement for the evolution o freemit testes is that it pertects the testes frae abdominal cavity pressur chynges caused bi jimpin an gallopin.[36]

4) Pertection against DNA damage. Mild, transient scrotal heat stress causes DNA damage, reduced fertility an abnormal embryonic development in mice.[37] DNA strand breaks war foond in spermatocytes recovered frae testicles subjectit tae 40 °C or 42 °C for 30 meenits.[37] Thir findings suggest that the freemit location o the testicles provides the adaptive benefit o pertectin spermatogenic cells frae heat-induced DNA damage that coud itherwise lead tae infertility an germline mutation.

Cross section o rabbit testis, photographed in bricht field microscopy at 40× magnification.

The relative size o testes is eften influenced bi matin seestems.[38] Testicular size as a proportion o bouk wecht varies widely. In the mammalian kinrick, there is a tendency for testicular size tae correspond wi multiple mates (e.g., harems, polygamy). Production o testicular ootput sperm an spermatic fluid is lairger in polygamous ainimals, possibly a spermatogenic competeetion for survival. The testes o the richt whaul are likely tae be the maist muckle o ony animal, ilk wechtin aroond 500 kg (1,100 lb).[39]

Amang the Hominidae, gorillas hae little female promiscuity an sperm competeetion an the testes are smaw compared to bouk wecht (0.03%). Chimpanzees hae heich promiscuity an muckle testes compared tae bouk weight (0.3%). Human testicular size faws atween thir extremes (0.08%).[40]

Internal structur


Unner a teuch membranous shell cried the tunica albuginea, the testis o amniotes, as well as some teleost fish, conteens verra fine coiled tubes cried seminiferous tubules.

Amphibians an maist fish dae nae possess seminiferous tubules. Instead, the sperm are produced in spherical structures cried sperm ampullae. Thir are saisonal structurs, releasin thair contents in the breedin saison, an then bein reabsorbed bi the bouk. Afore the next breedin saison, new sperm ampullae begin tae form an ripen. The ampullae are itherwise essentially identical tae the seminiferous tubules in heicher vertebrates, includin the same range o cell teeps.[21]




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