View source for Testicle

{{Short description|Internal organ in the male reproductive system}}
{{Pp-move-indef}}
{{Pp-vandalism|small=yes}}
{{Infobox anatomy
| Name        = Testicle
| Latin       = testis
| Image       = Testicle_hariadhi.svg
| Caption     = Diagram of inner structures of a human testicle (the labelling "seminal vesicle lobules" is incorrect and should be "testicular lobules" instead)
| Width       = 300
| Image2      = Gray1144.png
| Caption2    = Diagram of the external features and surrounding structures of the testicles of an adult male
| Precursor   =
| System      =
| Artery      = [[Testicular artery]]
| Vein        = [[Testicular vein]], [[pampiniform plexus]]
| Nerve       = [[Spermatic plexus]]
| Lymph       = [[Lumbar lymph nodes]]
}}
[[File:Testis.gif|thumb|right|202px| Animation of the migration of spermatozoa from their origin as germ cells to their exit from the [[vas deferens]].  A) Blood vessels; B) Head of [[epididymis]]; C) Efferent ductules; D) [[Seminiferous tubule]]s; E) Parietal lamina of [[tunica vaginalis]]; F) Visceral lamina of tunica vaginalis; G) Cavity of tunica vaginalis; H) Tunica albuginea; I) Lobule of testis; J) Tail of epididymis; K) Body of epididymis; L) [[Mediastinum testis]]; M) Vas deferens.]]

A '''testicle''' or '''testis''' ({{plural form}}: '''testes''') is the male [[gonad]] in all [[bilateria]]ns, including humans. It is [[Homology (biology)|homologous]] to the female [[ovary]]. The functions of the testicles are to produce both [[sperm]] and [[androgen]]s, primarily [[testosterone]]. Testosterone release is controlled by the anterior pituitary [[luteinizing hormone]], whereas sperm production is controlled both by the [[anterior pituitary]] [[follicle-stimulating hormone]] and gonadal testosterone.

==Structure==
===Appearance===
[[File:Male and female gonads 1.png|thumb|Male gonad (testes, left) and female gonad ([[ovaries]], right)]]

Males have two testicles of similar size contained within the [[scrotum]], which is an extension of the [[abdominal wall]].<ref name="Steger">{{Cite book|last1=Steger|first1=Klaus|last2=Weidner|first2=Wolfgang|title=Practical Urology: Essential Principles and Practice|chapter=Anatomy of the Male Reproductive System|chapter-url=https://books.google.com/books?id=A9m8TkdCUqEC&pg=PA57|date=2011|publisher=Springer Science & Business Media|isbn=978-1-84-882034-0|pages=57–59|access-date=2022-06-01|archive-date=2023-06-29|archive-url=https://web.archive.org/web/20230629155358/https://books.google.com/books?id=A9m8TkdCUqEC&pg=PA57|url-status=live}}</ref> 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.<ref name="Steger"/> For 85% of men, the right testis hangs lower than the left one.<ref name="Steger"/> 

===Measurement and volume===
The volume of the testicle can be estimated by palpating it and comparing it to [[ellipsoid]]s (an [[orchidometer]]) of known sizes. Another method is to use calipers, a ruler, or 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: 
:<math>Volume = \frac{4}{3} \cdot \pi \cdot \frac{length}{2} \cdot \frac{width}{2} \cdot \frac{depth}{2}</math>
:<math>\approx length \cdot width \cdot depth \cdot 0.52</math>

However, the most accurate calculation of actual testicular volume is gained from the formula:<ref name="Lao">{{Cite book|last1=Lao|first1=Michael|last2=Smith|first2=Shannon|last3=Gilbert|first3=Bruce R.|title=Practical Urological Ultrasound|chapter=Male Reproductive Ultrasound|chapter-url=https://books.google.com/books?id=QU8AEAAAQBAJ&pg=PA298|date=2020|publisher=Springer Nature|isbn=978-3-03-052309-1|page=298|access-date=2022-07-05|archive-date=2023-06-29|archive-url=https://web.archive.org/web/20230629155354/https://books.google.com/books?id=QU8AEAAAQBAJ&pg=PA298|url-status=live}}</ref>

:<math>\approx length \cdot width \cdot depth \cdot 0.71</math>

An average adult testicle measures up to {{convert|5|×|2|×|3|cm|in|frac=4|abbr=on}}. The [[Tanner scale#Genitals (male)|Tanner scale]], 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&nbsp;cm<sup>3</sup>; to stage V, a volume greater than 20&nbsp;cm<sup>3</sup>. Normal volume is 15 to 25&nbsp;cm<sup>3</sup>; the average is 18&nbsp;cm<sup>3</sup> per testis (range 12–30&nbsp;cm<sup>3</sup>).<ref name="Steger"/>

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]] as a result.<ref name="Rhoades">{{Cite book|last1=Rhoades|first1=Rodney A.|last2=Bell|first2=David R.|title=Medical Physiology: Principles for Clinical Medicine|date=2012|publisher=Lippincott Williams & Wilkins|isbn=978-1-60-913427-3|page=681|url=https://books.google.com/books?id=1kGcFOKCUzkC&pg=PA681|access-date=2022-07-05|archive-date=2023-06-29|archive-url=https://web.archive.org/web/20230629155405/https://books.google.com/books?id=1kGcFOKCUzkC&pg=PA681|url-status=live}}</ref> As such, men with larger testicles produce on average more sperm cells in each [[ejaculate]], as testicular volume is positively correlated with semen profiles.<ref>{{cite journal | doi=10.1155/2013/145792 | doi-access=free | title=Relationship between Testicular Volume and Conventional or Nonconventional Sperm Parameters | year=2013 | last1=Condorelli | first1=Rosita | last2=Calogero | first2=Aldo E. | last3=La Vignera | first3=Sandro | journal=International Journal of Endocrinology | volume=2013 | pages=1–6 | pmid=24089610 | pmc=3780703 }}</ref>

===Internal structure===
[[Image:transversetestis.png|thumb|Transverse section through the left side of the scrotum and the left testis]]

====Duct system====
The testes are covered by a tough fibrous shell called the [[Tunica albuginea of testis|tunica albuginea]].<ref name="Cho">{{Cite book|last1=Cho|first1=S|last2=Bae|first2=J.H.|title=Clinical Regenerative Medicine in Urology|chapter=Penis and Testis|chapter-url=https://books.google.com/books?id=J9s1DwAAQBAJ&pg=PA281|date=2017|publisher=Springer|isbn=978-9-81-102723-9|page=281|access-date=2022-06-01|archive-date=2023-06-29|archive-url=https://web.archive.org/web/20230629155730/https://books.google.com/books?id=J9s1DwAAQBAJ&pg=PA281|url-status=live}}</ref> Under the tunica albuginea, the testes contain very fine-coiled tubes called [[seminiferous tubule]]s.<ref name="Cho"/> The tubules are lined with a layer of cells ([[germ cell]]s) that develop from [[puberty]] through old age into [[sperm]] [[cell (biology)|cells]] (also known as [[spermatozoa]] or male [[gamete]]s).<ref name="Cho"/> 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]]).<ref name="Pocock">{{Cite book|last1=Pocock|first1=Gillian|last2=Richards|first2=Christopher D.|last3=Richards|first3=David A.|title=Human Physiology|date=2018|publisher=Oxford University Press|isbn=978-0-19-873722-3|page=766|url=https://books.google.com/books?id=NcQ9DwAAQBAJ&pg=PA766|access-date=2022-06-02|archive-date=2023-06-29|archive-url=https://web.archive.org/web/20230629155730/https://books.google.com/books?id=NcQ9DwAAQBAJ&pg=PA766|url-status=live}}</ref> The sperm move into the [[vas deferens]], and are eventually expelled through the [[urethra]] and out of the [[External urethral orifice (male)|urethral orifice]] through muscular contractions.<ref name="Pocock"/>

====Primary cell types====
Within the seminiferous tubules, the germ cells develop into [[spermatogonia]], [[spermatocyte]]s, [[spermatid]]s and spermatozoa through the process of spermatogenesis. The gametes contain DNA for fertilization of an [[ovum]].<ref>[https://books.google.com/books?id=FoSiGTXn6BUC&q=testes+OR+testis+OR+testicle+OR+testicles Histology, A Text and Atlas] {{Webarchive|url=https://web.archive.org/web/20230629155731/https://books.google.com/books?id=FoSiGTXn6BUC&q=testes+OR+testis+OR+testicle+OR+testicles |date=2023-06-29 }} by Michael H. Ross and Wojciech Pawlina, Lippincott Williams & Wilkins, 5th ed, 2006{{page needed|date=April 2022}}</ref> Sertoli cells{{Snd}}the true epithelium of the seminiferous epithelium, critical for the support of germ cell development into spermatozoa. Sertoli cells secrete [[inhibin]].<ref name="Huhtaniemi">{{Cite book|last=Huhtaniemi|first=Ilpo|title=Encyclopedia of Endocrine Diseases|date=2018|publisher=Academic Press|isbn=978-0-12-812200-6|page=667|url=https://books.google.com/books?id=nVh7DwAAQBAJ&pg=RA1-PA667|access-date=2022-06-02|archive-date=2023-06-29|archive-url=https://web.archive.org/web/20230629155732/https://books.google.com/books?id=nVh7DwAAQBAJ&pg=RA1-PA667|url-status=live}}</ref> [[Peritubular myoid cells]] surround the seminiferous tubules.<ref name="Schlegel">{{Cite book|last1=Schlegel|first1=P.N.|last2=Katzovitz|first2=M.A.|title=Urologic Principles and Practice|chapter=Male Reproductive Physiology|chapter-url=https://books.google.com/books?id=bNHHDwAAQBAJ&pg=PA50|date=2020|publisher=Springer Nature|isbn=978-3-03-028599-9|page=50|access-date=2022-06-02|archive-date=2023-06-29|archive-url=https://web.archive.org/web/20230629155757/https://books.google.com/books?id=bNHHDwAAQBAJ&pg=PA50|url-status=live}}</ref>

Between tubules (interstitial cells) exist [[Leydig cells]]<ref name="Bitzer">{{Cite book|last1=Bitzer|first1=Johannes|last2=Mahmood|first2=Tahir A.|title=Handbook of Contraception and Sexual Reproductive Healthcare|date=2022|publisher=Cambridge University Press|isbn=978-1-10-895863-9|page=16|url=https://books.google.com/books?id=WWtYEAAAQBAJ&pg=PA16|access-date=2022-07-05|archive-date=2023-06-29|archive-url=https://web.archive.org/web/20230629155732/https://books.google.com/books?id=WWtYEAAAQBAJ&pg=PA16|url-status=live}}</ref>{{Snd}}cells localized between seminiferous tubules that produce and secrete [[testosterone]] and other [[androgen]]s important for [[puberty]] (including [[secondary sexual characteristics]] like facial hair), [[Human sexual activity|sexual behavior]], and [[libido]]. Sertoli cells support spermatogenesis.<ref name="Miell">{{Cite book|last1=Miell|first1=John|last2=Davies|first2=Zoe|title=Clinical Biochemistry: Metabolic and Clinical Aspects|chapter=Reproductive function in the male|chapter-url=https://books.google.com/books?id=2FkXAwAAQBAJ&pg=PA451|date=2014|publisher=Elsevier Health Sciences|isbn=978-0-70-205478-5|page=451|access-date=2022-07-05|archive-date=2023-06-29|archive-url=https://web.archive.org/web/20230629160234/https://books.google.com/books?id=2FkXAwAAQBAJ&pg=PA451|url-status=live}}</ref> Testosterone controls testicular volume.

Immature Leydig cells and interstitial [[macrophage]]s 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]], and the [[artery to the ductus deferens]].<ref name="Goldenberg">{{Cite book|last1=Goldenberg|first1=Etai|last2=Benjamin|first2=Tavya G.R|last3=Gilbert|first3=Bruce R.|title=Practical Urological Ultrasound|chapter=Scrotal Ultrasound|date=2020|publisher=Springer Nature|isbn=978-3-03-052309-1|page=80|chapter-url=https://books.google.com/books?id=QU8AEAAAQBAJ&pg=PA80|access-date=2022-07-06|archive-date=2023-06-29|archive-url=https://web.archive.org/web/20230629160234/https://books.google.com/books?id=QU8AEAAAQBAJ&pg=PA80|url-status=live}}</ref> Blood supply and [[lymphatic drainage]] of the testes and scrotum are distinct:
* The paired testicular arteries arise directly from the [[abdominal aorta]] 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]]).<ref name="Steger 2"/><ref name="Tortora">{{Cite book|last1=Tortora|first1=Gerard J.|last2=Nielsen|first2=Mark|title=Principles of Human Anatomy|date=2017|publisher=John Wiley & Sons|isbn=978-1-11-944446-6|page=486|url=https://books.google.com/books?id=e0rcDwAAQBAJ&pg=PA486|access-date=2022-07-06|archive-date=2023-06-29|archive-url=https://web.archive.org/web/20230629160234/https://books.google.com/books?id=e0rcDwAAQBAJ&pg=PA486|url-status=live}}</ref>
* The testis has collateral blood supply from the cremasteric artery (a branch of the [[inferior epigastric artery]], 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).<ref name="Pua">{{Cite book|last1=Pua|first1=Bradley B.|last2=Covey|first2=Anne M.|last3=Madoff|first3=David C.|title=Interventional Radiology: Fundamentals of Clinical Practice|date=2018|publisher=Oxford University Press|isbn=978-0-19-027625-6|page=533|url=https://books.google.com/books?id=edB2DwAAQBAJ&pg=PA533|access-date=2022-07-06|archive-date=2023-06-29|archive-url=https://web.archive.org/web/20230629160235/https://books.google.com/books?id=edB2DwAAQBAJ&pg=PA533|url-status=live}}</ref><ref name="Berney">{{Cite book|last1=Berney|first1=Daniel M|last2=Ulbright|first2=Thomas M.|title=Genitourinary Pathology: Practical Advances|chapter=Anatomy of the Testis and Staging of its Cancers: Implications for Diagnosis|date=2015|publisher=Springer|isbn=978-1-49-392044-0|page=436|chapter-url=https://books.google.com/books?id=97RnBwAAQBAJ&pg=PA436|access-date=2022-07-06|archive-date=2023-06-29|archive-url=https://web.archive.org/web/20230629160304/https://books.google.com/books?id=97RnBwAAQBAJ&pg=PA436|url-status=live}}</ref> Therefore, if the testicular artery is ligated, e.g., during a Fowler-Stevens [[orchiopexy]] for a high undescended testis, the testis will usually survive on these other blood supplies.<ref name="Aboumarzouk">{{Cite book|last=Aboumarzouk|first=Omar M.|title=Blandy's Urology|date=2019|publisher=John Wiley & Sons|isbn=978-1-11-886336-7|page=747|url=https://books.google.com/books?id=WsaKDwAAQBAJ&pg=PA747|access-date=2022-07-06|archive-date=2023-06-29|archive-url=https://web.archive.org/web/20230629160244/https://books.google.com/books?id=WsaKDwAAQBAJ&pg=PA747|url-status=live}}</ref>
* Lymphatic drainage of the testes follows the testicular arteries back to the [[paraaortic lymph nodes]], while lymph from the scrotum drains to the [[inguinal lymph nodes]].<ref name="Steger 2"/><ref name="Berney"/>

====Layers====
[[File:Layers of the Scrotum.ogg|thumb|3D anatomy of the layers surrounding the testis]]

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]].<ref name="Steger"/> The [[cremasteric muscle]] arises from the [[internal oblique muscle]].<ref name="Steger"/><ref name="Tubbs">{{Cite book|last1=Tubbs|first1=R. Shane|last2=Shoja|first2=Mohammadali M.|last3=Loukas|first3=Marios|title=Bergman's Comprehensive Encyclopedia of Human Anatomic Variation|date=2016|publisher=John Wiley & Sons|isbn=978-1-11-843068-2|page=1393|url=https://books.google.com/books?id=U_0dDAAAQBAJ&pg=PT1393|access-date=2022-06-03|archive-date=2023-06-29|archive-url=https://web.archive.org/web/20230629160506/https://books.google.com/books?id=U_0dDAAAQBAJ&pg=PT1393|url-status=live}}</ref>

====The blood–testis barrier====
{{main|Blood–testis barrier}}
Large molecules cannot pass from the blood into the lumen of a seminiferous tubule due to the presence of [[tight junction]]s between adjacent Sertoli cells.<ref name="Steger 2">{{Cite book|last1=Steger|first1=Klaus|last2=Weidner|first2=Wolfgang|title=Practical Urology: Essential Principles and Practice|chapter=Anatomy of the Male Reproductive System|chapter-url=https://books.google.com/books?id=A9m8TkdCUqEC&pg=PA63|date=2011|publisher=Springer Science & Business Media|isbn=978-1-84-882034-0|page=63|access-date=2022-06-05|archive-date=2023-06-29|archive-url=https://web.archive.org/web/20230629160505/https://books.google.com/books?id=A9m8TkdCUqEC&pg=PA63|url-status=live}}</ref> 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.<ref name="Steger 2"/>

The function of the blood–testis barrier may be to prevent an [[auto-immune]] reaction.<ref name="Steger 2"/> Mature sperm (and their [[antigen]]s) emerge significantly after immune tolerance is set in infancy.<ref name="Steger 2"/> 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.<ref name="Steger 2"/>

Injection of sperm antigens causes inflammation of the testis (auto-immune orchitis) and reduced fertility.<ref name="Steger 2"/> The blood–testis barrier may reduce the likelihood that sperm proteins will induce an immune response.<ref name="Wiser">{{Cite book|last1=Wiser|first1=Herbert J.|last2=Sandlow|first2=Jay|last3=Kohler|first3=Tobias S.|title=Male Infertility: Contemporary Clinical Approaches, Andrology, ART & Antioxidants|chapter=Causes of Male Infertility|chapter-url=https://books.google.com/books?id=W8pplg2vbEUC&pg=PA8|date=2012|publisher=Springer Science & Business Media|isbn=978-1-46-143335-4|page=8|access-date=2022-07-10|archive-date=2023-06-29|archive-url=https://web.archive.org/web/20230629160509/https://books.google.com/books?id=W8pplg2vbEUC&pg=PA8|url-status=live}}</ref>

===Temperature regulation and responses===
[[Carl Richard Moore]] in 1926 <ref name="Moore 1926 pp. 4–50">{{cite journal |last=Moore |first=Carl R. |date=1926 |title=The Biology of the Mammalian Testis and Scrotum |journal=The Quarterly Review of Biology |volume=1 |issue=1 |pages=4–50 |doi=10.1086/394235 |issn=0033-5770}}</ref> proposed that testicles were external due to [[spermatogenesis]] being enhanced at temperatures slightly less than core body temperature outside the body. The spermatogenesis is less efficient at lower and higher temperatures than 33&nbsp;°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.<ref name="Cho"/> The temperature of the testes is maintained at 34.4&nbsp;°C, a little below body temperature, as temperatures above 36.7&nbsp;°C impede spermatogenesis.<ref name="Steger"/><ref name="Cho"/> There are a number of mechanisms to maintain the testes at the optimum temperature.<ref name="Coad">{{Cite book|last1=Coad|first1=Jane|last2=Pedley|first2=Kevin|last3=Dunstall|first3=Melvyn|title=Anatomy and Physiology for Midwives E-Book|date=2019|publisher=Elsevier Health Sciences|isbn=978-0-70-206665-8|pages=53–54|url=https://books.google.com/books?id=YnetDwAAQBAJ&pg=PA53|access-date=2022-06-17|archive-date=2023-06-29|archive-url=https://web.archive.org/web/20230629160508/https://books.google.com/books?id=YnetDwAAQBAJ&pg=PA53|url-status=live}}</ref>

The cremasteric muscle covers the testicles and the [[spermatic cord]].<ref name="de Jong">{{Cite book|last=de Jong|first=M. Robert|title=Sonography Scanning E-Book: Principles and Protocols|date=2020|publisher=Elsevier Health Sciences|isbn=978-0-32-376425-4|page=343|url=https://books.google.com/books?id=BvcCEAAAQBAJ&pg=PA343|access-date=2022-06-05|archive-date=2023-06-29|archive-url=https://web.archive.org/web/20230629160507/https://books.google.com/books?id=BvcCEAAAQBAJ&pg=PA343|url-status=live}}</ref> 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.<ref name="de Jong"/> When cooling is required, the cremasteric muscle relaxes and the testicles lower away from the warm body and are able to cool.<ref name="de Jong"/> Contraction also occurs in response to [[Stress (biology)|physical stress]], such as blunt trauma; the testicles withdraw and the scrotum shrinks very close to the body in an effort to protect them.<ref name="Song">{{Cite book|last1=Song|first1=David H|last2=Neligan|first2=Peter C|title=Plastic Surgery E-Book: Volume 4: Trunk and Lower Extremity|date=2017|publisher=Elsevier Health Sciences|isbn=978-0-32-335707-4|page=293|url=https://books.google.com/books?id=cR0xDwAAQBAJ&pg=PA293|access-date=2022-06-10|archive-date=2023-06-29|archive-url=https://web.archive.org/web/20230629160507/https://books.google.com/books?id=cR0xDwAAQBAJ&pg=PA293|url-status=live}}</ref>

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===
{{Further |Bioinformatics#Gene and protein expression}}

The [[human genome]] includes approximately 20,000 protein coding genes: 80% of these [[Gene expression|genes are expressed]] in adult testes.<ref name="Uhlén 1260419">{{Cite journal|last1=Uhlén|first1=Mathias|last2=Fagerberg|first2=Linn|last3=Hallström|first3=Björn M.|last4=Lindskog|first4=Cecilia|last5=Oksvold|first5=Per|last6=Mardinoglu|first6=Adil|last7=Sivertsson|first7=Åsa|last8=Kampf|first8=Caroline|last9=Sjöstedt|first9=Evelina|date=2015-01-23|title=Tissue-based map of the human proteome|journal=Science|language=en|volume=347|issue=6220|pages=1260419|doi=10.1126/science.1260419|issn=0036-8075|pmid=25613900|s2cid=802377}}</ref> The testes have the highest fraction of tissue type-specific genes compared to other organs and tissues.<ref name="Djureinovic 476–488">{{Cite journal|last1=Djureinovic|first1=D.|last2=Fagerberg|first2=L.|last3=Hallström|first3=B.|last4=Danielsson|first4=A.|last5=Lindskog|first5=C.|last6=Uhlén|first6=M.|last7=Pontén|first7=F.|date=2014-06-01|title=The human testis-specific proteome defined by transcriptomics and antibody-based profiling|journal=MHR: Basic Science of Reproductive Medicine|volume=20|issue=6|pages=476–488|doi=10.1093/molehr/gau018|pmid=24598113|issn=1360-9947|doi-access=free}}</ref> About 1000 of them are highly specific for the testes,<ref name="Uhlén 1260419"/> 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.<ref name="Djureinovic 476–488"/> 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]]. Sperm cell flagella and fallopian tube cilia are [[Homology (biology)|homologous]] structures. The testis-specific proteins that show the highest level of expression are [[protamine]]s.<ref name="Hammoud">{{Cite book|last1=Hammoud|first1=S|last2=Carrell|first2=D.T.|title=Biennial Review of Infertility: Volume 2, 2011, Volume 2;Volume 2011|chapter=The Emerging Role of the Sperm Epigenome and its Potential Role in Development|chapter-url=https://books.google.com/books?id=4LgOGlCK_FMC&pg=PA184|date=2011|publisher=Springer Science & Business Media|isbn=978-1-44-198456-2|page=184|access-date=2022-06-16|archive-date=2023-06-29|archive-url=https://web.archive.org/web/20230629160804/https://books.google.com/books?id=4LgOGlCK_FMC&pg=PA184|url-status=live}}</ref>

==Development==
{{Main|Development of the gonads#Testis}}
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.<ref>Online textbook: "[https://www.ncbi.nlm.nih.gov/books/NBK9983/?depth=2 Developmental Biology] {{Webarchive|url=https://web.archive.org/web/20180405085524/https://www.ncbi.nlm.nih.gov/books/NBK9983/?depth=2 |date=2018-04-05 }}" 6th ed. By Scott F. Gilbert (2000) published by Sinauer Associates, Inc. of Sunderland (MA).</ref> 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.<ref name="Steger"/><ref name="Khurana">{{Cite book|last1=Khurana|first1=Indu|last2=Khurana|first2=Arushi|title=Textbook of Medical Physiology - E-book|date=2015|publisher=Elsevier Health Sciences|isbn=978-8-13-124254-4|page=807|url=https://books.google.com/books?id=eTFgDwAAQBAJ&pg=PA807|access-date=2022-06-14|archive-date=2023-06-29|archive-url=https://web.archive.org/web/20230629160817/https://books.google.com/books?id=eTFgDwAAQBAJ&pg=PA807|url-status=live}}</ref> 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.<ref name="Steger"/> 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 [[Anti-Müllerian hormone|AMH]]), which lead to development of the male phenotype, including directing development of the early bipotential gonad toward the male path of development.<ref name="Steger"/> 

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]].<ref name="Moore">{{Cite book|last1=Moore|first1=Keith L.|last2=Persaud|first2=T. V. N.|last3=Torchia|first3=Mark G.|title=The Developing Human - E-Book: Clinically Oriented Embryology|date=2018|publisher=Elsevier Health Sciences|isbn=978-0-32-361156-5|page=259|url=https://books.google.com/books?id=OTaBDwAAQBAJ&pg=PA259|access-date=2022-06-11|archive-date=2023-06-29|archive-url=https://web.archive.org/web/20230629160821/https://books.google.com/books?id=OTaBDwAAQBAJ&pg=PA259|url-status=live}}</ref> In most cases (97% full-term, 70% [[preterm]]), both testes have descended by birth.<ref name="Moore"/><ref name="Winkler">{{Cite book|last1=Winkler|first1=Stephan|last2=Dalkowski|first2=Katja|last3=Mair|first3=Jörg|last4=Klebe|first4=Sonja|title=Sobotta Anatomy Textbook: English Edition with Latin Nomenclature|date=2018|publisher=Elsevier Health Sciences|isbn=978-0-72-067617-4|page=374|url=https://books.google.com/books?id=8uh8DwAAQBAJ&pg=PA374|access-date=2022-06-13|archive-date=2023-06-29|archive-url=https://web.archive.org/web/20230629160801/https://books.google.com/books?id=8uh8DwAAQBAJ&pg=PA374|url-status=live}}</ref> 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]].<ref name="Winkler"/> 

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==
{{anchor|Disease}}

===Protection and injury===
{{Further|Testicular pain}}
* 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]], which is the primary nerve of each testicle.<ref name="Kulkarni">{{Cite book|last=Kulkarni|first=Neeta V|title=Clinical Anatomy: A Problem Solving Approach Author|date=2015|publisher=JP Medical Ltd|isbn=978-9-35-152966-8|page=621|url=https://books.google.com/books?id=vrlEDwAAQBAJ&pg=PA621|access-date=2022-07-07|archive-date=2023-06-29|archive-url=https://web.archive.org/web/20230629160801/https://books.google.com/books?id=vrlEDwAAQBAJ&pg=PA621|url-status=live}}</ref> 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.<ref name="Ovadia">{{Cite book|last1=Ovadia|first1=Aaron E|last2=Yang|first2=Hailiu|last3=Neiderberger|first3=Craig S.|last4=Ho|first4=Christina|last5=Sabia|first5=Michael|last6=Seftel|first6=Allen D.|title=Urogenital Pain: A Clinicians Guide to Diagnosis and Interventional Treatments|chapter=Scrotal Pain|date=2017|publisher=Springer|isbn=978-3-3-1945794-9|page=108|chapter-url=https://books.google.com/books?id=ZPKhDgAAQBAJ&pg=PA108|access-date=2022-07-12|archive-date=2023-06-29|archive-url=https://web.archive.org/web/20230629160807/https://books.google.com/books?id=ZPKhDgAAQBAJ&pg=PA108|url-status=live}}</ref>
* [[Testicular rupture]] is severe trauma affecting the tunica albuginea.<ref name="Bucci">{{Cite book|last1=Bucci|first1=Stefano|last2=Rizzo|first2=Michele|last3=Liguori|first3=Giovanni|last4=Umari|first4=Paolo|last5=Chiriaco|first5=Giovanni|last6=Bertolotto|first6=Michele|title=Atlas of Ultrasonography in Urology, Andrology, and Nephrology|chapter=The Testicles: Trauma, Inflammation and Testicular Torsion|chapter-url=https://books.google.com/books?id=fa-MDgAAQBAJ&pg=PA500|date=2017|publisher=Springer|isbn=978-3-31-940782-1|page=500|access-date=2022-07-07|archive-date=2023-06-29|archive-url=https://web.archive.org/web/20230629161041/https://books.google.com/books?id=fa-MDgAAQBAJ&pg=PA500|url-status=live}}</ref>
* Penetrating injuries to the scrotum may cause [[castration]], 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 (medicine)|saline]] and [[bacitracin]] help remove debris and foreign objects from the wound.<ref name="Wessells">{{Cite book|last=Wessells|first=Hunter|title=Urological Emergencies: A Practical Approach|date=2013|publisher=Springer Science & Business Media|isbn=978-1-62-703423-4|page=96|url=https://books.google.com/books?id=RapDAAAAQBAJ&pg=PA96|access-date=2022-07-12|archive-date=2023-06-29|archive-url=https://web.archive.org/web/20230629161013/https://books.google.com/books?id=RapDAAAAQBAJ&pg=PA96|url-status=live}}</ref>
* [[Jockstrap]]s support and protect the testicles.

===Diseases and conditions===
{{Infobox medical condition (new)
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 |specialty     = [[Urology]], [[Reproductive medicine]]
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}}
* 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.<ref name="DK">{{Cite book|title=Medical Symptoms|date=2022|publisher=Penguin|isbn=978-0-74-406302-8|page=211|url=https://books.google.com/books?id=nbVYEAAAQBAJ&pg=PA211|access-date=2022-07-13|archive-date=2023-06-29|archive-url=https://web.archive.org/web/20230629161300/https://books.google.com/books?id=nbVYEAAAQBAJ&pg=PA211|url-status=live}}</ref><ref name="Kamaya">{{Cite book|last1=Kamaya|first1=Aya|last2=Wong-You-Cheong|first2=Jade|title=Diagnostic Ultrasound: Abdomen and Pelvis E-Book Diagnostic Ultrasound|date=2021|publisher=Elsevier Health Sciences|isbn=978-0-32-379403-9|page=938|url=https://books.google.com/books?id=SydHEAAAQBAJ&pg=PA938|access-date=2022-07-13|archive-date=2023-06-29|archive-url=https://web.archive.org/web/20230629161034/https://books.google.com/books?id=SydHEAAAQBAJ&pg=PA938|url-status=live}}</ref>
* [[Hydrocele testis]] 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.<ref name="Kumar">{{Cite book|last1=Kumar|first1=Vinay|last2=Abbas|first2=Abul K.|last3=Aster|first3=Jon C.|title=Robbins Basic Pathology E-Book Robbins Pathology|date=2017|publisher=Elsevier Health Sciences|isbn=978-0-32-339413-0|page=692|url=https://books.google.com/books?id=YYZMDgAAQBAJ&pg=PA692|access-date=2022-07-13|archive-date=2023-06-29|archive-url=https://web.archive.org/web/20230629161106/https://books.google.com/books?id=YYZMDgAAQBAJ&pg=PA692|url-status=live}}</ref>
* [[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.<ref name="Kamaya"/>
* [[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.<ref name="Jameson">{{Cite book|last1=Jameson|first1=J. Larry|last2=De Groot|first2=Leslie J.|title=Endocrinology: Adult and Pediatric|date=2015|publisher=Elsevier Health Sciences|isbn=978-0-32-332195-2|page=2363|url=https://books.google.com/books?id=xmLeBgAAQBAJ&pg=PA2363|access-date=2022-07-14|archive-date=2023-06-29|archive-url=https://web.archive.org/web/20230629161307/https://books.google.com/books?id=xmLeBgAAQBAJ&pg=PA2363|url-status=live}}</ref> Other genetic conditions can result in the loss of the [[Wolffian ducts]] and allow for the persistence of [[Müllerian ducts]]. Both excess and deficient levels of [[estrogen]]s can disrupt [[spermatogenesis]] and cause [[infertility]].<ref>{{Cite journal
| last1 = Sierens | first1 = J. E.
| last2 = Sneddon | first2 = S. F.
| last3 = Collins | first3 = F.
| last4 = Millar | first4 = M. R.
| last5 = Saunders | first5 = P. T.
| title = Estrogens in Testis Biology
| doi = 10.1196/annals.1336.008
| journal = Annals of the New York Academy of Sciences
| volume = 1061
| pages = 65–76
| year = 2005
| issue = 1
| pmid = 16467258
| bibcode = 2005NYASA1061...65S
| s2cid = 24905596
}}</ref>
* [[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. Those with Bell-clapper are at a higher risk of testicular torsion.<ref name="Soto">{{Cite book|last1=Soto|first1=Jorge A|last2=Lucey|first2=Brian|title=Emergency Radiology: The Requisites E-Book|date=2016|publisher=Elsevier Health Sciences|isbn=978-0-32-339008-8|page=202|url=https://books.google.com/books?id=6MiUCwAAQBAJ&pg=PA202|access-date=2022-07-14|archive-date=2023-06-29|archive-url=https://web.archive.org/web/20230629161307/https://books.google.com/books?id=6MiUCwAAQBAJ&pg=PA202|url-status=live}}</ref><ref name="Melmed">{{Cite book|last1=Melmed|first1=Shlomo|last2=Koenig|first2=Ronald|last3=Rosen|first3=Clifford|last4=Auchus|first4=Richard|last5=Goldfine|first5=Allison|title=Williams Textbook of Endocrinology E-Book|date=2019|publisher=Elsevier Health Sciences|isbn=978-0-32-371154-8|page=669|url=https://books.google.com/books?id=j-u9DwAAQBAJ&pg=PA669|access-date=2022-07-14|archive-date=2023-06-29|archive-url=https://web.archive.org/web/20230629161307/https://books.google.com/books?id=j-u9DwAAQBAJ&pg=PA669|url-status=live}}</ref>
* [[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.<ref name="Winkler"/>
*Testicular enlargement is an [[sensitivity and specificity|unspecific]] sign of various testicular diseases, and can be defined as a testicular size of more than 5&nbsp;cm ([[Anatomical terms of location#Axes|long axis]]) × 3&nbsp;cm (short axis).<ref>[https://books.google.com/books?id=safNmcP3lakC&pg=PA559 Page 559] in: {{cite book|title=Introduction to Vascular Ultrasonography|author=John Pellerito, Joseph F Polak|edition=6|publisher=Elsevier Health Sciences|year=2012|isbn=9781455737666}}</ref>
*[[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.

Scientists are working on developing lab-grown testicles that might help infertile men in the future.<ref>{{Cite web |last=Dewan |first=Pandora |date=2024-02-19 |title=Scientists create lab-grown testicles |url=https://www.newsweek.com/scientists-creat-lab-grown-testicles-1871324 |access-date=2024-02-20 |website=Newsweek |language=en}}</ref>

===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.

Stimulation of testicular functions via [[gonadotropin|gonadotropic-like hormones]] may enlarge their size. Testes may shrink or atrophy during [[Hormone therapy|hormone replacement therapy]] or through [[chemical castration]].

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

==Society and culture==
{{further|Testicles as food|Sex selection}}
[[File:Yoshitoshi tanuki.jpg|thumb|Depiction of ''[[bake-danuki]]'' with oversized testicles]]
The testicles of [[Calf (animal)|calves]], [[Sheep|lambs]], [[rooster]]s, [[Turkey as food|turkeys]], and other animals are eaten in many parts of the world, often under [[euphemistic]] [[culinary name]]s. Testicles are a by-product of the castration of young animals raised for meat, so they might have been a late-spring seasonal specialty.<ref name="Mason-2014">{{Cite book|last=Mason|first=Laura|url=https://books.google.com/books?id=RL6LAwAAQBAJ&dq=spanish+cooking+testicles&pg=PA816|title=The Oxford Companion to Food|publisher=Oxford University Press|year=2014|isbn=9780199677337|editor-last=Davidson|editor-first=Alan|pages=816|language=en|access-date=2021-12-05|archive-date=2023-06-29|archive-url=https://web.archive.org/web/20230629161308/https://books.google.com/books?id=RL6LAwAAQBAJ&dq=spanish+cooking+testicles&pg=PA816|url-status=live}}</ref> In modern times, they are generally frozen and available year-round.

In the [[Middle Ages]], 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.<ref>{{cite web |url=http://genetics.thetech.org/ask/ask76 |title=Understanding Genetics |website=[[The Tech Interactive]] |archive-date=9 November 2013 |archive-url=https://web.archive.org/web/20131109115247/http://genetics.thetech.org/ask/ask76 |url-status=dead}}</ref> As early as 330 BC, [[Aristotle]] prescribed the ligation (tying off) of the left testicle in men wishing to have boys.<ref>Hoag, Hannah. [http://dfire.org/x1405.xml I'll take a girl, please... ''Cherry-picking from the dish of life'']. Drexel University Publication. {{webarchive |url=https://web.archive.org/web/20110831122518/http://dfire.org/x1405.xml |date=August 31, 2011 }}</ref>

===Etymology and slang===
One theory about the etymology of the word ''testis'' is based on [[Roman law]]. The original [[Latin]] word {{Lang|la|testis}}, "witness", was used in the firmly established legal principle "{{Lang|la|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|date=May 2008}}

Another theory says that ''testis'' is influenced by a loan translation, from Greek {{Transliteration|el|parastatēs}} "defender (in law), supporter" that is "two glands side by side".<ref>[[The American Heritage Dictionary of the English Language]], Fourth Edition</ref>

There are multiple slang terms for the testes. They may be referred to as "balls". Frequently, "[[Nut (fruit)|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 [[Deez Nuts (satirist)|satirical political candidate in 2016]]. 

In [[Spanish language|Spanish]], the term {{Lang|es|huevos}} is used, which is Spanish for [[egg]]s.

==Other animals==
[[File:Hühner-Organ.jpg|thumb|Testicles of a [[rooster]]]]
===External appearance===
In [[seasonal breeder]]s, the weight of the testes often increases during the breeding season.<ref name="Johnson2012">{{cite book|author=A.D. Johnson|title=Development, Anatomy, and Physiology|url=https://books.google.com/books?id=yAJMlA0FkAUC&q=%22seasonal+breeders%22|date=2 December 2012|publisher=Elsevier|isbn=978-0-323-14323-3|access-date=19 October 2020|archive-date=29 June 2023|archive-url=https://web.archive.org/web/20230629161308/https://books.google.com/books?id=yAJMlA0FkAUC&q=%22seasonal+breeders%22|url-status=live}}</ref> The [[Dromedary#Testicles|testicles of a dromedary camel]] are {{convert|7|-|10|cm|in|abbr=on}} long, {{convert|4.5|cm|in|abbr=on}} deep and {{convert|5|cm|in|abbr=on}} in width. The right testicle is often smaller than the left.<ref name=anatomy>{{cite book|last=Mukasa-Mugerwa|first=E.|title=The Camel (Camelus dromedarius): A Bibliographical Review|pages=11–3}}</ref>

In [[shark]]s, the testicle on the right side is usually larger. In many bird and mammal species, the left may be larger. [[Fish_anatomy#Testes|Fish usually have two testes]] of a similar size. The primitive [[jawless fish]] have only a single testis, located in the midline of the body, although this forms from the fusion of paired structures in the embryo.<ref name=VB/>

===Location===

====Internal====
The basal condition for mammals is to have internal testes.<ref name="Kleisner">{{cite journal |last1= Kleisner |first1= Karel |last2= Ivell |first2= Richard |last3= Flegr |first3= Jaroslav |date= March 2010|title= The evolutionary history of testicular externalization and the origin of the scrotum |url=http://www.ias.ac.in/article/fulltext/jbsc/035/01/0027-0037  |journal= Journal of Biosciences |publisher= Indian Academy of Sciences |volume= 35 |issue= 1 |pages= 27–37 |doi=  10.1007/s12038-010-0005-7 |pmid= 20413907 |s2cid= 11962872 |access-date=8 December 2018 |archive-url=https://web.archive.org/web/20180225094002/http://www.ias.ac.in/article/fulltext/jbsc/035/01/0027-0037 |archive-date=25 February 2018 |url-status=dead }}</ref> The testes of [[monotremes]],<ref name="Griffiths2012">{{cite book|author=Mervyn Griffiths|title=The Biology of the Monotremes|url=https://books.google.com/books?id=W7nu16_DkTgC&q=testes+OR+scrotum|date=2 December 2012|publisher=Elsevier Science|isbn=978-0-323-15331-7|access-date=19 October 2020|archive-date=29 June 2023|archive-url=https://web.archive.org/web/20230629161534/https://books.google.com/books?id=W7nu16_DkTgC&q=testes+OR+scrotum|url-status=live}}</ref><ref name="NowakWalker1999">{{cite book|author1=Ronald M. Nowak|author2=Ernest Pillsbury Walker|title=Walker's Mammals of the World|url=https://archive.org/details/walkersmammalsof0001nowa|url-access=registration|quote=testes.|date=29 July 1999|publisher=JHU Press|isbn=978-0-8018-5789-8}}</ref> [[xenarthra]]ns,<ref name="NowakWalker1999"/> and [[afrotheria]]ns<ref name="FowlerMikota2008">{{cite book|author1=Murray Fowler|author2=Susan K. Mikota|title=Biology, Medicine, and Surgery of Elephants|url=https://books.google.com/books?id=A3GFYySwY1cC&q=testes+OR+testis+OR+testicles|date=9 January 2008|publisher=John Wiley & Sons|isbn=978-0-470-34411-8|access-date=19 October 2020|archive-date=29 June 2023|archive-url=https://web.archive.org/web/20230629161516/https://books.google.com/books?id=A3GFYySwY1cC&q=testes+OR+testis+OR+testicles|url-status=live}}</ref> remain within the abdomen ([[testicondy]]). There are also some [[marsupial]]s with external testes<ref name="Hunsaker2012">{{cite book|author=Don II Hunsaker|title=The Biology of Marsupials|url=https://books.google.com/books?id=cESCLrRJGm0C&q=testes+OR+testis+OR+testicles|date=2 December 2012|publisher=Elsevier Science|isbn=978-0-323-14620-3|access-date=19 October 2020|archive-date=29 June 2023|archive-url=https://web.archive.org/web/20230629161542/https://books.google.com/books?id=cESCLrRJGm0C&q=testes+OR+testis+OR+testicles|url-status=live}}</ref><ref name="Tyndale-Biscoe2005">{{cite book|author=C. Hugh Tyndale-Biscoe|title=Life of Marsupials|url=https://books.google.com/books?id=KqtlPZJ9y8EC&q=testes+OR+testis+OR+testicles|year=2005|publisher=Csiro Publishing|isbn=978-0-643-06257-3|access-date=2020-10-19|archive-date=2023-06-29|archive-url=https://web.archive.org/web/20230629161517/https://books.google.com/books?id=KqtlPZJ9y8EC&q=testes+OR+testis+OR+testicles|url-status=live}}</ref><ref name="Tyndale-BiscoeRenfree1987">{{cite book|author1=Hugh Tyndale-Biscoe|author2=Marilyn Renfree|title=Reproductive Physiology of Marsupials|url=https://books.google.com/books?id=HpjovN0vXW4C&q=testes|date=30 January 1987|publisher=Cambridge University Press|isbn=978-0-521-33792-2|access-date=19 October 2020|archive-date=29 June 2023|archive-url=https://web.archive.org/web/20230629161518/https://books.google.com/books?id=HpjovN0vXW4C&q=testes|url-status=live}}</ref> and [[boreoeutheria]]n mammals with internal testes, such as the rhinoceros.<ref name="rhinoresourcecenter.com">Schaffer, N. E., et al. "[http://www.rhinoresourcecenter.com/pdf_files/125/1258623199.pdf Ultrasonography of the reproductive anatomy in the Sumatran rhinoceros (Dicerorhinus sumatrensis)] {{Webarchive|url=https://web.archive.org/web/20170923194033/http://www.rhinoresourcecenter.com/pdf_files/125/1258623199.pdf |date=2017-09-23 }}." Journal of Zoo and Wildlife Medicine (1994): 337-348.</ref> [[Cetaceans]] such as whales and dolphins also have internal testes.<ref name="WürsigThewissen2017">{{cite book|author1=Bernd Würsig|author2=J.G.M. Thewissen|author3=Kit M. Kovacs|title=Encyclopedia of Marine Mammals|url=https://books.google.com/books?id=T3zGDgAAQBAJ&q=testes+OR+testis+OR+testicles|date=27 November 2017|publisher=Elsevier Science|isbn=978-0-12-804381-3|access-date=19 October 2020|archive-date=29 June 2023|archive-url=https://web.archive.org/web/20230629161516/https://books.google.com/books?id=T3zGDgAAQBAJ&q=testes+OR+testis+OR+testicles|url-status=live}}</ref><ref name="rommel2007">{{cite book | last1 = Rommel | first1 = S.A. | first2 = D.A. | last2 = Pabst | first3 = W.A. | last3 = McLellan | chapter = Functional anatomy of the cetacean reproductive system, with comparisons to the domestic dog | title = Reproductive Biology and Phylogeny of Cetacea: Whales, Porpoises and Dolphins | date = 2007 | pages = 127–145 | editor-last = Miller | editor-first = D.L. | chapter-url = https://www.researchgate.net/publication/286334625 | doi = 10.1201/b11001| isbn = 9780429063626 }}</ref> 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.<ref name="rommel1998">{{cite magazine | last1 = Rommel | first1 = S.A. | first2 = D.A. | last2 = Pabst | first3 = W.A. | last3 = McLellan | title = Reproductive Thermoregulation in Marine Mammals | journal = American Scientist | volume = 86 | number = 5 | date = 1998 | pages = 440–448 | jstor = 27857097 | url = https://uncw.edu/mmsp/documents/rommeletal1998reproductivethermoregulation.pdf | url-status = live | archive-date = 22 November 2021 | archive-url = https://web.archive.org/web/20211122202231/https://uncw.edu/mmsp/documents/rommeletal1998reproductivethermoregulation.pdf}}</ref><ref name="Pabst, D. Ann 1998">{{cite book | last1 = Pabst | first1 = D.A. | last2 = Sentiel | first2 = A.R | first3 = W.A. | last3 = McLellan | chapter = Evolution of thermoregulatory function in cetacean reproductive systems | title = The Emergence of Whales | publisher = Springer US | date = 1998 | pages = 379–397 | series = Advances in Vertebrate Paleobiology | doi = 10.1007/978-1-4899-0159-0_13 | isbn = 978-1-4899-0161-3 | editor-last = Thewissen | editor-first = J.G.M.}}</ref> In [[odobenid]]s and [[phocid]]s, the location of the testes is para-abdominal, though [[otariid]]s have scrotal testes.<ref name="GullandDierauf2018">{{cite book|author1=Frances M.D. Gulland|author2=Leslie A. Dierauf|author3=Karyl L. Whitman|title=CRC Handbook of Marine Mammal Medicine|url=https://books.google.com/books?id=b1ZSDwAAQBAJ|date=20 March 2018|publisher=CRC Press|isbn=978-1-351-38416-2|access-date=23 January 2020|archive-date=29 June 2023|archive-url=https://web.archive.org/web/20230629161820/https://books.google.com/books?id=b1ZSDwAAQBAJ|url-status=live}}</ref>

====External====
[[Boreoeutheria]]n land mammals, the large group of mammals that includes humans, have externalized testes.<ref name="MillsMarchant-Forde2010">{{cite book|author1=D. S. Mills|author2=Jeremy N. Marchant-Forde|title=The Encyclopedia of Applied Animal Behaviour and Welfare|url=https://books.google.com/books?id=vrueZDfPUzoC&q=%22boreoeutherian+OR+boreoeutheria%22+%28testes+OR+testicles%29+-wikipedia+-%22There+are+also+some+Boreoeutherian+mammals%22&pg=PA293|year=2010|publisher=CABI|isbn=978-0-85199-724-7|pages=293–|access-date=2020-10-19|archive-date=2023-06-29|archive-url=https://web.archive.org/web/20230629161816/https://books.google.com/books?id=vrueZDfPUzoC&q=%22boreoeutherian+OR+boreoeutheria%22+%28testes+OR+testicles%29+-wikipedia+-%22There+are+also+some+Boreoeutherian+mammals%22&pg=PA293|url-status=live}}</ref> 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.{{citation needed|date=July 2017}}

Researchers have wondered why birds, despite having very high core body temperatures, have internal testes and did not evolve external testes.<ref name=bio>[http://www.biolreprod.org/content/56/6/1570.full.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)] {{webarchive|url=https://web.archive.org/web/20150923190501/http://www.biolreprod.org/content/56/6/1570.full.pdf |date=2015-09-23 }}</ref> 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.<ref name="bio"/>

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.<ref>{{cite web |url=http://www.askabiologist.org.uk/answers/viewtopic.php?id=653 |title=Ask a Biologist Q&A / Human sexual physiology – good design? |publisher=Askabiologist.org.uk |date=4 September 2007 |access-date=25 October 2010 |archive-date=27 October 2010 |archive-url=https://web.archive.org/web/20101027155029/http://askabiologist.org.uk/answers/viewtopic.php?id=653 |url-status=live }}</ref>

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.<ref>{{cite web |url=http://old.richarddawkins.net/videos/865-the-human-body-as-an-evolutionary-patchwork/comments |title='The Human Body as an Evolutionary Patchwork' by Alan Walker, Princeton.edu |publisher=RichardDawkins.net |date=2007-04-10 |access-date=25 October 2010 |url-status=dead |archive-url=https://web.archive.org/web/20131109120731/http://old.richarddawkins.net/videos/865-the-human-body-as-an-evolutionary-patchwork/comments |archive-date=9 November 2013 }}</ref> This might have led to enzymes involved in spermatogenesis, spermatogenic [[DNA polymerase beta]] and [[recombinase]] 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 descended testes in mammals|evolution of external testes]] is that it protects the testes from abdominal cavity pressure changes caused by jumping and galloping.<ref>{{cite web|url=https://www.newscientist.com/article/mg15120432.000-science--bumpy-lifestyle-led-to-external-testes.html|title=Science : Bumpy lifestyle led to external testes|access-date=2017-09-15|archive-date=2015-02-22|archive-url=https://web.archive.org/web/20150222041925/http://www.newscientist.com/article/mg15120432.000-science--bumpy-lifestyle-led-to-external-testes.html|url-status=live}}</ref>

Mild, transient scrotal heat stress causes DNA damage, [[infertility|reduced fertility]] and abnormal embryonic development in mice.<ref name="pmid18390691">{{cite journal |vauthors=Paul C, Murray AA, Spears N, Saunders PT |title=A single, mild, transient scrotal heat stress causes DNA damage, subfertility and impairs formation of blastocysts in mice |journal=Reproduction |volume=136 |issue=1 |pages=73–84 |year=2008 |pmid=18390691 |doi=10.1530/REP-08-0036 |doi-access=free }}</ref>  DNA strand breaks were found in spermatocytes recovered from testicles subjected to 40&nbsp;°C or 42&nbsp;°C for 30 minutes.<ref name="pmid18390691" /> 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===
[[Image:Rabbit testis.jpg|thumb|Cross section of rabbit testis, photographed in [[bright-field microscopy]] at 40× magnification]]

The [[Monogamy in animals#Testis size|relative size of the testes]] is often influenced by [[mating system]]s.<ref name="Pitcher, Dunn, Whittingham, 2005">{{cite journal | last1 = Pitcher | first1 = T.E. | last2 = Dunn | first2 = P.O. | last3 = Whittingham | first3 = L.A. | year = 2005 | title = Sperm competition and the evolution of testes size in birds | journal = Journal of Evolutionary Biology | volume = 18 | issue = 3| pages = 557–567 | doi=10.1111/j.1420-9101.2004.00874.x| pmid = 15842485 | s2cid = 18331398 | doi-access = free }}</ref> 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 [[Sperm competition|spermatogenic competition]] for survival. The testes of the [[right whale]] are likely to be the largest of any animal, each weighing around 500&nbsp;kg (1,100&nbsp;lb).<ref>{{cite web |url=http://animaldiversity.ummz.umich.edu/site/accounts/information/Eubalaena_glacialis.html |author=Crane, J. |author2=Scott, R. |year=2002 |title=Eubalaena glacialis |work=Animal Diversity Web |access-date=1 May 2009 |archive-date=28 March 2022 |archive-url=https://web.archive.org/web/20220328231412/https://animaldiversity.org/site/accounts/information/Eubalaena_glacialis.html |url-status=live }}</ref>

Among the [[Hominidae]], [[gorilla]]s have little [[female promiscuity]] and sperm competition and the testes are small compared to body weight (0.03%). [[Chimpanzee]]s have high promiscuity and large testes compared to body weight (0.3%). Human testicular size falls between these extremes (0.08%).<ref name=Shackelford>{{Cite journal | last1 = Shackelford | first1 = T. K. | last2 = Goetz | first2 = A. T. | doi = 10.1111/j.1467-8721.2007.00473.x | title = Adaptation to Sperm Competition in Humans | journal = Current Directions in Psychological Science | volume = 16 | pages = 47–50 | year = 2007 | s2cid = 6179167 }}</ref>

Testis weight also varies in seasonal breeders like [[red fox]]es,<ref name="s537">{{Harvard citation no brackets|Heptner|Naumov|1998|p=537}}</ref> [[golden jackal]]s,<ref name="h154">{{Harvard citation no brackets|Heptner|Naumov|1998|pp=154–155}}</ref> and [[coyote]]s.<ref name="Johnson2012"/>

===Internal structure===
[[Amphibian]]s 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.<ref name=VB>{{cite book |author=Romer, Alfred Sherwood |author2=Parsons, Thomas S.|year=1977 |title=The Vertebrate Body |location=Philadelphia |publisher=Holt-Saunders International |pages= 385–386|isbn= 978-0-03-910284-5}}</ref>

==Gallery==
<gallery class="center">
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
</gallery>

==See also==
{{div col|colwidth=20em}}
* [[Anorchia]]
* [[Anther]]
* [[Antheridium]]
* [[Bollocks]]
* [[Cryptorchidism]] (cryptorchismus)
* [[Ejaculation]]
* [[Eunuch]]s
* [[Gelding]]
* [[List of homologues of the human reproductive system]]
* [[Neutering]]
* [[Perineum]]
* [[Polyorchidism]]
* [[Sterilization (medicine)|Sterilization (surgical procedure)]], [[vasectomy]]
* [[Testicular nubbin]]
{{div col end}}

== General and cited references ==
* {{Cite book |last1=Heptner |first1=V. G. |last2=Naumov |first2=N. P. |year=1998 |title=Mammals of the Soviet Union Vol. II Part 1a, SIRENIA AND CARNIVORA (Sea cows; Wolves and Bears) |url=https://archive.org/details/mammalsofsov211998gept |location=Enfield, NH |publisher=Science Publishers |isbn=978-1-886106-81-9 |oclc=490089621 |access-date=9 November 2013}}

== Citations ==
{{Reflist|33em}}

== External links ==
{{Commons category|Testicles}}
* [[wikt:Thesaurus:testicles|List of synonyms and slang words for testicles in many languages]] at [[wikt:Wiktionary:Thesaurus|Wiktionary's Thesaurus]] 
* [https://www.proteinatlas.org/humanproteome/testis Testis at the Human Protein Atlas]

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[[Category:Testicle| ]]
[[Category:Endocrine system]]
[[Category:Glands]]
[[Category:Human male reproductive system]]
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[[Category:Scrotum]]
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