Testicle
A testicle, also called a testis, is the male gonad in all gonochoric animals. It functions as the primary producer of sperm and the secretor of androgens like testosterone. Male humans typically possess two testicles of similar size housed within the scrotum. Scrotal asymmetry occurs frequently because one testicle often hangs lower than the other. For 85% of men, the right testis extends farther down into the scrotum than the left one. This difference stems from variations in the vasculature's anatomy.
The volume of an adult human testicle measures up to 25 cubic centimeters on average. The Tanner scale assigns maturity stages based on calculated volume ranging from less than 1.5 cm3 to greater than 20 cm3. Normal volume sits between 15 and 25 cm3 with an average of 18 cm3 per testis. Men with larger testicles produce more sperm cells in each ejaculate due to higher counts of seminiferous tubules and Sertoli cells.
The testes are covered by a tough fibrous shell known as the tunica albuginea. Under this layer lie very fine-coiled tubes called seminiferous tubules. These tubules contain germ cells that develop into spermatozoa through puberty until old age. Developing sperm travel through these tubules to the rete testis located in the mediastinum testis. They then move to the efferent ducts and finally to the epididymis where they mature.
Blood supply comes from three distinct sources including the testicular artery and the cremasteric artery. The paired testicular arteries arise directly from the abdominal aorta and descend through the inguinal canal. Lymphatic drainage follows the testicular arteries back to the paraaortic lymph nodes while scrotal lymph drains to the inguinal lymph nodes.
Within the seminiferous tubules, germ cells develop into spermatogonia, spermatocytes, spermatids and spermatozoa. This process is called spermatogenesis and occurs continuously from puberty onward. The gametes contain DNA required for fertilization of an ovum. Sertoli cells act as the true epithelium of the seminiferous epithelium supporting germ cell development.
Sertoli cells secrete inhibin which plays a crucial role in regulating spermatogenesis. Inhibin helps control sperm production by inhibiting the secretion of follicle-stimulating hormone from the pituitary gland. Between the seminiferous tubules lie interstitial cells known as Leydig cells. These cells produce and secrete testosterone and other androgens important for puberty and sexual behavior.
Testosterone controls testicular volume and drives the entire reproductive cycle. Large molecules cannot pass from blood into the lumen of a seminiferous tubule due to tight junctions between adjacent Sertoli cells. Spermatogonia occupy the basal compartment deep to these tight junctions while mature forms like primary spermatocytes occupy the adluminal compartment.
The function of this blood-testis barrier may prevent an autoimmune reaction against sperm proteins. Mature sperm emerge significantly after immune tolerance is set during infancy. Since sperm are antigenically different from self-tissue, a male animal could react immunologically to his own sperm if not protected.
During mammalian development gonads are initially capable of becoming either ovaries or testes. In humans starting at about week 4 the gonadal rudiments appear within the intermediate mesoderm adjacent to developing kidneys. At about week 6 sex cords develop within forming testes made up of early Sertoli cells surrounding germ cells that migrate in shortly before sex determination begins.
In males the sex-specific gene SRY found on the Y chromosome initiates sex determination. This gene regulates downstream factors such as GATA4 SOX9 and AMH leading to development of the male phenotype. Testes follow the path of descent from high in the posterior fetal abdomen to the inguinal ring and beyond into the scrotum.
In most cases both testes have descended by birth for full-term infants though only 70% of preterm infants achieve this status. When one testis fails to descend it is called cryptorchidism. Most cases resolve themselves within the first half year of life but surgical anchoring may be required if they do not descend far enough.
The testes grow in response to the start of spermatogenesis and size depends on lytic function and interstitial fluid production. The testicles are fully descended before the male reaches puberty. Size also varies based on sperm production amount present in the testis.
Testicular torsion represents a medical emergency where time determines whether the organ can be saved. There is a 90% chance to save the testicle if de-torsion surgery occurs within six hours of onset. Longer delays increase ischemia and the likelihood that the testicle will be lost permanently.
Varicocele involves swollen veins usually affecting the left side while the testis itself remains normal. Hydrocele testis causes swelling around testes due to accumulation of clear liquid within a membranous sac. It stands as the most common cause of scrotal swelling overall.
Spermatocele forms when a tubule of the rete testis or head of epididymis distends with barely watery fluid containing spermatozoa. Endocrine disorders can affect both size and function of the testis through hormonal imbalances. Certain inherited conditions involving mutations in key developmental genes impair descent resulting in abdominal or inguinal testes.
Bell-clapper deformity allows the testicle to rotate freely on the spermatic cord within the tunica vaginalis. Those with this condition face higher risks of testicular torsion. Orchitis describes inflammation of the testicles while epididymitis refers to painful inflammation of the epididymis often caused by bacterial infection.
The basal condition for mammals is to have internal testes found in monotremes xenarthrans and afrotherians. Cetaceans such as whales and dolphins also possess internal testes kept cool by special circulatory systems placing arteries near veins bringing cooled venous blood from skin. Externalized testes appear in boreoeutherian land mammals including humans where they function best at temperatures lower than core body temperature.
One hypothesis suggests cooler temperatures allow more efficient fertile spermatogenesis since no enzymes operate at normal core body temperature as efficiently. Early mammals had lower body temperatures so their testes worked efficiently inside bodies. Boreotherian mammals may have developed higher body temperatures requiring external placement to keep them cool.
Researchers wonder why birds despite having very high core body temperatures maintain internal testes without evolving external ones. Some studies once theorized birds used air sacs to cool testes internally but later revealed they function at core body temperature. Seasonal breeders like red foxes keep testes internal until breeding season when they descend and increase size.
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 degrees Celsius or 42 degrees Celsius for thirty minutes.
The testicles of calves lambs roosters turkeys and other animals are eaten in many parts of the world under euphemistic culinary names. They serve as a by-product of castration of young animals raised for meat making them a late-spring seasonal specialty historically. In modern times these organs are generally frozen and available year-round regardless of season.
In the Middle Ages men who wanted boys sometimes had their left testicle removed based on belief that the right testicle made boy sperm while the left made girl sperm. As early as 330 BC Aristotle prescribed ligation of the left testicle in men wishing to have sons. This practice reflects ancient misconceptions about gender selection mechanisms.
One theory regarding etymology links the word testis to Roman law principles involving witness testimony requiring corroboration. Another theory suggests influence from Greek phrases meaning two glands side by side. Multiple slang terms exist including balls nuts and Deez Nuts used satirically during political campaigns in 2016.
Spanish slang uses huevos which means eggs typically referring to unfertilized chicken eggs despite biological differences. The male gonad produces sperm transporting male gametes to an egg rather than female gametes found in actual eggs. Perhaps the term exists because epididymus bodies resemble common bird eggs visually.
Common questions
What is a testicle and what does it produce?
A testicle, also called a testis, functions as the primary producer of sperm and the secretor of androgens like testosterone. It serves as the male gonad in all gonochoric animals.
How large are adult human testicles on average?
The volume of an adult human testicle measures up to 25 cubic centimeters on average with normal volume sitting between 15 and 25 cm3. The Tanner scale assigns maturity stages based on calculated volume ranging from less than 1.5 cm3 to greater than 20 cm3.
When do testes descend into the scrotum during development?
Testes follow the path of descent from high in the posterior fetal abdomen to the inguinal ring and beyond into the scrotum starting at about week 6. In most cases both testes have descended by birth for full-term infants though only 70% of preterm infants achieve this status.
Why do testicles hang lower on one side?
Scrotal asymmetry occurs frequently because one testicle often hangs lower than the other due to variations in the vasculature's anatomy. For 85% of men the right testis extends farther down into the scrotum than the left one.
What happens if a testicle twists causing torsion?
Testicular torsion represents a medical emergency where time determines whether the organ can be saved. There is a 90% chance to save the testicle if de-torsion surgery occurs within six hours of onset.