Testicle

The testicle, also known as the testis, is the male gonad in all gonochoric animals, including humans. It holds the distinction of being homologous to the ovary , the female gonad. Its primary biological mandates are the meticulous production of sperm and the precise secretion of androgens , with testosterone being the principal player.

The intricate dance of testosterone release is orchestrated by luteinizing hormone (LH), a signal dispatched from the anterior pituitary gland . Meanwhile, the genesis of sperm is a complex symphony controlled by follicle-stimulating hormone (FSH), also originating from the anterior pituitary, and crucially, by the very testosterone that the gonads themselves produce.

Structure

Appearance

In a typical adult male, two testicles of roughly equal size reside within the scrotum , a sac that is essentially an extension of the abdominal wall . It’s not uncommon to observe scrotal asymmetry, where one testicle hangs lower than the other. This variation is often attributed to the anatomical nuances of the vasculature. For the vast majority of men, approximately 85%, the right testis assumes a lower position than the left.

Measurement and Volume

Estimating the volume of a testicle can be achieved through palpation, comparing its size to a set of known ellipsoids using an orchidometer . Alternatively, calipers, a ruler, or an ultrasound image can be employed to capture the three key dimensions: length, width, and depth. These measurements can then be plugged into the formula for the volume of an ellipsoid:

$Volume = \frac{4}{3} \cdot \pi \cdot \frac{length}{2} \cdot \frac{width}{2} \cdot \frac{depth}{2} \approx length \cdot width \cdot depth \cdot 0.52$

However, for a more precise determination of the actual testicular volume, the following approximation is generally preferred:

$\approx length \cdot width \cdot depth \cdot 0.71$

An average adult testicle typically measures up to 5 cm × 2 cm × 3 cm (2 in × 3⁄4 in × 1⁄4 in). The Tanner scale , a tool used to gauge the maturity of male genitalia, assigns a maturity stage based on calculated volume, ranging from stage I (less than 1.5 cm³) to stage V (greater than 20 cm³). A normal testicular volume falls within the range of 15 to 25 cm³, with an average of 18 cm³ per testis (the broader range being 12–30 cm³).

The quantity of spermatozoa produced by an adult human male is directly correlated with testicular volume. Larger testicles possess a greater number of seminiferous tubules and Sertoli cells , consequently leading to a higher sperm output per ejaculate . Research indicates a positive correlation between testicular volume and semen profiles.

Internal structure

A transverse section through the left side of the scrotum and left testis reveals a complex internal landscape.

Duct system

The testes are ensheathed by a robust fibrous layer known as the tunica albuginea . Beneath this protective layer lie the seminiferous tubules, exquisitely coiled, microscopic tubes. These tubules are lined by germ cells which, from puberty through senescence, undergo a remarkable transformation into sperm cells , also referred to as spermatozoa or male gametes . The journey of these developing sperm takes them through the seminiferous tubules to the rete testis , situated within the mediastinum testis . From there, they proceed to the efferent ductules , and subsequently to the epididymis , where they undergo maturation, a process known as spermatogenesis . Following maturation, the sperm migrate into the vas deferens , eventually being expelled from the body via the urethra and the urethral orifice through a series of muscular contractions.

Primary cell types

Within the intricate network of seminiferous tubules, germ cells embark on their developmental path, transforming sequentially into spermatogonia , spermatocytes , spermatids , and finally, spermatozoa. These gametes carry the essential DNA required for the fertilization of an ovum . The Sertoli cells constitute the true epithelium of the seminiferous epithelium, playing a critical role in nurturing the development of germ cells into mature spermatozoa. Sertoli cells are also responsible for secreting inhibin , a hormone vital for regulating spermatogenesis. Inhibin achieves this by suppressing the secretion of FSH from the pituitary gland, thereby maintaining a delicate balance in sperm production. Surrounding the seminiferous tubules are the peritubular myoid cells .

Nestled between the seminiferous tubules are the interstitial cells , more commonly known as Leydig cells . These cells are the primary site of production and secretion for testosterone and other androgens . These hormones are indispensable for male sexual development during puberty , influencing characteristics such as secondary sexual characteristics (e.g., facial hair), driving sexual behavior , and modulating libido . Testosterone also plays a crucial role in maintaining testicular volume. Additionally, immature Leydig cells, interstitial macrophages , and epithelial cells are present in this interstitial space.

Blood supply and lymphatic drainage

The testicular blood supply is a tripartite affair, originating from the testicular artery , the cremasteric artery , and the artery to the ductus deferens . The lymphatic drainage of the testes and scrotum follows distinct pathways:

• The paired testicular arteries originate directly from the abdominal aorta and embark on a descent through the inguinal canal . In contrast, the scrotum and the remainder of the external genitalia receive their blood supply from the internal pudendal artery , a branch of the internal iliac artery .
• The testis benefits from a collateral blood supply provided by the cremasteric artery, which branches from the inferior epigastric artery (itself a branch of the external iliac artery ), and the artery to the ductus deferens, a branch of the inferior vesical artery (which stems from the internal iliac artery). This extensive collateral circulation is crucial; for instance, if the testicular artery is ligated during a procedure like a Fowler-Stevens orchiopexy for an undescended testis, the testis can often survive due to these alternative blood sources.
• Lymphatic drainage from the testes mirrors the arterial supply, tracing the testicular arteries back to the paraaortic lymph nodes . The lymph from the scrotum, however, drains into the inguinal lymph nodes .

Layers

The anatomical features of the adult testis bear the imprint of its developmental journey within the abdomen . The layers of tissue that envelop each testicle are, in fact, derived from the corresponding layers of the anterior abdominal wall . The cremasteric muscle , for example, originates from the internal oblique muscle .

The blood–testis barrier

The blood–testis barrier is a critical physiological structure. Large molecules are prevented from traversing from the bloodstream into the lumen of a seminiferous tubule, a feat achieved by the presence of tight junctions that seal adjacent Sertoli cells. This barrier effectively divides the seminiferous tubule into two compartments: the basal compartment, which houses the spermatogonia, and the adluminal compartment, where more mature germ cell forms, such as primary and secondary spermatocytes and spermatids, reside.

The hypothesized function of the blood–testis barrier is to preempt an auto-immune response. Mature sperm, and their associated antigens , emerge relatively late in development, after the immune system has established its tolerance mechanisms during infancy. Since sperm possess antigenic properties distinct from the body’s own tissues, an untreated male could potentially mount an immune response against his own sperm, leading to the production of auto-antibodies. Indeed, the injection of sperm antigens has been shown to induce testicular inflammation (auto-immune orchitis) and compromise fertility. The blood–testis barrier is thought to mitigate the likelihood of sperm proteins triggering such an adverse immune reaction.

Temperature regulation and responses

The external location of the testicles is intrinsically linked to temperature regulation, a concept first explored by Carl Richard Moore in 1926. His hypothesis suggested that testicles are positioned externally because spermatogenesis functions optimally at temperatures slightly below the core body temperature. Temperatures exceeding 33°C can impair this process. The scrotum, through its musculature and vascularization, actively participates in maintaining this delicate thermal balance.

The cremasteric muscle, which envelops the testicles and the spermatic cord , plays a key role. When this muscle contracts, it shortens the cord, drawing the testicles closer to the body and thereby increasing their temperature. Conversely, when cooling is needed, the muscle relaxes, allowing the testicles to descend away from the body’s warmth. This thermoregulatory mechanism is crucial, as temperatures above 36.7°C can significantly impede spermatogenesis. The optimal temperature for the testes is maintained around 34.4°C.

Beyond temperature regulation, the cremasteric muscle also contracts in response to physical stress , such as blunt trauma. This reflex action withdraws the testicles and constricts the scrotum, pulling them tightly against the body for protection. The cremasteric reflex is an involuntary response that elevates the testicles. Voluntary elevation is also possible, involving the pubococcygeus muscle, which engages related musculature.

Gene and protein expression

The human genome, comprising roughly 20,000 protein-coding genes, exhibits a remarkable concentration of expression in adult testes, with approximately 80% of these genes being active. The testes stand out for having the highest proportion of tissue type-specific genes compared to other organs and tissues. Around 1,000 of these genes are highly specific to the testes, and an additional 2,200 show an elevated expression pattern. The majority of these genes are involved in spermatogenesis within the seminiferous tubules. Notably, sperm cells express proteins that orchestrate the development of flagella ; coincidentally, these same proteins are found in the cells lining the female fallopian tube , where they drive the development of cilia . The flagella of sperm and the cilia of the fallopian tubes are considered homologous structures. Among the most highly expressed testis-specific proteins are protamines .

Development

The growth of the testes occurs in two significant phases: the embryonic phase and the pubertal phase.

During mammalian development, the gonads initially possess the potential to develop into either ovaries or testes. In humans, beginning around week 4 of gestation, gonadal rudiments emerge from the intermediate mesoderm adjacent to the developing kidneys. By approximately week 6, sex cords begin to form within the nascent testes. These cords are composed of precursor Sertoli cells, which provide nourishment and support to the germ cells that migrate into the gonads shortly before the onset of sex determination. In males, the SRY gene, located on the Y chromosome , initiates the cascade of male sex determination. This involves the downstream regulation of key sex-determining factors, such as GATA4 , SOX9 , and AMH , ultimately directing the development of the early bipotential gonad towards the male phenotype.

A critical aspect of testicular development is their descent. They follow a specific migratory path, originating high in the posterior fetal abdomen, traversing the inguinal ring, and continuing through the inguinal canal to settle within the scrotum . In the majority of full-term births (97%) and a significant portion of preterm births (70%), both testes have descended by the time of birth. When descent is incomplete, often affecting only one testis, the condition is termed cryptorchidism . While this condition frequently resolves spontaneously within the first six months of life, if the testes do not descend sufficiently into the scrotum, surgical intervention may be necessary to mitigate the risks of infertility and testicular cancer .

Testicular growth accelerates with the onset of spermatogenesis at puberty. The final size of the testes is influenced by factors such as lytic function, the intensity of sperm production, the amount of interstitial fluid , and the fluid production by Sertoli cells. The testes are typically fully descended well before an individual reaches puberty.

Clinical significance

Protection and injury

The testicles are acutely sensitive to impact and injury. Pain signals from the testicles travel upwards through the spermatic plexus , the primary nerve supply, into the abdominal cavity, often manifesting as pain in the hip and back. This pain typically subsides within minutes.

Testicular torsion represents a dire medical emergency. The longer the delay in seeking medical intervention, the greater the risk of losing the testicle due to prolonged ischemia. Prompt surgical de-torsion within six hours of symptom onset offers a 90% chance of saving the testicle.

Testicular rupture signifies severe trauma that compromises the integrity of the tunica albuginea. Penetrating injuries to the scrotum can lead to castration , involving the physical separation or destruction of the testes, potentially accompanied by partial or complete loss of the penis, resulting in permanent sterility if prompt reattachment is not achieved. To combat severe infection, liberal application of saline and bacitracin is recommended for wound debridement and removal of foreign bodies. Jockstraps are often employed to provide support and protection to the testicles.

Diseases and conditions

Medical condition

A range of medical conditions can affect the testicles. Regular testicular self-examination is strongly advised to facilitate early detection of testicular cancer , neoplasms , and other health issues.

• Varicocele involves the swelling of veins within the scrotum, typically affecting the left side, though the testis itself usually remains unaffected.
• Hydrocele testis is characterized by swelling around the testes due to the accumulation of clear fluid within a membranous sac. This is the most common cause of scrotal swelling, and the testis itself is usually normal.
• Spermatocele is a cyst that forms from a blockage in the tubules of the rete testis or the head of the epididymis , distended with a fluid containing spermatozoa.
• Endocrine disorders can exert a significant impact on both the size and function of the testes.
• Certain inherited genetic conditions, stemming from mutations in crucial developmental genes, can impede testicular descent, leading to abdominal or inguinal testes. These undescended testes are typically non-functional and carry an increased risk of developing cancer. Other genetic anomalies can result in the failure of the Wolffian ducts to develop and the persistence of Müllerian ducts . Imbalances in estrogen levels, whether excessive or deficient, can disrupt spermatogenesis and lead to infertility .
• The Bell-clapper deformity describes a condition where the testicle is not securely attached to the scrotal walls, allowing it to rotate freely within the tunica vaginalis. Individuals with this deformity face a heightened risk of testicular torsion.
• Orchitis refers to inflammation of the testicles.
• Epididymitis is a painful inflammation of the epididymis, often caused by bacterial infection but sometimes of unknown etiology.
• Anorchia signifies the congenital absence of one or both testicles.
• Cryptorchidism , or “undescended testicles,” occurs when a testicle fails to descend into the scrotum of a male infant.
• Testicular enlargement, defined as a testicular size exceeding 5 cm (long axis) × 3 cm (short axis), serves as a non-specific indicator of various testicular diseases.
• Blue balls is a colloquial term for a condition involving temporary fluid congestion in the testicles and prostate region, often resulting from prolonged sexual arousal.

Testicular prostheses are available to restore the appearance and feel of absent testicles, whether due to injury or as part of gender-affirming care for gender dysphoria . Implantation in canines has also been documented. Scientists are actively pursuing the development of lab-grown testicles, a breakthrough that could offer hope to infertile men in the future.

Effects of exogenous hormones

It is possible, to a degree, to alter testicular size. Beyond direct injury or exposure to adverse conditions like excessive heat, testicular shrinkage can be induced by exogenous hormonal interventions that compete with the body’s intrinsic hormonal function. The use of steroidal hormones, particularly anabolic steroids for muscle enhancement, frequently leads to the undesirable side effect of testicular shrinkage. Conversely, stimulation of testicular functions through gonadotropic-like hormones can lead to an increase in size. Testicular atrophy and shrinkage can also occur during hormone replacement therapy or as a result of chemical castration . In all instances, a reduction in testicular volume is accompanied by a decrease in spermatogenesis.

Society and culture

The testicles of various animals, including calves , lambs , roosters , and turkeys , are consumed in numerous cultures worldwide, often under euphemistic culinary terms. Historically, testicles were a by-product of animal castration for meat production, potentially making them a seasonal delicacy. Today, they are typically frozen and available year-round.

In the Middle Ages , a curious practice existed among men desiring a son: the removal of the left testicle. This stemmed from the prevailing belief that the right testicle produced “boy” sperm and the left produced “girl” sperm. This notion dates back to at least 330 BC, with Aristotle himself advocating for the ligation (tying off) of the left testicle in men seeking male offspring.

Etymology and slang

The etymology of the word “testis” is subject to a fascinating theory rooted in Roman law . The original Latin term, testis, meant ‘witness’. This is thought to derive from the legal principle “Testis unus, testis nullus” (one witness equals no witness), which stipulated that the testimony of a single individual in court was inadmissible unless corroborated by at least one other witness. Consequently, it became common practice to present two witnesses, often bribed, to testify identically in lawsuits with ulterior motives. As these witnesses invariably came in pairs, the term testis was extended, frequently in its diminutive form (testiculus, testiculi), to refer to the two testicles.

An alternative theory suggests that testis might be influenced by a loan translation from Greek parastatēs, meaning ‘defender (in law), supporter’, humorously interpreted as “two glands side by side.”

Numerous slang terms exist for the testes, with “balls” being a common one. The term “nuts” is also frequently used, often with intentional misspelling as “nutz,” owing to the perceived geometric resemblance. A notable variant, “Deez Nuts,” gained notoriety through a satirical political candidate in 2016 .

In Spanish , the slang term huevos is employed, which translates to “eggs.” This is a somewhat non-sequitur, or perhaps a false analogy, as the male gonad produces sperm, not the female gametes found in eggs. A more plausible explanation for the slang usage of huevos is that the overall shape of each epididymis within the scrotum bears a resemblance to a common bird egg.

Other animals

In seasonal breeders , the testes often increase in weight during the breeding season. The testicles of a dromedary camel are approximately 7–10 cm long, 4.5 cm deep, and 5 cm wide, with the right testicle frequently being smaller than the left.

In sharks , the right testicle is typically larger, while in many bird and mammal species, the left may be dominant. Fish generally possess two testes of similar size. However, primitive jawless fish have a single, midline testis, which develops from the fusion of paired embryonic structures.

Location

• Internal: The default state for mammals is to have internal testes. The testes of monotremes , xenarthrans , and afrotherians remain within the abdomen, a condition known as testicondy . Some marsupials also exhibit external testes, while certain boreoeutherian mammals, such as the rhinoceros, retain internal testes. Cetaceans , including whales and dolphins, also have internal testes. This internal positioning, while potentially increasing drag in water, is facilitated by specialized circulatory systems that cool arterial blood destined for the testes by routing it near veins carrying cooled venous blood from the skin. In odobenids and phocids , the testes are located para-abdominally, whereas otariids possess scrotal testes.

• External: Boreoeutherian land mammals, a diverse group encompassing humans, have externalized testes. This adaptation is linked to the optimal functioning of spermatogenesis at temperatures slightly below core body temperature. The testes are suspended outside the body within the scrotum, attached by the spermatic cord. Several hypotheses attempt to explain the evolution of external testes in most boreotherian mammals. One perspective suggests it’s an evolutionary artifact, with enzymes adapted for cooler temperatures due to external testes evolving for other reasons. Another view posits that the cooler temperature afforded by externalization simply enhances sperm production efficiency. The classic hypothesis centers on the idea that cooler testicular temperatures facilitate more efficient fertile spermatogenesis, as no enzymes operating at normal core body temperature can match the efficiency of those evolved for this slightly lower temperature range. It is proposed that early mammals had lower body temperatures, allowing their testes to function effectively internally. However, boreotherian mammals may have evolved higher core body temperatures, necessitating the development of external testes for cooling. Evidence supporting this includes the observation that mammals with internal testes, such as monotremes, armadillos, sloths, elephants, and rhinoceroses, generally have lower core body temperatures than their counterparts with external testes.

Researchers have long pondered why birds, despite their characteristically high core body temperatures, retain internal testes and have not evolved external ones. While it was once theorized that birds utilized their air sacs for internal testicular cooling, subsequent research indicates that avian testes can indeed function effectively at core body temperature. Some mammals exhibiting seasonal breeding patterns keep their testes internal until the breeding season commences, at which point they descend and enlarge, becoming external.

The ancestor of the boreoeutherian mammals might have been a small creature requiring exceptionally large testes for sperm competition , thus necessitating their externalization. This evolutionary pressure could have driven the development of enzymes involved in spermatogenesis, such as spermatogenic DNA polymerase beta and recombinase activities, to achieve a unique optimal temperature slightly below core body temperature. As boreoeutherian mammals diversified into larger forms or those facing less intense sperm competition, they retained these enzymes adapted for cooler temperatures, thus maintaining external testes. This explanation is somewhat challenged by the existence of external testes in the kangaroo , a non-boreoeutherian mammal, suggesting that the evolutionary path to external testes might have occurred independently or been driven by different selective pressures in large animals.

An alternative argument for the evolution of external testes suggests a protective function against the pressure fluctuations within the abdominal cavity during activities like jumping and galloping. Studies in mice have demonstrated that mild, transient scrotal heat stress can induce DNA damage, reduced fertility , and abnormal embryonic development. DNA strand breaks were observed in spermatocytes recovered from testicles subjected to temperatures of 40°C or 42°C for 30 minutes. These findings lend support to the notion that the external location of the testicles provides an adaptive advantage by shielding spermatogenic cells from heat-induced DNA damage, thereby preventing infertility and germline mutation .

Size

The relative size of the testes is often influenced by the prevailing mating system within a species. Testicular size, as a proportion of body weight, exhibits considerable variation across the mammalian kingdom. A general trend indicates that testicular size tends to be larger in species with multiple mating partners (e.g., harems, polygamy ). Consequently, the production of testicular sperm and seminal fluid is also greater in polygamous animals, likely as a result of spermatogenic competition for reproductive success. The testes of the right whale are believed to be the largest of any animal, each weighing approximately 500 kg (1,100 lb).

Within the Hominidae family, gorillas , characterized by low levels of female promiscuity and sperm competition, possess relatively small testes compared to their body weight (0.03%). In contrast, chimpanzees , exhibiting high promiscuity, have significantly larger testes relative to their body weight (0.3%). Human testicular size falls between these extremes, averaging around 0.08%. Testis weight also fluctuates seasonally in species such as red foxes , golden jackals , and coyotes .

Internal structure

In amphibians and most fish, seminiferous tubules are absent. Instead, sperm are produced within spherical structures called sperm ampullae. These are seasonal, releasing their sperm during the breeding season before being reabsorbed by the body. New sperm ampullae then begin to form and mature in preparation for the next breeding cycle. These ampullae are functionally analogous to the seminiferous tubules found in higher vertebrates , containing a similar array of cell types.

Gallery

A collection of images illustrating the testicle and its surrounding structures can be found in the gallery, offering visual representations of its anatomy and appearance.

See also

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

General and cited references

A comprehensive list of references, including works by Heptner and Naumov, provides detailed information on the subject.

Citations

The text is supported by numerous citations, each referencing specific scientific publications and textbooks that provide the factual basis for the information presented. These citations are crucial for verifying the accuracy and depth of the content.