Testicle
The testicle is a structure so ordinary in human anatomy that most people never think twice about it, yet it performs two of the most consequential jobs in biology: producing sperm and secreting testosterone. In humans, these paired organs sit outside the body entirely, suspended within the scrotum, a fact that strikes many people as a curious design choice. Why should organs so essential to reproduction be left outside the body's core, vulnerable to impact and temperature change? The answer to that question runs deep into evolutionary history, cutting across mammal species from rhinoceroses to right whales. Along the way, the testicle turns out to be one of the most genetically active organs in the human body, expressing roughly 80% of all protein-coding genes. What does it actually look like inside? How does it regulate its own temperature to within a fraction of a degree? And what happens when something goes wrong? These are the threads this documentary will follow.
Beneath a tough outer shell called the tunica albuginea, each testicle is threaded with coiled tubes so fine they are called seminiferous tubules. These tubules are lined with germ cells that transform, from puberty through old age, into fully formed sperm. The journey does not end there. Newly created sperm travel from the seminiferous tubules to a region called the rete testis, then through efferent ducts, and finally into the epididymis, where they complete their maturation in a process known as spermatogenesis.
Sertoli cells line the walls of the seminiferous tubules and act as the primary support system for developing sperm. They secrete a hormone called inhibin, which feeds back to the pituitary gland to help regulate how much sperm the body is producing at any given time. Nestled between the tubules are Leydig cells, which produce testosterone and other androgens. These androgens govern not only sexual behavior and libido but also the secondary sexual characteristics of puberty, including facial hair.
A specialized structure called the blood-testis barrier forms at tight junctions between adjacent Sertoli cells. Large molecules cannot cross from the bloodstream into the lumen of the seminiferous tubules. This barrier likely exists because sperm, which emerge long after the immune system has established its baseline tolerance in infancy, carry antigens that the body might otherwise attack. When sperm antigens are experimentally injected, the result is inflammation of the testis, a condition called auto-immune orchitis, and reduced fertility.
Carl Richard Moore proposed in 1926 that the external position of testicles exists because spermatogenesis operates best at temperatures slightly below core body temperature. The mechanism the body uses to meet that demand is precise. Testicular temperature is maintained at 34.4 degrees Celsius, which sits below body temperature, because temperatures above 36.7 degrees Celsius impede sperm production.
The cremasteric muscle, which wraps around the testicles and the spermatic cord, is central to this regulation. When the body is cold, the muscle contracts, pulling the testicles closer to the body's warmth. When cooling is needed, the muscle relaxes, lowering the testicles away from the core. The same muscle also responds to physical threat: blunt trauma triggers an immediate contraction that draws the testicles up toward the body as a protective reflex.
Mild heat stress, even temporary, carries measurable consequences. Studies in mice found DNA strand breaks in spermatocytes recovered from testicles exposed to temperatures of 40 or 42 degrees Celsius for just 30 minutes. Those breaks were linked to reduced fertility and abnormal embryonic development. The implication is that external positioning is not merely about efficiency but about protecting the genetic integrity of sperm from heat-induced DNA damage that could otherwise pass mutations to offspring.
Around week 4 of human fetal development, the structures that will become gonads are present within the intermediate mesoderm, adjacent to the developing kidneys. At this stage, they are bipotential, capable of becoming either ovaries or testes. By about week 6, a gene on the Y chromosome called SRY initiates the cascade toward male development, activating downstream factors including GATA4, SOX9, and AMH, which direct the early gonad down the male path.
From their starting position high in the posterior fetal abdomen, the testes descend through the inguinal ring, pass through the inguinal canal, and arrive in the scrotum. In 97% of full-term births and 70% of premature births, both testes complete this journey before the infant is born. When one or both fail to descend, the condition is called cryptorchidism. In many cases it resolves on its own within the first half year of life, but if the testes remain too high, surgery is required. The stakes are real: undescended testes carry a higher risk of infertility and testicular cancer.
Growth happens in two distinct surges. The first is during embryonic development, and the second comes with puberty, when spermatogenesis begins. Testicular volume, measured by an orchidometer or by calculating the dimensions of the organ using the formula for an ellipsoid, signals this maturity. The Tanner scale assigns stages from stage I, at a volume of less than 1.5 cubic centimeters, to stage V, at a volume greater than 20 cubic centimeters. Normal adult volume falls between 15 and 25 cubic centimeters, with an average of 18 cubic centimeters per testis.
The human genome contains approximately 20,000 protein-coding genes. Of those, 80% are expressed in the adult testes, giving the testes the highest fraction of tissue-specific genes of any organ in the body. Around 1,000 genes are highly specific to testicular tissue, and about 2,200 show an elevated pattern of expression there.
Most of these genes encode proteins expressed in the seminiferous tubules and connected directly to spermatogenesis. Sperm cells express proteins that drive the formation of flagella, the whip-like tails that propel them. Those same proteins appear in the cells lining the fallopian tube in females, where they instead form cilia. The shared molecular machinery behind sperm flagella and fallopian tube cilia marks them as homologous structures, analogous in origin and mechanism despite their different roles. The testis-specific proteins expressed at the highest levels are called protamines.
Testicular volume is directly proportional to sperm output. Larger testes contain more seminiferous tubules and more Sertoli cells, and men with greater testicular volume produce, on average, more sperm per ejaculate. Testosterone, secreted by the Leydig cells, regulates testicular volume itself, and the release of testosterone is governed by luteinizing hormone from the anterior pituitary gland.
The testes of a dromedary camel are 7 to 10 cm long, 4.5 cm deep, and 5 cm wide, with the right testicle often smaller than the left, a pattern that reverses in sharks, where the right testicle is usually larger. In many bird and mammal species, the left tends to be larger. Fish generally have two testes of similar size, and the most primitive jawless fish possess only a single testis, located at the body's midline, which forms from the fusion of paired embryonic structures.
Not all mammals place their testes outside the body. Monotremes, xenarthrans, and afrotherians retain internal testes, a condition called testicondy. Whales and dolphins also keep their testes internal, because external testes would increase hydrodynamic drag. They cool them instead through specialized circulatory systems that route arterial blood near veins carrying cooled venous blood from the skin's surface. Rhinoceroses, despite being large land mammals, also have internal testes.
Testicular size relative to body weight correlates strongly with mating systems. Gorillas, with little female promiscuity and minimal sperm competition, have testes that represent only 0.03% of body weight. Chimpanzees, in which promiscuity is high and sperm competition intense, have testes at 0.3% of body weight. Humans fall between these at 0.08%. The right whale holds the record for the largest testes of any known animal, each weighing around 500 kg. One leading hypothesis is that the ancestor of boreoeutherian mammals, the large group that includes humans, may have required very large testes for sperm competition, driving the evolution of enzymes that function best at cooler temperatures, an adaptation that persists even in species that no longer face the same competitive pressures.
Testicular torsion, in which the testicle twists on the spermatic cord and cuts off its own blood supply, is a medical emergency. De-torsion surgery performed within six hours of onset carries a 90% chance of saving the organ. As ischemia extends beyond that window, the chances of loss rise sharply. Bell-clapper deformity, in which the testicle is not anchored to the scrotal wall and can rotate freely within the tunica vaginalis, raises the risk of torsion significantly.
Penetrating injuries can cause rupture of the tunica albuginea or, in severe cases, castration. Treatment includes saline and bacitracin to clear debris and foreign matter. Conditions such as varicocele, hydrocele testis, and spermatocele affect the scrotum and surrounding structures without necessarily impairing the testis itself. Endocrine disorders and certain inherited mutations in developmental genes can impair testicular descent or disrupt spermatogenesis. Both excess and deficient estrogen levels can cause infertility.
Exogenous hormones directly affect testicular size. Anabolic steroids, taken for muscle enhancement, often cause testicular shrinkage as a side effect because they compete with the body's own hormonal signals. Testes may also atrophy during hormone replacement therapy or chemical castration, and in every case the loss in volume corresponds to a loss of spermatogenesis.
In the Middle Ages, men who wanted a son sometimes had their left testicle removed, acting on a belief that the right testis produced male sperm and the left produced female sperm. Aristotle, writing as early as 330 BC, prescribed the ligation of the left testicle in men hoping to father boys. Testicular prostheses, which mimic the appearance and feel of one or both testes, are now available after injury or as part of treatment for gender dysphoria, and have even been implanted in dogs. Scientists are also working on developing lab-grown testicles that might one day help infertile men.
Common questions
What are the primary functions of the testicle?
The testicle has two primary functions: producing sperm and secreting androgens, chiefly testosterone. Sperm production is controlled by follicle-stimulating hormone from the anterior pituitary and by testosterone produced within the gonads themselves.
Why are human testicles located outside the body?
Testicles are external because spermatogenesis operates best at temperatures slightly below core body temperature. Carl Richard Moore proposed this in 1926. Testicular temperature in humans is maintained at 34.4 degrees Celsius, and temperatures above 36.7 degrees Celsius impede sperm production.
What is cryptorchidism and why does it matter medically?
Cryptorchidism is the failure of one or both testicles to descend into the scrotum before birth. It occurs in about 3% of full-term births. If the testes do not descend far enough on their own within the first half year of life, surgery is required because undescended testes carry increased risks of infertility and testicular cancer.
What is testicular torsion and how quickly must it be treated?
Testicular torsion occurs when the testicle twists on the spermatic cord and cuts off its blood supply; it is a medical emergency. De-torsion surgery performed within six hours of onset carries a 90% chance of saving the testicle.
How does testicular size relate to sperm production?
The number of sperm produced is directly proportional to testicular volume. Larger testes contain more seminiferous tubules and Sertoli cells, resulting in more sperm per ejaculate. Normal adult testicular volume ranges from 15 to 25 cubic centimeters, with an average of 18 cubic centimeters per testis.
Which animal has the largest testicles of any known species?
The right whale has the largest testes of any known animal, with each testis weighing around 500 kg (1,100 lb). Testicular size across mammals correlates with mating systems and the intensity of sperm competition.
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