Inside each testicle is a tightly organized system of coiled tubes, hormone-producing cells, and a network of ducts, all wrapped in a tough protective shell. Despite being roughly the size of a small plum (averaging about 20 mL in volume), a single testicle contains an astonishing amount of internal architecture dedicated to two jobs: making sperm and producing testosterone.
The Outer Shell
The outermost functional layer of the testicle itself is the tunica albuginea, a dense, white, inelastic tissue that acts like a pressurized casing. It gives the testicle its firm, smooth feel. Surrounding that is a thinner membrane called the tunica vaginalis, which allows the testicle to move freely inside the scrotum without friction.
From the inner surface of the tunica albuginea, thin walls of connective tissue extend inward like the segments of an orange. These dividers split the interior into roughly 200 to 300 small compartments called lobules. Each lobule contains its own set of coiled tubes where sperm production takes place.
Seminiferous Tubules: Where Sperm Are Made
The bulk of the testicle’s interior is filled with seminiferous tubules, tightly coiled tubes where sperm cells develop. Each testicle contains somewhere between 800 and 1,600 of these tubules. If you could uncoil and lay them end to end, they’d stretch about 600 meters, roughly four times the height of the Empire State Building, all packed into something you can hold in your hand.
The walls of each tubule are lined with two key cell types. Germ cells are the precursors that gradually mature into sperm through a process called spermatogenesis. Sitting among them are Sertoli cells, sometimes called “nurse cells,” which support, nourish, and protect developing sperm at every stage. Sertoli cells also create a barrier that shields immature sperm from the immune system, which would otherwise attack them as foreign.
A single sperm cell takes about 64 days to fully develop inside these tubules. The process happens in waves, so the testicle is continuously producing sperm at different stages of maturity rather than in one big batch.
The Spaces Between the Tubes
In the tissue gaps between the seminiferous tubules sit Leydig cells, the testicle’s hormone factories. These cells are the body’s primary source of testosterone. What’s remarkable is the concentration: testosterone levels inside the testicle are roughly 180 times higher than in the bloodstream. That extreme local concentration is necessary because developing sperm need direct exposure to high levels of testosterone to mature properly. The hormone diffuses from the spaces between the tubules directly into them, bathing the Sertoli and germ cells.
Leydig cells do more than support sperm production. During fetal development, they produce the hormones that drive the formation of male reproductive anatomy. After puberty, the testosterone they release into the bloodstream maintains muscle mass, bone density, body hair, sex drive, and other characteristics typically associated with male biology.
The Internal Duct System
Once sperm cells finish developing inside the seminiferous tubules, they need a way out. The tubules converge toward a collecting area at the back of the testicle called the rete testis, a mesh-like network of channels. From there, sperm pass through a set of small connecting tubes called efferent ductules, which bridge the gap between the testicle and the epididymis, the long coiled tube that sits along the back of each testicle.
This transport isn’t passive. The efferent ductules are lined with tiny hair-like structures called cilia that actively sweep sperm forward. The ducts are also wrapped in smooth muscle cells that contract rhythmically, pushing sperm along. At the same time, cells in the ductule walls absorb excess fluid that was secreted during sperm development, concentrating the sperm as they move. Once in the epididymis, sperm undergo further maturation over several days before they’re capable of fertilization.
Blood Supply and Temperature Control
Sperm production is extremely sensitive to heat. It requires a temperature about 2 to 4 degrees Celsius below core body temperature, which is why the testicles sit outside the body in the first place. But even within the scrotum, there’s a built-in cooling mechanism.
The testicular artery, which carries warm blood from the abdomen, is wrapped by a web of small veins called the pampiniform plexus. This network acts as a countercurrent heat exchanger: cooler venous blood flowing away from the testicle absorbs heat from the warmer arterial blood flowing in. By the time arterial blood reaches the testicle, it has already been cooled to the right temperature. When this system fails, as in a condition called varicocele (where the veins become enlarged and don’t cool efficiently), sperm production can decline.
Nerve Supply and Sensation
The testicles develop near the kidneys during fetal life and descend into the scrotum before birth. Because of this origin, they share nerve pathways with the upper abdomen. That’s why a blow to the testicles produces deep, nauseating pain that radiates into the belly rather than staying local.
Three groups of autonomic nerves travel alongside the blood vessels and the vas deferens to reach the testicle. About 90% of this nerve supply is sympathetic, originating from the mid-spine (T10 through L1), with the remainder coming from parasympathetic fibers lower in the spinal cord. These nerves regulate blood flow, smooth muscle contractions in the duct system, and pain sensation. A separate nerve, the genitofemoral nerve, controls the cremaster muscle that raises and lowers the testicle in response to temperature changes or touch.
How It All Works Together
The interior of the testicle is essentially a compact factory with two production lines running simultaneously. The seminiferous tubules produce sperm, supported by Sertoli cells inside them and fueled by the high testosterone environment created by Leydig cells outside them. The duct system collects and transports that sperm, the vascular network maintains the precise temperature required, and the tough outer shell holds everything under the slight internal pressure needed to keep fluid moving through the system.
A healthy adult testicle averages about 20 mL in volume. Below roughly 12 mL is considered atrophic, a threshold that can signal hormonal problems and is associated with higher risk for certain conditions including testicular cancer. Size varies naturally between individuals and even between a person’s two testicles, with slight asymmetry being completely normal.