Inside each testicle is a soft, yellowish-tan tissue packed with hundreds of tightly coiled tubes, all wrapped in a tough white outer shell. From the outside, testicles appear smooth and uniform, but their interior is surprisingly complex, organized into distinct compartments with a built-in plumbing system designed to produce and transport sperm.
The Outer Shell
Each testicle is encased in a dense, white, inelastic layer called the tunica albuginea. Think of it like the rind on a citrus fruit. It’s smooth, fibrous, and gives the testicle its firm, oval shape. This shell doesn’t stretch much, which is partly why a direct hit to the testicles is so painful: the tissue inside swells against a casing that won’t give.
From the back side of this shell, thin walls of tissue extend inward like the spokes of an orange, dividing the interior into roughly 250 small compartments called lobules. Each lobule is a self-contained pocket of sperm-producing tissue.
What the Tissue Looks Like
If you sliced a healthy testicle open, the interior tissue would appear soft and yellowish-tan, with a uniform, slightly wet texture. It’s much softer than the tough outer shell, closer in consistency to a ripe avocado than to muscle. Pathologists describe normal tissue as homogeneous, meaning it looks the same throughout without lumps, hard spots, or color changes.
On ultrasound, which is the most common way doctors look inside a testicle without surgery, healthy tissue shows up as an even, medium-gray image. Both testicles should look nearly identical in shade and texture when scanned side by side. Any dark patches, bright spots, or uneven areas can signal problems like inflammation, injury, or a mass.
The Coiled Tube System
The bulk of what’s inside each testicle is an enormous network of tiny, tightly coiled tubes called seminiferous tubules. Each of the 250 lobules contains one to four of these coiled tubes. If you could unravel all of them and lay them end to end, the total length in a single testicle would stretch hundreds of meters. This is where sperm cells are actually made, starting at the outer wall of each tube and gradually maturing as they move toward the center.
Lining these tubes are support cells that nourish developing sperm, feeding them nutrients and regulating the local environment so sperm can mature properly. Scattered in the spaces between the tubes are hormone-producing cells that release testosterone into the bloodstream. So the interior of the testicle is doing two jobs at once: manufacturing sperm inside the tubes and producing hormones in the tissue between them.
How Sperm Exits the Testicle
The coiled tubes in each lobule straighten out and converge into a network of small channels located near the back of the testicle, in a region called the mediastinum. Picture a drainage system: hundreds of tiny streams feeding into a central collecting area. From there, sperm passes through a series of small, thin-walled, highly convoluted tubules that connect to the epididymis, the comma-shaped structure you can feel along the back of each testicle. The transition between these areas is abrupt. The lining of the collecting channels is flat and simple, then suddenly shifts to taller, more active cells designed to move fluid and concentrate sperm before it enters the epididymis for final storage and maturation.
The Blood Supply
Testicles need to stay about 2 to 4 degrees cooler than core body temperature for sperm production to work properly. The blood vessel arrangement inside the spermatic cord, the rope-like structure connecting each testicle to the body, is specifically built for temperature control. A single artery carrying warm blood from the abdomen is tightly surrounded by a web of veins carrying cooler blood back from the testicle. This design works like a heat exchanger: warm arterial blood transfers heat to the cooler venous blood before it ever reaches the testicle. A dense capillary network embedded within the artery wall itself connects the artery to the surrounding veins, making the cooling process even more efficient.
Inside the testicle, smaller blood vessels branch throughout the tissue between the seminiferous tubules, delivering oxygen and picking up testosterone for distribution to the rest of the body.
Normal Size and Variations
A typical adult testicle measures up to 5 cm long, 3 cm wide, and 2 to 3 cm in height, with an average volume of about 20 milliliters (roughly four teaspoons of fluid). The left and right testicles are usually very close in size. In one study measuring both sides, the average right testicle was 19.8 ml and the left was 20.1 ml, though individual volumes ranged widely from about 7 to 36 ml.
Testicles smaller than 12 ml are considered atrophic, meaning they’ve shrunk below the normal functional range. This can happen from hormonal issues, prior injury, or other conditions, and it may slightly increase the risk of testicular cancer.
One common incidental finding on ultrasound is tiny calcium deposits scattered through the tissue, appearing as bright white specks smaller than 3 mm. These show up in less than 1% of men who get scrotal ultrasounds and are usually harmless, though doctors may recommend periodic follow-up imaging depending on other risk factors.
What Abnormal Tissue Looks Like
When something goes wrong inside a testicle, the uniform appearance changes. Inflammation causes the tissue to swell and become uneven on ultrasound, losing its normal homogeneous look. Torsion, where the testicle twists on its blood supply, produces similar swelling and patchiness as tissue becomes starved of oxygen. Internal bleeding from an injury shows up as a loss of the normal even texture, eventually developing into dark fluid-filled areas. A tumor typically appears as a distinct mass with a different density than the surrounding tissue.
This is why doctors compare both testicles side by side during an ultrasound. The healthy side serves as a built-in reference point, making it easier to spot subtle differences in size, texture, or blood flow on the affected side.