Testicles have two primary jobs: producing sperm and producing testosterone. These two functions happen simultaneously but in different compartments of the organ, driven by hormonal signals from the brain. Understanding how it all fits together starts with the basic anatomy and works outward from there.
The Two Compartments Inside Each Testicle
Each testicle is packed with tightly coiled tubes called seminiferous tubules, where sperm are made. If you unraveled all of them, they’d stretch hundreds of feet. Nestled in the tissue between those tubes are specialized cells called Leydig cells, which produce testosterone. These two compartments work side by side but serve different purposes, and each responds to a different hormonal signal from the brain.
How Testosterone Gets Made
Testosterone production starts with a signal from the brain. The pituitary gland releases luteinizing hormone (LH), which travels through the bloodstream and binds to Leydig cells in the testicles. When LH arrives, it triggers Leydig cells to pull cholesterol into their cellular machinery and convert it, through a series of enzymatic steps, into testosterone. The process moves cholesterol through the inner structures of each cell, transforming it step by step into the final hormone.
Normal testosterone levels in adult males range from about 193 to 824 ng/dL, though results vary depending on the lab. Before puberty, levels sit below 25 ng/dL. The dramatic jump during adolescence, when levels can climb as high as 830 to 1,010 ng/dL in the mid-teen years, is what drives the development of deeper voice, facial hair, increased muscle mass, and other changes associated with male puberty.
Testosterone doesn’t just affect physical appearance. It plays a role in bone density, red blood cell production, fat distribution, mood, and libido. It also feeds back into sperm production itself: the Sertoli cells inside the seminiferous tubules need local testosterone to support the development of sperm.
How Sperm Are Produced
Sperm production, called spermatogenesis, happens inside the seminiferous tubules in three broad stages. First, stem cells at the outer edge of the tubes divide by mitosis to create copies of themselves, ensuring there’s always a fresh supply. Second, those cells undergo two rounds of meiosis, the type of cell division that cuts the chromosome count in half, going from 46 chromosomes down to 23. This yields small, round cells called spermatids. Third, in a process called spermiogenesis, those round spermatids reshape themselves into the streamlined, tail-bearing sperm cells most people picture.
The entire cycle from stem cell to finished sperm takes roughly 74 days. Supporting cells called Sertoli cells act as scaffolding and caretakers throughout the process. They physically cradle developing sperm at every stage, feed them nutrients, clear away cellular debris, and release the mature sperm into the center of the tube when they’re ready. Sertoli cells are sometimes called the “biological clock” of the testis because they regulate the pace and timing of the whole operation. They’re activated by follicle-stimulating hormone (FSH) from the pituitary gland, which is the second major brain signal the testicles rely on.
How the Brain Controls the Testicles
The hypothalamus and pituitary gland form a feedback loop with the testicles, often called the HPG axis. The hypothalamus releases a signaling hormone that tells the pituitary to secrete LH and FSH. LH drives Leydig cells to make testosterone. FSH drives Sertoli cells to support sperm production.
When testosterone levels in the blood rise high enough, the hypothalamus detects this and dials back its signaling, which reduces LH release and slows testosterone production. Sertoli cells contribute their own brake pedal by secreting a hormone called inhibin, which specifically suppresses FSH. This dual feedback system keeps both testosterone and sperm production within a functional range without the brain needing conscious input.
Why Temperature Matters
Sperm production works best at about 34°C, which is 2 to 3 degrees below core body temperature. That’s why the testicles sit outside the body in the scrotum rather than inside the abdomen. Prolonged exposure to higher temperatures is considered a risk factor for reduced fertility.
The scrotum has built-in climate control. A layer of smooth muscle beneath the scrotal skin contracts in cold conditions, wrinkling the skin and pulling the testicles closer to the body. This reduces the surface area exposed to cold air and shortens the distance between the testicles and the warmth of the torso. In warm conditions, the same muscle relaxes, letting the scrotum hang lower and allowing more heat to dissipate. A second muscle, the cremaster, runs along the spermatic cord and can also pull each testicle upward or let it drop. Together, these muscles make constant, automatic adjustments throughout the day.
How Sperm Mature After Leaving the Testicle
Sperm that exit the seminiferous tubules aren’t yet capable of swimming or fertilizing an egg. They travel into a tightly coiled tube called the epididymis, which sits along the back of each testicle. Transit through the epididymis takes 10 to 15 days, and during that time sperm undergo critical changes. Their DNA becomes more tightly packed, their outer membrane composition shifts, and their head narrows. Most importantly, they gain the ability to swim and acquire surface proteins needed to bind to and penetrate an egg.
The tail end of the epididymis also serves as a storage site. Sperm can wait here until ejaculation, at which point they’re propelled through the vas deferens, mix with fluids from the seminal vesicles and prostate, and exit the body as semen.
How the Immune System Stays Out of the Way
Sperm cells carry only half the body’s normal genetic material, which makes them look foreign to the immune system. If immune cells encountered developing sperm freely, they could mount an attack against them. The testicles solve this problem with a structure called the blood-testis barrier, formed by tight junctions between neighboring Sertoli cells. This barrier divides the inside of each seminiferous tubule into two zones: a basal compartment near the outer wall, where the earliest stem cells live, and an inner compartment where meiosis and later development take place. The barrier prevents immune cells and antibodies from reaching the more mature, genetically distinct sperm cells in that inner zone.
Interestingly, the barrier alone doesn’t explain the full picture. Some early-stage cells with potentially foreign markers sit on the exposed side of the barrier, yet the immune system still leaves them alone. Additional local immune-suppressing mechanisms within the testis help maintain this tolerance.
What Normal Output Looks Like
The World Health Organization’s most recent reference values, published in 2021, define the lower end of normal for semen quality. A typical ejaculate contains at least 39 million total sperm. At least 42% of those sperm should be motile (moving), and at least 30% should be swimming forward in a sustained direction, known as progressive motility. These are fifth-percentile cutoffs, meaning 95% of men who conceived naturally within a year had values at or above these numbers. Falling below them doesn’t guarantee infertility, but it does flag reduced odds.
How Testicle Function Changes With Age
Testosterone levels begin a gradual decline starting around age 30. The drop is slow enough that most men don’t notice symptoms for years, if ever. Sperm count begins to decrease more noticeably from around age 41, and daily sperm production drops by more than 30% in men over 50. Sperm motility declines by about 1.2% every five years, and progressive motility in men over 50 is roughly half of what it is in men between 40 and 50.
These changes don’t mean fertility disappears. Men can and do father children well into their later decades. But the probability of conception per cycle decreases, and the time it takes to achieve pregnancy tends to increase. Sperm DNA quality also tends to decline with age, which is associated with slightly higher risks of certain conditions in offspring.