The testes have two main jobs: producing sperm and producing testosterone. These two functions drive male fertility, shape physical development during puberty, and influence everything from bone strength to mood throughout life. Though each testis is only about the size of a small plum in adulthood, the internal machinery is remarkably complex.
Sperm Production
Sperm are made inside tightly coiled tubes called seminiferous tubules. Each testis contains roughly 250 small compartments (lobules), and each lobule holds one to four of these coiled tubes. If you stretched them all out, the total length of tubules in both testes would reach several hundred meters.
The production process takes about 64 to 72 days from start to finish and unfolds in three broad stages. First, early stem cells multiply through standard cell division, creating a large supply of starting material. Second, those cells go through two rounds of a special division that cuts the chromosome count in half, so each resulting cell carries only 23 chromosomes instead of 46. Third, those round, immature cells reshape themselves into the streamlined form you’d recognize under a microscope: a compact head carrying DNA and a long tail for swimming. This final reshaping step is called spermiogenesis.
Once sperm leave the seminiferous tubules, they travel into a coiled storage tube called the epididymis, which sits along the back of each testis. There, they undergo final maturation and gain the ability to swim and fertilize an egg. A healthy ejaculate contains at least 39 million sperm in total, based on the World Health Organization’s current reference values.
Why Temperature Matters
There’s a reason the testes sit outside the body. Sperm production requires a temperature 2 to 6 °C below core body temperature. The scrotum acts as a climate-controlled chamber, moving the testes closer to or farther from the body depending on ambient conditions. Sustained overheating from tight clothing, laptops on the lap, or prolonged hot tub use can temporarily reduce sperm quality.
Testosterone Production
Scattered between the seminiferous tubules are specialized cells called Leydig cells. These are the body’s primary testosterone factory. The process starts when the brain’s pituitary gland releases a signaling hormone (LH) into the bloodstream. When LH reaches the Leydig cells, it triggers them to convert cholesterol into testosterone through a chain of chemical steps inside their internal structures.
The brain constantly monitors testosterone levels in the blood. When levels drop, the pituitary releases more LH to boost production. When levels rise, the signal dials back. This feedback loop keeps testosterone within a functional range. A separate pituitary hormone, FSH, works alongside testosterone inside the testes to keep sperm production running smoothly. So the two core functions of the testes, sperm and hormone production, are tightly linked.
What Testosterone Does Beyond Reproduction
Testosterone is best known for driving the changes of male puberty: deepening of the voice, growth of facial and body hair, increased muscle mass, and genital development. Testicular growth is actually the very first visible sign of puberty, typically beginning when the testes reach about 4 milliliters in volume (roughly 2.5 cm long). By the end of puberty, each testis usually exceeds 20 milliliters.
But testosterone’s role extends well past adolescence. It directly stimulates the cells that build new bone tissue and suppresses the cells that break bone down. This is why men with low testosterone are at greater risk for fractures, not only because their bones become less dense, but because testosterone also supports muscle mass and strength. Weaker muscles mean more falls, compounding the fracture risk.
Testosterone also plays a role in red blood cell production, fat distribution, energy levels, and mood regulation. When testosterone drops significantly, the cluster of symptoms can include persistent fatigue, depressed mood, irritability, reduced sex drive, loss of muscle mass, and increased body fat around the midsection.
How Testicular Function Changes With Age
Testosterone levels typically decline by about 1% per year after age 30 to 40. For most men, this gradual drop doesn’t cause noticeable problems. But in some cases, levels fall low enough to produce symptoms. This is sometimes called late-onset hypogonadism. Sperm production also decreases with age, though many men remain fertile well into their later decades.
Signs of Reduced Testicular Function
When the testes underperform, whether from aging, injury, infection, or a genetic condition, the result is called hypogonadism. It’s generally diagnosed when morning testosterone levels fall below 300 ng/dL on at least two separate blood tests.
The most telling symptoms include fewer spontaneous and morning erections, low sex drive, unexplained fatigue, and noticeably smaller testes (below about 4 cm in length or 20 mL in volume). Infertility and breast tissue growth are also more common when the problem originates in the testes themselves rather than in the brain’s signaling system. If the testes are the source, LH levels in the blood will typically be elevated, because the brain is ramping up its signal in an attempt to get the testes to respond.
The Two Functions Work Together
It’s easy to think of sperm production and testosterone production as separate jobs, but they depend on each other. The high concentration of testosterone inside the testes, which is far greater than what circulates in the bloodstream, is essential for sperm cells to develop properly. When testosterone production drops, sperm production usually suffers too. This is why conditions that damage Leydig cells often cause both low testosterone symptoms and fertility problems at the same time.