What is Sperm? A 101 Guide

Our guide to sperm will provide a technical explanation of what exactly sperm is, how it works, and debunk some common misconceptions about sperm.

When was the last time you thought about your sperm? For many people, the answer is effectively never, and that’s entirely normal. But understanding how sperm works is important, especially when trying to conceive. Healthy sperm are essential for having biological children, and is also an important marker of your overall health.

Sperm production, called spermatogenesis, begins at puberty, usually around age 12. Once sperm production starts, it will typically continue uninterrupted until death, though discernible defects in sperm quality and sperm quantity increase with age.

A healthy male reproductive system will produce around 50–100 million viable sperm each day. The whole process of spermatogenesis, from newly formed sperm to a fully mature one, usually takes around 74 days.

Spermatogenesis takes place in the testes, starting in vessels called “seminiferous tubules.” There, a germ cell (a “diploid” cell with 46 chromosomes) divides three times to produce multiple haploid spermatids (immature sperm). These divisions of the sperm cells create genetic variation. At this point, the sperm cells that have divided are still connected with cytoplasm.

In the next step, the spermatids begin to mature, a process driven by the male hormone testosterone. They form a tail and an acrosome, their DNA becomes more compact, and excess cytoplasm is removed to create individual structures.

At this point, sperm are still immotile (unable to move on their own). They are transported via muscle contractions to the epididymis, a coiled tube behind each testicle, where they mature further, gaining the ability to move on their own and fertilize an egg.

Relatively cool temperatures are important for spermatogenesis. That’s why, during puberty, the testicles descend from the body to hang between the legs. This keeps them about 4ºF cooler than the usual body temperature of 98.6ºF.

Fertilization is a “race to the top” of the female reproductive system, from the vagina to the fallopian tubes, where sperm meet the egg and fertilization occurs. On average, anywhere between 80 and 300 million sperm cells are released per ejaculation.

However, most are eliminated along the way; any abnormal or otherwise defective sperm fall to the many dangers of the journey, along with a good percentage of healthy sperm. In fact, less than 1 in a million from the original ejaculate will reach the egg at the time of fertilization.

The journey to the egg is complex, full of obstacles, and at times hostile. The acidic environment of the vagina will eliminate any sperm that linger for too long. Once sperm reach the cervix, they must navigate through a web of mucus that can trap many. A great deal of sperm can be lost to “crypts,” side-channels where they can survive for several days.

Once they reach the Fallopian tubes, many become bound to its surface; only a select few are freed and continue on to the egg — that is, if they’ve chosen the correct tube. It is hypothesized that this long, arduous journey is nature’s way of weeding out the weak, so only the healthiest, strongest sperm survive.

When a sperm reaches the upper part of the female reproductive system, it undergoes the final steps that allow it to fertilize the egg: chemical changes to the surface of the sperm allow the acrosome to access the egg and dissolve its outer layer, and the tail begins to “beat” faster to facilitate egg penetration.