When people think of fertility, they most often think of women, yet men play an equal role in fertility and may be the cause of childlessness. In fact, more than three million American men also experience infertility. Male infertility can be a temporary issue caused by a reaction to certain conditions, say, a sexually transmitted disease or even a tumor. On the other hand, infertility may be a permanent condition caused by a fundamental issue with the sperm itself, including too few sperm produced, misshapen sperm, motility (when sperm that cannot swim properly), or blockages that prevent sperm from joining with semen.
To better understand infertility, then, scientists commonly investigate sperm, the specialized cell that is so small — 30 times more petite than the female egg — it contains little more than DNA and molecular motors. From head to tail, a sperm cell is invisible to the naked eye, just about 1/500th of an inch. For this reason, even more than other cells, DNA must be tightly packaged to fit within its miniscule boundaries.