MayoClinic.com states that about 15 percent of couples are infertile and that about half of the cases originate with the man. For normal fertility, a man must produce sperm that can swim up the female reproductive tract to fertilize the egg. Although it might seem straightforward, sperm movement, or motility, is a complex phenomenon that scientists still do not completely understand.
Significance
A mature sperm consists of the "head," containing the DNA, a midpiece housing the energy-producing mitochondria and a powerful whip-like tail that provides the propulsion for swimming. A sperm's ability to swim is absolutely critical to normal fertility. While only a few thousandths of a millimeter long, the sperm must travel over 100,000 times its own length to reach and fertilize an egg, according to "Endocrinology: An Integrated Approach."
Phases
A sperm within the testis cannot swim. It matures as it travels through the male duct works, gaining the ability to swim by the time it reaches the vas deferens, the tube that leads to the urethra carrying sperm through the penis. After ejaculation, a sperm begins to swim using energy from fructose and citric acid contained within the semen. The sperm tail beats in an even back and forth motion to propel it forward; this is called progressive motility. Once in the fallopian tube, or oviduct, sperm undergo a series of changes called capacitation that make them capable of fertilizing the egg.
Hyperactivation
Hyperactivation, a different type of sperm motility, is a critical part of capacitation. Sperm must hyperactivate to swim through the thick mucus in the fallopian tube and to penetrate the membrane that surrounds the egg. The tails of hyperactivated sperm beat rapidly and asymmetrically, swinging more to one side than the other to produce a figure-eight motion that might aid penetration. Susan Suarez, Ph.D. cites evidence in her 2008 article in "Human Reproduction Update" that sperm from infertile men show lower levels of hyperactivation compared to sperm from fertile men.
Causes
Men with a varicocele--a swollen vein in the scrotum, the sac that holds the testes outside the body--might be infertile because the testis is too hot for optimal sperm production. Men with a varicocele make fewer sperm, and those sperm might have abnormal motility. Contagious infections or inflammation of the male reproductive tract can temporarily make sperm less motile. New evidence reviewed by Dr. Suarez in "Human Reproduction Update" shows that some infertile men have mutations in genes that regulate hyperactivation so that their sperm cannot penetrate the egg.
Treatment
There is no clear treatment for a man diagnosed with abnormal sperm motility. Intracytoplasmic sperm injection might be an option. In this procedure, sperm and eggs are collected into lab dishes and a single sperm is injected directly into the egg to fertilize it, bypassing the need for sperm to swim and penetrate the egg on their own.
References
- MayoClinic.com: Male Infertility
- "Endocrinology: An Integrated Approach"; Chapter 9: The Gonad; S.S. Nussey and S.A. Whitehead; 2001.
- Human Reproduction Update; "Control of hyperactivation in sperm"; S. Suarez; 2008.
