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A reason for male infertility when sperm counts and sperm motility are normal is found. The absence of a protective protein surrounding sperm cells makes fertilization of the egg impossible:25% men has two copies of the recessive gene causing this defect.
Defective Gene Makes Fatherhood Less Likely But Not ImpossibleA group of scientists in China and the USA report that they have found a reason for male infertility when sperm counts and sperm motility are normal. The absence of a protective protein surrounding sperm cells, they say, makes fertilization of the egg impossible. And one in four men has two copies of the recessive gene that causes the sperm defect.
Why Sperm Need a Protective Coat
When semen is ejaculated in sexual intercourse, the individual sperm cells typically land in the mucus lining the cervix. Cervical mucus can provide a medium in which the sperm "swims" to the fallopian tubes to fertilize the egg, or it can present a barrier to conception. Either way, sperm have to be able to bind to the mucus to use it as a vehicle to reach the egg.
The way sperm bind to cervical mucus is through a protein known as a lectin. These kinds of proteins are very common in both plants and animals. In plants, some lectins bind to minerals the plant needs from the soil. Other lectins serve to protect the plant from predators by binding to receptors in the grazing animal's digestive tract, or the human digestive tract, as the case may be. Lectins in beans, for example, cause bloat in animals and flatulence in humans, discouraging the eating of the bean's "children."
In animals, lectins activate enzyme. One of the lectins on the surface of sperm activates an enzyme that dissolves protein in the cervical mucus to make a path to reach the egg. It dissolves just enough of the mucus to allow the sperm to move without interfering with the ability of the mucus to protect it.
The lectin beta-defensin helps the sperm move through the cervix and keeps it from being attacked by the female partner's immune system. A single defective gene for making beta-defensin in the father has no effect on the ability of sperm to reach the fallopian tubes. Two defective genes, however, keep the sperm from making beta-defensin and greatly reduce the ability of a sperm to reach and fertilize the egg. If the egg is not fertilized within about 24 hours after its release, it is necessary to wait until the next ovulation to try again.
Genetic Defect Does Not Make Fatherhood ImpossibleNearly one-quarter of all men, the Chinese and American scientists report, have not just one but two defective copies of the gene for beta-defensin. Many of these men, however, can become fathers if they just try long enough.
Recessive genes that fail to code for beta-defensin do not mean that a man can never become a father, but rather that conditions have to be ideal for conception. There are many things the female partner can do to help with timing intercourse and changing the consistency of cervical mucus to compensate for problems with sperm. Not every woman wants to monitor her fertility in as much depth as may help compensate for problems with sperm, but there are very simple interventions that help over 90% of couples trying to conceive.
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