The secret of a sperm's wiggle
The secret of a sperm's wiggleGiving sperm a boost as they near their target (Image: Dr. David M. Phillips/Getty)
The switch that transforms sperm into super-swimmers (see below) in the race towards the egg has been identified at last. The discovery could lead to drugs that boost male fertility and new forms of male contraceptives. It could also explain why zinc deficiency and smoking cannabis can impair male fertility.
Sperm are relatively immobile before ejaculation. But once they get inside the female reproductive tract, they start swimming and speed up as they near the egg.
Although we've known for a while that sperm are prompted to swim by a rise in their internal pH caused by protons flowing out of the cell, the mechanism that regulates swimming was unknown.
To investigate, Yuriy Kirichok at the University of California in San Francisco and his colleagues probed human sperm cells with "patch clamping", normally used to study nerve cells. This enabled them to monitor the flow of protons across the membrane.
They found that the sperm's tail contained large numbers of proton channels called Hv1 channels that respond to changes in pH and zinc concentration outside the cell. When the channels open, protons stream out of the sperm cells, causing the pH inside the cell to rise and the sperm to swim.
Since the uterus has a higher pH than semen, this difference may trigger the sperm to start swimming inside the female reproductive tract. High concentrations of zinc, such as those found in the male reproductive tract and in seminal fluid will keep channels from opening prematurely until the sperm are ready to start swimming. And because the concentration of zinc drops sharply and pH rises as sperm reach the fallopian tubes, this may trigger an extra spurt of swimming power as they near the egg.
The zinc-sensitive channels in the sperm cell membrane may also help explain why men with deficiencies in this mineral are less fertile.
"Zinc keeps sperm not quite active," says Kirichok. "You can imagine that if you don't have enough zinc, sperm cells could be activated prematurely and may burn out."
A similar process may explain why cannabis saps male fertility. Kirichok's team found that the pores also open in response to a substance called anandamide, which is found in high levels near the egg. Anandamide binds to the same type of receptors as cannabis does. So the drug might cause sperm to swim too soon – though this idea has yet to be tested.
The discovery of the Hv1 channels in sperm also raises the prospect of creating drugs that block or open them.
"Now that we know what this ion channel is, it could lead us to either develop a novel contraceptive for men, or perhaps find a way to improve the sperm motility for men whose sperm don't swim as well as they should," says Allan Pacey at the University of Sheffield, UK, who was not involved in the current study.
And since other important sperm processes also rely on changes in the pH inside the cell – including the reaction that enables sperm to penetrate the egg – Hv1 channels may yet play an even broader role in male fertility.
Journal reference: Cell, DOI: 10.1016/j.cell.2009.12.053
Whether sperm fly at high speed or laze their way towards an egg might depend on how much competition they face, suggests a new analysis of sperm samples. The study reveals that promiscuous primate species have faster sperm than their more monogamous counterparts.
Jaclyn Nascimento at the University of California in San Diego, US, and her colleagues received sperm samples from humans, gorillas, chimpanzees and rhesus monkeys for analysis. Researchers had collected sperm samples from the latter two species using artificial vaginas, while the gorillas were trained to give up sperm (with the helping hand of a researcher) in exchange for candy.
Nascimento, an electrical engineer, and her collaborators focused in on specific sperm within the diluted samples, and recorded the activity of these individual sperm on film. Next, they used a sophisticated computer algorithm specifically created to determine the speed of a given sperm by tracking its head.
After examining numerous sperm from the two men who provided sperm samples, the team calculated that human sperm travel at about 0.2 metres per hour, a finding within the same range as some previous studies.
By comparison, the sperm from chimpanzees and macaques - which are much more sexually promiscuous than humans - appears to travel at a rate of 0.7 m/h. Sperm from gorillas - a relatively monogamous species in which females tend to mate with just one male - is "ridiculously slow" and clocks in at just 0.1 m/h, according to Nascimento.
Then, scientists assessed the force with which the sperm move. This step involved using used a laser technology known as "optical tweezers" to try to hold sperm in place using light. If the sperm moved with enough force, they could break past this resistance field and go forward - otherwise, they continued to push against it without luck.
The experiment showed that not only do the sperm from chimps and macaques move faster, they also move with greater force, at about 50 piconewtons. By comparison, human sperm swim with a force of about 5 piconewtons, and gorillas with a measly 2 piconewtons. (Watch movies of the sperm caught in the optical tweezers.)
Nascimento says the findings suggest that sperm from promiscuous species such as chimps, where a female might mate with multiple males within an hour, have evolved to move faster as a result of competition. "The first ones to make it to the egg" succeed, she says.
Gorilla mating, on the other hand, follows a harem-like pattern: females are more likely to mate with only the dominant male. "You are the best by default," Nascimento says of the leisurely sperm in the less promiscuous primates.
The speed at which sperm travel is partly determined by a motor structure inside the cell that powers the movement of their tails. In humans, certain compounds such as caffeine and anti-impotence drugs can alter how fast sperm move.
Journal reference: Journal of the Royal Society Interface (DOI:10.1098/rsif.2007.1118)