Scientists Find Sperm's Sex Potion

Sperm like it warm and perfumed -- with a spcial turn-on.

A protein, vital to conception, may be the key to better birth control.
By Faye Flam
Inquirer Staff Writer

Once sperm get into the female body, the hard part of their mission has just begun. When you're 20 microns long and blind, navigating four to five inches of the female reproductive tract must rival sailing the Northwest Passage. Only a few will succeed, while millions die trying.

How any of them get to an egg remains an active area of scientific research, and some interesting clues have emerged in the last several years. It now appears that sperm carry chemical and thermal sensory equipment, and can swim with different strokes depending on whether they're gliding up the uterus or breaking down the thick zona pellucida that surrounds and protects the ultimate target.

Some feminist scholars have argued that our picture of reproduction is skewed by male bias to make sperm look like macho explorers, eggs like coy, passive receptacles. The egg is in fact active, they argue, guiding and then grabbing the sperm.

But who has time to argue when at this very moment thousands of young women across the country are in a panic as unwanted sperm make their unstoppable way to their fallopian tubes - where fertilization happens.

We have to face reality. Whatever active role the egg plays in conception, it doesn't cover for indiscretions. And sperm are hard to turn back.

Hope for a better birth control, especially a better morning-after pill, rests on basic researchers like Harvard's David Clapham and Yuriy Kurichok's unveiling the secret lives of sperm.

Sperm don't just swim randomly. They home in on their targets like heat-seeking missiles. Apparently the female reproductive tract gets warmer as you get closer to the fallopian tubes, said Kurichok, and a few years ago scientists discovered sperm will swim from colder places to warmer.

Even more surprising, he said, was the discovery that sperm may be able to smell. We humans have more than 1,000 different molecular smell sensors, called olfactory receptors. Most of them are expressed in the nose, he said. But to the surprise of scientists, some crop up in sperm.

Scientists still don't know what the chemical attractant might be, or whether it comes from the egg itself or from some nearby region of the female body. But in experiments, they found sperm will swim toward increasing concentrations of a synthetic compound called bourgeonal, which Kirichok says smells floral.

Once they get all that way to the fallopian tubes, about a 15-minute swim, sperm can hold out for around five days waiting for an egg to emerge.

But that's not nearly the end of the job. Ordinary tail wiggling won't get a sperm through the zona pellucida, a thick, jellylike barrier, said Kirichok. He and Clapham showed this last feat depends on a protein called CatSper. Once the sperm gets close enough to the egg, it senses something that activates this CatSper, allowing calcium ions into the sperm.

The incoming calcium triggers a hard whiplike tail motion that pushes it to its ultimate goal.

The researchers are now working with a biotech company to develop a contraceptive that would disable sperm by blocking CatSper. There is no CatSper elsewhere in the body, so it's possible a woman could take such a blocker with no side effects.

Such a contraceptive might even work as a morning-after pill that would avoid disrupting an already fertilized egg. Therefore it would get around the political and religious controversy that's keeping so-called Plan B from being sold over the counter. Our best bet for emergency contraception may lie not in Plan B, but in basic research leading to Plans C, D and E.