Medicine has advanced to the point where a severed finger can be reattached or a hand, crushed by an industrial press, can be surgically repaired. Patients expect this. But when Dr. Paul Cook, a Riverside Methodist Hospital vascular microsurgeon, tells his patients that a leech - yes, a leech - will help them regain the use of their severely damaged digits, he has a little more explaining to do.
"It's disconcerting to most people," Cook said. "When you tell them 'Either we do this or your finger is going to die,' most people will say, 'Do what you've got to do.' "
Hirudo medicinalis have come a long way.
In ancient times, leeches were used to draw headaches and foul "humors" from patients.
Today, leeches are approved by the Food and Drug Administration as medical devices and are valued for their ability to draw blood and keep it from clotting.
Past research involving leeches has helped to create blood-thinners. Researchers also have examined an anti-inflammatory compound that leeches inject into their hosts to keep wounds from swelling and restricting blood flow.
But it's their genetic structure that excites scientists now. What they teach us could help explain why some animals can regenerate lost limbs, and how nervous systems transmit messages that help animals and people make decisions.
One of those researchers is Bill Kristan.
"It's closing in on 40 years," said Kristan, a University of California, San Diego neurobiologist. "I keep finding ever-more interesting things in their nervous systems."
There are more than 500 known species of leeches, which are members of the worm family.
Let it bleed
Although leeches fell out of favor in modern medicine, they are back and routinely are used during reattachment surgeries.
Leeches draw excess blood from severed or crushed fingers. A component of leech saliva, called hirudin, keeps the blood from clotting.
All of this, Cook said, gives the reattached digit time to regrow capillaries needed to keep the tissue alive.
Riverside keeps about 40 leeches in a jar in the hospital's basement pharmacy. To prevent infection issues, each leech is used once and discarded, said Michele Holley, the pharmacy operations manager.
A genetic link
Deciphering the leech's genetic code could help describe how a whole range of invertebrate animals including earthworms and mussels evolved, said David Weisblat, a cell-development biologist at the University of California, Berkeley.
Weisblat said that although a leech's genetic code is similar to that of an earthworm, the differences could provide the biggest benefit to science.
An earthworm cut in half by a shovel, for example, can grow into two separate worms.
"You cut a leech in two, you have a dead leech," Weisblat said. "We're pretty sure it's an evolutionary loss in the development of leeches."
Studies that compare the two could help scientists identify the genes involved in regeneration, Weisblat said. That could, in turn, provide clues to why some amphibians can regenerate lost limbs while humans cannot.
Nerves and free will
Kristan said the leech's nervous systems could reveal similar secrets about how animals and humans make choices.
"What I'm interested in is how a nervous system causes behavior," he said.
The leech is ideal to study, Kristan said, because its nervous system is organized simply, with a nerve cluster, or "ganglion," in each segment and two "brains" - one in the head and the other in the tail.
"You can find the same cells again and again and again in each segment of the leech," he said.
Early research identified a cell in each ganglion that works like a switch. Switch it on, and the leech swims.
Researchers stimulated the cell with a microelectrode and got the same reaction. When a similar cell in the leech's brain was stimulated, the leech would either try to swim or wriggle.
More work found that the depth of water the leech was in triggered the swim or wriggle. If the water was deep, the leech would swim. Shallow water triggered wriggling.
Kristan said his research might help unravel how nervous system messages help animals and humans make choices. He was quick to say, however, that nerves don't control what choice is made.
"My core belief is that the leech is using the same kind of (nervous system) mechanisms that we use, but is doing it at only a single level," Kristan said. "In our brains there are maybe 30 levels, and it becomes very abstract."