Read the text and find out the nature of experiments made by different scientists.

Most of our cells are continually being replaced. Old cells die and new ones are formed. This is especially important when we are injured. We would quickly die if our bodies had no ability to repair the damage.

Most of our nerve cells, however, do not repair or replace themselves so successfully. And the cells in our central nervous system – those in our brain and spinal cord – repair themselves very poorly. This is why so many people are unable to move again after an injury to the spinal cord. It also is why brain damage is often permanent.

For many years scientists believed that central nerve cells could not grow again after being cut or crushed. Or if the nerves grew, they could not connect with other nerves to restore the use of that part of the body. In recent years, much new evidence has caused scientists to change their opinions. They are learning not only how some nerve cells regrow, but also what guides them in the right direction.

More than 60 years ago a Spanish scientist R. Cajal observed that spinal cord nerve cells, in fact, try to regenerate. Cajal studied the axon, the long fiber that carries messages between nerve cells. He noted that when the axon was cut or crushed, it would shrink away from the wound on both sides. The ends would die. Soon after, the axon would begin growing. But the new growth stopped at the injury, and the axon died again.

Cajal`s experiments showed that cells in the central nervous system could not regenerate. In the late 1970s, however, American scientists were able to show that a sea creature called lamprey could regenerate cut spinal cord. Unlike Cajal, the later scientists had a tool which proved that nerve cells grew across the cut, connected with other nerves and reestablished body movement. The tool was a special enzyme found in the horseradish plant. The scientists put the enzyme into the lamprey`s brain above the spinal cord. So the nerves grew back, the enzyme flowed down with them.

Scientists were excited at this proof that some simple animals could regenerate central nerve cells. But they wondered. Did more developed animals, including humans, have that ability?

Albert Aguayo, working at McGill University in Canada, was interested in the difference between central nerve cells and peripheral nerves. The peripheral nerves carry the messages of movement and feeling from our spine to the rest of our body. Unlike central nerves, they grow well after an injury. Doctor Aguayo thought peripheral nerves might contain something that might help central nerves to regenerate.

He experimented with mice. First, he cut the animals` spinal cords. Then he placed the injured central nerve cells inside a group of peripheral nerve cells which had been

transplanted in the mice. The transplant served as a bridge over the injury. The central nerve axons grew very well along this bridge. And they continued to grow down the spine as far as four centimeters. But when Doctor Aguayo tried to reconnect the axons to undamaged spinal nerves below the break, the axons stopped growing. And the ends died.

Doctor Aguayo`s experiment provided important evidence that the ability to grow is not lost even in damaged central nerve cells. Instead, something in the central nerve cell`s environment blocks its growth.

Scientists say a number of things could be stopping central nerve cell growth. They are sure that an important part of the answer is the chemistry of the central nervous system. There may be natural chemicals present that stop regrowth. Or the central nervous system lacks the chemical that causes growth in the peripheral nervous system.

In the past 20 years, researchers have discovered hundreds of nerve growth chemicals. They say there could be thousands, each used by a different kind of nerve cell. Researchers now know very little about these nerve growth chemicals. But they believe that the key to nerve regeneration may be to place such chemicals at the nerve where it is injured.

How do nerve cells know which way to go? To find out, scientists are studying the central nerve systems of unborn, simple animals. They think these early nerves are guided long paths made of supporting cells. Each axon grows in one direction by recognizing chemicals along the path and on the target cells. Scientists are not sure if these pathways remain when the animals are fully grown. If so, it might be possible for nerves to regenerate in the same way as they developed.

Researchers at many universities also are experimenting with brain tissue transplants. They are investigating if new connections can be formed between the old and the new brain cells, and if this will restore activity to damaged areas.

A team led by Doctor Richard Wyatt at Saint Elizabeth`s Hospital in Washington, experimented with rats. First, they used a drug to damage the part of the rats` brain that controls movement. The rats were no longer able to walk in a straight line. Instead, they could move only in circles. Next, the researchers removed a similar piece of tissue from the brains of unborn rats. They transplanted this new tissue into the damaged area of the rats` brain. Some of the rats regained normal movement, and most improved greatly. A chemical marker showed that the new tissue had connected well with the old brain cells.

There is another way that nerve cells can regenerate. Scientists have long known that in simple animals, nerve cells not only repair themselves but can replace themselves completely. The old cells divide, creating two new cells. These cells then establish new connections. The body regains whatever abilities it had lost because of the nerve damage.

Nineteenth-century scientists believed that the olfactory nerve – the one for smelling – could regenerate in simple animals by dividing in this way. But the idea could not be proved until the 1950s. Then American researchers Pasquale and Giuseppina Graziadei discovered a protein in the olfactory nerves of octopuses that is present only during cell division.

Researchers since have found that nerve cells divide and replace themselves in the olfactory nerves of full-grown mice, cats and monkeys. And they say there is every reason to believe it also happens in humans.

In their experiment the Graziadeis wanted to see if olfactory nerves could continue to replace themselves in areas of the brain in which they are not usually found. Working with rats, they transplanted olfactory nerve cells to a strange part of the brain. One year later, the transplanted cells were alive and healthy. They continued to divide in their new position and replaced themselves eight or nine times.

The Graziadeis and other researchers are now searching for a good chemical marker for examining the connections made by the transplanted cells.

Leaders in nerve regeneration research say there has been an explosion of understanding in the past 10 years. They are not yet sure where the new discoveries will lead. But most now think it is only a matter of time before scientists will know how to guide nerve regeneration in humans.