Two separate teams of scientists, using different methods and different strains of mice, have demonstrated that transplanted bone marrow cells can transform themselves naturally into neurons - cells that carry nerve impulses - and install themselves seamlessly into the brain.
However, it could take years before the researchers can prove that such transplants can be effective and safe for treating human brain disorders, they said.
The researchers said the finding suggests that converting bone marrow cells into brain neurons may be part of a previously unknown natural healing action the body uses to replace failed brain cells.
"It may be a repair mechanism that is going all the time at a low level," said Helen M. Blau, a professor at Stanford University and senior author of one of two studies appearing Friday in the journal Science.
Blau said the repair mechanism may not be powerful enough to correct "a really severe insult, like an injury or Parkinson's disease," but medical science may find a way to enlist this potential to replace neurons destroyed by disease or injury.
Dr. Eva Mezey of the National Institute of Neurological Disorders and Stroke, first author of the other study in Science, agreed that the researchers may have found a way the body replaces neurons.
"On a certain level, there might be a repair going on throughout life," said Mezey. She said there's no awareness of the repair because the process replaces a neuron as soon as the cell fails. This process could be similar to the way the body constantly replaces dead skin cells with new cells, she said.
The studies are the latest in a hot new field of research exploring the ability of adult stem cells to transform themselves into other types of cells and to fill new roles in the body. Bone marrow, which consists of at least two types of stem cells, has long been known to be the source of blood cells. Recent studies have shown that these cells also can convert themselves into muscle, bone and liver. There also have been studies showing that neural stem cells can convert into muscle or make a variety of brain cells.
In Blau's study at Stanford, researchers obtained bone marrow from a strain of mice that carry in each cell a protein that glows in the dark.
This bone marrow was injected into the tail vein of mice whose natural bone marrow had been killed by radiation. Later, the mice were killed and their brains examined.
Blau said her team was stunned to find that some cells in the test animals' brains glowed in the dark, proving that bone marrow cells from the donor mice had migrated to the brains of the test mice and changed into neural tissue.
"That was a total surprise to us. We did not expect it," said Blau. We were going to do experiments looking at muscle" but curiosity prompted them to look at the brain as well.
"We spent two years convincing ourselves that what we were seeing was right," and even enlisted scientists from other labs to confirm the discovery, Blau said.
In Mezey's lab, the researchers used bone marrow from male mice and injected it into a strain of female mice born without bone marrow. Using the male cells provided a marker since male cells contain a Y chromosome and female cells do not. The mice brains were examined later and the researchers found Y chromosome cells incorporated into several brain structures.
Additional tests in both studies indicated the new brain cells were neurons. Both Blau and Mezey, however, said more studies are needed to prove that such neurons made the right connections to become a working part of the brain.
Before they can prove the method is safe and effective in humans, science must identify the proteins or other factors in the body that signal the bone marrow to transform into brain cells, the researchers said.
"What we need to learn is the rules of the game," said Blau. "We need to know the factors, what is calling those cells to the brain."
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