Refuting previous research that adult bone marrow stem cells can transform themselves into brain cells, Baylor College of Medicine researchers are reporting that the phenomenon is an uncommon occurrence and unlikely to lead to therapy in the foreseeable future.

In an unusual publication of "negative" data in Friday's issue of the journal Science, the Baylor team described its futile efforts to persuade stem cells to produce neural cells, those found in the brain and nervous system.

The previous suggestion that they could be transformed was hailed as holding promise for treating brain disorders such as Parkinson's and Alzheimer's diseases. "Our data suggest that the 'bone to brain' (transformation) may not be a general phenomenon but may depend on the experimental system in which the hypothesis is tested," said Dr. David Shine, an associate professor in Baylor's Center for Cell and Gene Therapy and the lead author of the paper.

"Any therapeutic promise is not on the horizon."

The contradictory findings are part of ongoing research into the therapeutic potential of adult stem cells - historically viewed as limited but suddenly seen as promising, as scientists have reported greater flexibility than thought.

If the promise is realized, adult stem cells would provide a noncontroversial alternative to embryonic stem cells, which are considered medicine's next frontier despite ethical concerns because the embryo has to be killed to obtain the cells.

Stem cells are the body's building blocks. In embryonic form, they are capable of becoming any sort of tissue the body needs. In adult form, they are best known for replenishing cells in a particular organ that wears out. In the past few years the cells have been found to be capable of turning into other tissue, such as muscle or bone.

In one of the biggest discoveries, reported in Science in December 2000, two teams of researchers, using different methods and different strains of mice, showed that converting bone marrow cells into brain neurons may be part of a previously unknown healing action the body uses to replace failed brain cells.

The attempts by Shine's team to coax highly purified and unpurified bone marrow stem cells to transform in normal mice and in those with brain injuries, produced no evidence of brain cells derived from the original source.

The only cells related to the original source were a few similar to blooding-forming cells located near blood vessels in the brain.

"This calls into question the notion a patient's bone marrow cells could be converted to brain cells to treat diseases of the nervous system," said Shine.

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