A new HHS study suggests that some cells from bone marrow can enter the human brain and generate new neurons and other types of brain cells. If researchers can find a way to control these cells and direct them to damaged areas of the brain, this finding may lead to new treatments for stroke, Parkinson's disease, and other neurological disorders.

"This study shows that some kind of cell in bone marrow, most likely a stem cell, has the capacity to enter the brain and form neurons," said Èva Mezey, M.D., Ph.D., from HHS' National Institute of Neurological Disorders and Stroke (NINDS), who led the study.

Earlier scientific work has shown that bone marrow cells can enter the mouse brain and produce new neurons. However, the new study is the first to show that this phenomenon can occur in the human brain. The study appears in the January 20, 2003, online early edition of the Proceedings of the National Academy of Sciences1.

In the study, Dr. Mezey and colleagues examined brain tissue taken at autopsy from four female patients who had received bone marrow transplants from male donors. The investigators searched the autopsied brain tissue for male cells, which contain a Y chromosome. The Y chromosomes in these cells served as a useful way of distinguishing donor-derived cells from those of the female transplant recipients. The researchers found cells with Y chromosomes in brain tissue from all four of the patients.

Most of the bone marrow-derived cells in the brain tissue were glia (support cells) and other non-neuronal cells. However, a small number of neurons from each brain also contained Y chromosomes, showing that those cells had developed from the transplanted male bone marrow. Most of these neurons were found in the cerebral cortex - the outer layer of the brain, which is responsible for conscious thought - and in the hippocampus, a region that helps with memory and other functions.

The Y chromosome-positive cells within each patient's brain appeared in clusters, rather than being randomly dispersed throughout the brain tissue. The clusters sometimes contained both neuronal and non-neuronal cells. This suggests that a single bone marrow-derived stem cell may migrate into an "area of need" within the brain and then change, or differentiate, into several other kinds of cells. The clusters also might result from a large number of marrow cells that are "called" to specific parts of the brain. Previous studies have suggested that stem cells can respond to signals from within the brain that guide them to damaged regions.

Scientists must now determine what growth factors or other signals prompt the bone marrow cells to enter the brain and develop into neurons. This may lead to new ways of treating Parkinson's disease or other disorders where neurons lost to disease are not normally replaced.

"These studies are very much the beginning, but scientists should start to look down this road and find out if and how we can go further," says Dr. Mezey.