Stem cells from bone marrow make new fat.

Some body fat comes from stem cells that migrate out of bone marrow, a new study suggests.

Bone marrow acts as one of the body's most prolific stem cell factories, pumping out cells that circulate to different parts of the body through the bloodstream. Once these cells reach their destination, they can morph into new cell types--ranging from blood to heart muscle--that fill a specific need or replace cells lost to wear and tear.

Previous research had shown that certain bone marrow cells can be coaxed into becoming fat cells in the lab. However, it's been unclear whether these stem cells naturally contribute to new fat in the body.

To investigate this question, Dwight Klemm of the University of Colorado Health Sciences Center in Denver and his colleagues worked with mice treated so that their bone marrow cells glowed green. Some of the mice ate normal, lean laboratory chow. Other mice ate chow supplemented with a drag that increases the bone marrow's output of stem cells. A third group ate high-fat diets geared to pack on weight.

Klemm's team reports in the December Journal of Clinical Investigation that fat deposits sampled from all three groups of mice were speckled with green fat cells, indicating that they came from bone marrow. Fat in the drug-supplemented animals and those on the high-fat diets showed significantly more of the glowing cells.--C.B.