Bone formation appears to be controlled by serotonin, a chemical previously known mainly for its entirely separate role in the brain, researchers are reporting.

The discovery could have enormous implications, osteoporosis experts say, because there is an urgent need for osteoporosis treatments that actually build bone.

Osteoporosis affects 10 million Americans over age 50. It results in bone loss, and its hallmark is fragile bones that break easily. With one exception, current treatments only slow further bone loss rather than increase bone formation. And the exception, parathyroid hormone, given by injection, is recommended only for short-term use and costs about $6,700 a year.

But in a paper published online Wednesday in the journal Cell, a team led by Dr. Gerard Karsenty, chairman of the department of genetics and development at the Columbia University College of Physicians and Surgeons, reports the discovery of an unexpected system that appears to control bone formation.