A chemical that stimulates growth of human bones has been isolated, scientists have reported.

Similar substances have been found in animals and chickens, and a crude extract from human bones studied last year seemed to contain a substance responsible for bone growth. But this is the first time that a chemical that is being called the "skeletal growth factor" in humans has been isolated.

The growth factor has been used to stimulate human bone cells grown outside the body. In those, taken from hip bones, it increased the growth of samples by as much as 10 times the normal rate, according to David Baylink and John Farley of Loma Linda University, authors of the paper in the current Biochemistry, a journal published by the American Chemical Society.

The discovery may prove useful in treating such bone disorders as periodontal disease, bone loss around the teeth that afflicts some 30 million Americans, and osteoporosis, a brittling of the bones that affects as many as 10 million older women.

The next and most critical step in this line of research -- the step that will prove or disprove the use of this substance -- will be to determine whether the chemical will have the same effect in living animals. Right now, too little of the substance has been collected to test it.

In normal bodies, bone is both grown and destroyed continuously. Old or diseased bone is chemically degraded as new bone is formed to take its place. Usually the amount of bone created and destroyed is equal, but when it is not, diseases are believed to result.

Baylink says he believes it likely that the bone growth factor is the key chemical in these diseases. It is possible that the chemical's release in the body is triggered by the decay of bone, and triggered in amounts just right to replace the lost bone.

This "coupling" theory is controversial, and the new research does not prove that the bone growth factor is necessarily the chemical that accomplishes it. But Baylink believes the chemical is a logical candidate to be the "coupler."

The chemical the two researchers found is a large protein molecule, and a potent stimulator of bone growth. Only three-tenths of a microgram per milliliter is needed to cause a tenfold jump in the growth of human bone tissue.