The painful knees and hips experienced by so many people with osteoarthritis result from a loss of cartilage, which serves as a sort of cushioning in the joints. It had long been thought that cartilage, once gone, cannot grow back.

Now researchers at Stanford University have grown new cartilage in the joints of arthritic mice. Primitive cells that can be transformed into new cartilage lie dormant at the ends of bones, the researchers reported in Nature Medicine. The cells just have to be awakened and stimulated to grow.

The researchers say the next step is to try to grow cartilage in larger animals, like dogs or pigs. They are optimistic that the finding could eventually lead to treatments to prevent the often debilitating pain that arises when cartilage erodes away.

“It is really a major advance in field of osteoarthritis,” said Dr. Gerard Karsenty, a bone specialist at Columbia University who was not involved in the research.

Although scientists often question whether findings in mice may apply to humans, diseases of the skeleton often do, he added. “When you demonstrate something in the mouse, I don’t know of any example where it has not applied to humans,” Karsenty said.

But Dr. Robert Marx, an orthopedic surgeon, cautions that the path to a treatment that helps patients may be long and unpredictable.

Scientists will need to determine not only whether the method is safe and effective but also to learn which patients are likely to be helped — those early or later in the course of arthritis — and how long the treatment will last.

An estimated 50 million Americans have osteoarthritis; the lifetime risk of getting the diagnosis is 40%. Once the degeneration of the cartilage lining a joint begins, there is no treatment available to restore it.

The new research was conducted with mice with knee arthritis so severe they had trouble walking and with human bone transplanted into mice. Normal cartilage was grown in both settings.