CHICAGO - Meet Zoey Walsh, a teenage stem-cell recipient who is pushing the frontier of medical science.

He's a dog.

Unable to alleviate his pain with drugs and unwilling to risk another hip surgery on a dog so old, Zoey's owners turned to a treatment that involved injecting stem cells, which had been extracted from Zoey's fat, back into the animal. The stem cells stimulate repairs.

The therapy is gaining momentum as a treatment option for pets. But it also holds promise for humans, researchers and companies involved say.

"It did wonders," Zoey's owner, Raymond Walsh of Palos Heights, Ill., said of his 14-year-old American Eskimo's first injection six months ago. "He is having another round. But had we not given the first shot back in August, we might have had to put him under."

Although still early in its use with more than 1,500 dogs and cats since marketing began last year, the technology developed by California-based Vet-Stem Inc. is being embraced by pet owners.

Take Daisy Benik, a 7-year-old golden retriever who had stem cells injected into both hind legs last summer after being unable to walk on her left hind leg.

"She is back to what she was. Before she had been holding her foot up, favoring it," said Daisy's owner, Mary Benik of Crest Hill, Ill. Such treatments aren't cheap. An injection and anesthesia to sedate a dog before and after stem-cell extraction and injection can cost between $2,500 and $3,500. A follow-up treatment usually costs less than half that because stem cells from the original extraction often are left over.

The cost is worth it to Daisy's and Zoey's owners.

Zoey had been urinating in the house because he couldn't go outside. Zoey soon will get a second injection in both hind legs because he has trouble climbing stairs. "If it prolongs his life, it was worth it," Walsh said.

Here's how Vet-Stem's product works: A small amount of the animal's fat, the equivalent of two or three tablespoons, is surgically removed and shipped overnight to Vet-Stem's labs in Southern California, where the stem cells are isolated. The cells are returned to the veterinarian two days later in the form of concentrated regenerative stem cells.

The stem cells are injected back into the animal, where they home in on the injury and stimulate "resident" cells to become more active in making repairs. Stem cells are derived from building-block cells and are able to develop into many different types of specialized cells, serving as a sort of repair system for the body. Theoretically, they divide without limit to replenish other cells as long as the person or animal is alive.

Just how long a repair lasts is unclear, however, and that can mean additional injections may be necessary.

"So far, we have been pretty lucky with the treatment," said Dr. Andrea Hayes of Animal Surgical Services in Western Springs, who treated Zoey and other pets. "Because it's so new, we don't know how long it is going to last."

Similar technologies using stem cells derived from fat are being studied in experimental stages in humans. The goal is to go beyond alleviating pain to restoring joint cartilage as a way to avoid hip, knee or other joint surgeries.

"When you look at a Labrador retriever with arthritis in his hips, that is the same disease you and I have," said Dr. Bob Harman, chief executive of Vet-Stem. "Fractures are similar too, even though they are four-legged."

Vet-Stem's data and work with pets is helping other companies progress toward human clinical trials.

San Diego-based Cytori Therapeutics Inc., for example, has worked with Vet-Stem and uses a machine to harvest stem cells from fat tissue for use in breast reconstructive surgery. In Europe, Cytori's machine has been approved to help breast cancer patients.

"Our first application is taking the cells out and combining them with fat to allow fat to be used as a natural filler in breast reconstruction," said Tom Baker, Cytori director of investor relations.

Cytori executives say the company is working with the Food and Drug Administration in hopes of beginning clinical trials in humans later this year. "The question is how much clinical work is going to be required," Baker said.

Most products on the market are derived from cells from cadavers that stimulate bone growth. They do not require clinical trials and FDA approval for marketing because they are essentially cells that are harvested, processed and stored for a later use, much like organs used for transplant.

One person with a keen interest in stem-cell research and their applications for humans is Daisy's owner, Mary Benik. "I hope it continues to work (on the dog) because I have bad knees and I would do it if it could help."