Believe it or not, the cure to what ails you could already be inside you. Welcome to the dawning world of stem cell therapy, in which researchers are exploring the possibilities of growing new body parts and healing old ones by using patients' own stem cells. The idea is that these unspecialized cells (which can also be harvested from a donor) can be induced to develop into heart or lung or brain cells, say, and be injected to replace those damaged by disease or injury. While bone marrow stem cell therapies for certain cancers date back several decades, the approach is now being used or studied for a wide array of diseases and injuries.

"Stem cells could be the new antibiotics," says Joshua Hare, director of the Interdisciplinary Stem Cell Institute at the University of Miami Miller School of Medicine. "When you put the pieces of the puzzle together, stem cells could touch just about every area of medicine."

For the record: The cells Hare is talking about are not taken from embryos, a practice mired in controversy. They come from an adult's body tissue, usually the bone marrow, fat or skin. These "master cells" operate as a kind of internal repair system because they can replicate in a continuous fashion to replenish other cells or morph into cells with specialized functions.

Recently, headlines have cast a shadow over stem cell therapy by drawing attention to private clinics offering unproven interventions – and sometimes unfortunate outcomes. For example, an article in the New England Journal of Medicine this spring described three women who experienced vision loss after injections of stem cells derived from their fat tissue for age-related macular degeneration. At this point, most stem cell therapies are experimental, and patients should be looking for rigorous research. But "if you take stem cells from a patient and re-inject them back into the patient, there's no regulation of that," explains Donald Zack, director of the Johns Hopkins Center for Stem Cell and Ocular Regenerative Medicine, which is preparing to test whether such therapies can treat conditions like glaucoma, macular degeneration and retinitis pigmentosa.

Among the most established therapies to date are those for musculoskeletal problems such as arthritis of the knees, hips and shoulders; meniscus tears; and Achilles tendonitis. Stem cells from a patient's bone marrow or belly fat are injected into an injured joint in an outpatient procedure that lasts about 45 minutes. "It's not a panacea, but it offers another opportunity to delay a joint replacement," says Laith Jazrawi, chief of the division of sports medicine at NYU Langone Medical Center. Delay is good "because there's a longevity issue with joint replacement."

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"I didn't want that type of surgery," says Mike Barnet, 62, an avid cyclist and squash player from Margate, New Jersey, who'd been advised to have both knees replaced for bone-on-bone arthritis and a torn meniscus in his right one. He contacted Jazrawi, and a few weeks after an injection into his right knee of stem cells from his pelvic bone, the knee was free of pain and swelling. That was two and a half years ago. "I might need a knee replacement down the road," says Barnet, "but I'm hoping that this will be a long-term fix."

At the Cleveland Clinic, researchers are conducting trials to see if stem cells can address stress urinary incontinence. "We're trying to regenerate the muscle cells, so the sphincter muscle can close and prevent leakage," explains Courtenay Moore, a physician at the Clinic's Glickman Urological Institute. Early findings have shown that after a single injection into the urethra, over two-thirds of women improved by more than 50 percent, compared to a 75 to 80 percent cure rate for surgery. "Three of the women I've injected run on the treadmill at my gym," Moore reports. "And they run faster than me!"

Precisely how stem cell therapies work is still being uncovered. What's emerging is a realization that "they teach the body to heal from within," explains Atta Behfar, a cardiologist and director of the cardiac regenerative medicine program at the Mayo Clinic. Stem cells delivered into a heart injured by a heart attack don't "serve as the brick and mortar to repair the heart," he says. "It's the proteins and other substances they secrete that tell your body to heal."

Research on stroke, for example, has suggested that when stem cells are injected into the brain, they set off a release of factors that contribute to the recovery process, notes Lawrence Wechsler, chairman of the department of neurology at the University of Pittsburgh School of Medicine. (Mayo Clinic researchers in Florida are investigating whether the healing power might extend to brain cancer. In rodents, the effects of stem cells range from a shrinking of the tumors to complete remission. This is noteworthy because few drugs can cross the blood-brain barrier. Stem cells injected into the carotid artery get through, "act as Trojan horses and begin to kill cancer cells," says Alfredo Quinones-Hinojosa, chair of neurosurgery and director of Mayo Clinic's Brain Tumor Stem Cell Research Laboratory in Jacksonville.)

Not surprisingly, results aren't always dramatic. That was the case for Kate Brock of Gambrills, Maryland, 57, whose 2010 stroke left her right side semiparalyzed. In 2013, she had stem cells injected into her brain at Pittsburgh. She gained mobility and strength in her arm, but it didn't last. "The only thing that really changed a lot," says Brock, is that "my legs work better together."

Besides targeted injections, intravenous infusion of stem cells from a healthy donor is being studied as a way to deal with the frailty associated with aging. Preliminary findings from the University of Miami indicate that the therapy significantly reduces systemic inflammation, improves immune function, and boosts physical performance. "Look, I don't want to exaggerate these effects – I can't leap over buildings," says Phillip George, a retired plastic surgeon whose aches and pains, especially after playing golf, led him to participate in the frailty program. "But I have more energy and I feel much more comfortable." Six weeks after treatment, he performed dramatically better on pulmonary function tests and increased his distance on a six-minute walking test. His need for anti-inflammatory drugs is down by 90 percent.

With the field still developing, it's vital that people investigate a treatment's safety and efficacy for their condition. "There's a lot of excitement about stem cells," says Zack, "but we have to do this in a smart way." What that means, he says, is that a treatment should be based on science, not simply hope or beliefs.