Stem cells are both in and out. It was once thought that if these cells were injected into joints, they would turn into cartilage. Two problems arose. First, patients (unless they are infants) have very few stem cells left. Second, the injected cells died after releasing their instructional growth factors, never turning into cartilage. But what we have learned is that an increasing number of concentrated growth factors, immune stimulants, and even some drugs can induce the migration of the body’s own progenitor cells—some of them stem cells—cells that direct healing—to the injured area. These cells then optimize the environment around the injury to effect healing.