Abstract
From the outset, it was apparent that developing new therapies with mesenchymal stem/stromal cells (MSCs) was not a simple or easy task. Among the earliest experiments was administration of MSCs from normal mice to transgenic mice that developed brittle bones because they expressed a mutated gene for type 1 collagen isolated from a patient with osteogenesis imperfecta. The results prompted a clinical trial of MSCs in patients with severe osteogenesis imperfecta. Subsequent work by large numbers of scientists and clinicians has established that, with minor exceptions, MSCs do not engraft or differentiate to a large extent in vivo. Instead the cells produce beneficial effects in a large number of animal models and some clinical trials by secreting paracrine factors and extracellular vesicles in a "hit and run" scenario. The field faces a number of challenges, but the results indicate that we are on the way to effective therapies for millions of patients who suffer from devastating diseases.