Hematopoietic stem cell origin of mesenchymal cells: opportunity for novel therapeutic approaches.

There has been a general belief that there are two types of adult stem cells, i.e., hematopoietic stem cells (HSCs) and mesenchymal stem cells (MSCs), each with distinctly different functions. According to this dogma, HSCs produce blood cells, while MSCs are thought to generate a number of non-hematopoietic cells including fibroblasts, adipocytes, chondrocytes and bone cells. Recently, a number of studies, including those in our laboratory based on single HSC transplantation, blurred the clear distinction between HSCs and MSCs and strongly suggested an HSC origin of the adult mesenchymal tissues. This review summarizes the experimental evidence for this new paradigm and the literature pointing out the vagary in the stem cell nature of MSCs. The concept of the HSC origin of mesenchymal cells will have many immediate and long-term impacts on the therapies of diseases and injuries of the connective tissues.