Abstract
Hematopoietic stem cells are rare cells found in the bone marrow, peripheral blood, placenta, and elsewhere that sustain the hematopoietic system. The current review will focus on the evolving views of the biology of these cells. Because hematopoietic stem cells have been studied and transplanted for several decades, lessons learned from studying these cells may provide useful paradigms for the study of mesenchymal stem cells.