Phenotypic and functional characterization of hematopoietic stem cells.

This article summarizes recent progress in understanding the developmental hierarchy of the stem cell compartment in hematopoietic organs and discusses assays and phenotypic markers that allow their identification. In the past few years, we have witnessed significant progress in understanding the complexity of the embryonic origin of the hematopoietic system and the developmental migration of hematopoietic stem cells, which involves multiple anatomical sites in an embryo before adult hematopoiesis is established in bone marrow. New data shed more light on the developmental hierarchy of the stem cell compartment in bone marrow and there is growing evidence that bone marrow may also contain a pluripotent stem cell that gives rise to the long-term repopulating of hematopoietic stem cells, mesenchymal stromal cells and endothelial progenitors. Furthermore, rare populations of primitive hematopoietic stem cells have been identified in bone marrow that engraft after intrabone injection only. New strategies to isolate/purify hematopoietic stem cells are developed based on employing surface markers and metabolic properties. Noteworthy, expression of some surface markers may change during their activation and isolation. Hematopoietic stem cells are heterogeneous and differ in the expression of both surface markers and the assays by which they can be detected and quantified.