Hematopoietic stem cell characterization and isolation.

Hematopoietic stem cells (HSCs) are defined by the capabilities of multi-lineage differentiation and long-term self-renewal. Both these characteristics contribute to maintain the homeostasis of the system and allow the restoration of hematopoiesis after insults, such as infections or therapeutic ablation. Reconstitution after lethal irradiation strictly depends on a third, fundamental property of HSCs: the capability to migrate under the influence of specific chemokines. Directed by a chemotactic compass, after transplant HSCs find their way to the bone marrow, where they eventually home and engraft. HSCs represent a rare population that primarily resides in the bone marrow with an estimated frequency of 0.01% of total nucleated cells. Separating HSCs from differentiated cells that reside in the bone marrow has been the focus of intense investigation for years. In this chapter, we will describe in detail the strategy routinely used by our laboratory to purify murine HSCs, by exploiting their antigenic phenotype (KSL), combined with the physiological capability to efficiently efflux the vital dye Hoechst 33342, generating the so-called Side Population, or SP.