An evolving model of hematopoietic stem cell functional identity.

Easily accessed via venipuncture, blood has been an object of study for centuries. Direct sampling of the hematopoietic tissues in the bone marrow also presents a rather low bar for biopsy acquisition from living donors, especially when compared to other systems in the body such as the heart or brain. This relatively straight-forward ability to obtain cells from the primary anatomical locations of blood cell genesis and differentiation combines with a reliable transplantation assay and well-described surface markers to make the hematopoietic stem cell (HSC) the best understood of all tissue stem cells. HSC biology has been extensively though incompletely investigated over the years. The field continually refreshes itself as new findings require us to reevaluate our understanding of hematopoiesis. After providing a brief overview of the hematopoietic system in general, this review will touch on recent findings in three areas: (1) the niche, (2) HSC migration, and (3) challenges to the "classical" model of hematopoietic ontogeny.