Hematopoietic stem and progenitor cells HSPCs

At the time of birth and throughout adulthood, the bone marrow is the primary site for hematopoiesis and is the location of most of the HSC population. A small percentage of HSCs are mobilized and found in circulating or peripheral blood (PB). Additionally, other hematopoietic sites function to further differentiate and mature hematopoietic stem and progenitor cells (HSPCs). The thymus differentiates HSPCs into T cells, and the spleen functions to differentiate into B cells (in mice and humans). 

In its natural environment, hematopoiesis resides in a well-defined microenvironment characterized by local geometry (structure and vasculature), by stromal cells (accessory cells of mixed origin), and by an extracellular matrix composed of coUagen-like molecules and proteoglycans, produced by stromal cells (Nielsen, 1999). Thus, it is bkely that hematopoietic stem and progenitor cells (HSPCs) are influenced by accessory cells and the microenvironment they create in several ways. 

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