Assessment and characterization of hemopoietic stem cells

The adequate production of blood cells is sustained by pluripotent hemopoietic progenitors whose behavior is influenced by a permissive hemopoietic microenvironment. Hemopoietic progenitors have in common the expression of CD34 surface molecules, but are heterogeneous with respect to other properties. It is commonly accepted that primitive progenitors, considered to be candidates for marrow repopulating cells, are HLA-DR-, without expressing lineage-specific determinants. They are usually quiescent with respect to their cell cycle status. In addition to CD34, they may display adhesion molecules on their surface and express receptors for ligands such as c-kit, FLT2/FLK3 and various other cytokines. Some of these are expressed constitutively, while others emerge as the cells progress through their regular maturation program. This process appears to include a gradual reduction of their proliferative capabilities as demonstrated by a progressive loss of the length of their telomeric structures.