ASSESSMENT AND CHARACTERIZATION OF HEMATOPOIETIC STEM-CELLS

The adequate production of blood cells is sustained by pluripotent hem opoietic progenitors whose behavior is influenced by a permissive hemo poietic microenvironment. Hemopoietic progenitors have in common the e xpression of CD34 surface molecules, but are heterogeneous with respec t to other properties, It is commonly accepted that primitive progenit ors, considered to be candidates for marrow repopulating cells, are HL A-DR(-), without expressing lineage-specific determinants, They are us ually quiescent with respect to their cell cycle status, In addition t o CD34, they may display adhesion molecules on their surface and expre ss receptors for ligands such as c-kit, FLT2/FLK3 and various other cy tokines, Some of these are expressed constitutively, while others emer ge as the cells progress through their regular maturation program, Thi s process appears to include a gradual reduction of their proliferativ e capabilities as demonstrated by a progressive loss of the length of their telomeric structures.