Cyclic hematopoiesis: disorders of primitive hematopoietic stem cells.

Utilizing both in vivo and in vitro techniques, a great deal of information has been obtained on the structure and regulation of the hematopoietic cell lineages. A number of hematopoietic stem cells and regulators have now been well characterized and their possible physiologic relevance at least in part established. More recently, new "alternative" or primitive stem cells have been described which may provide important insights into the nature of hematopoietic regeneration and regulation. These include late CFUs, high proliferative potential colony-forming cells, colony-forming unit diffusion chamber and both bipotent and blast colonies assayed in in vitro culture systems. Regulators active at these stem cell levels are also under study and in large part appear to be produced by monocytes or lymphocytes. Cyclical hematopoiesis can be viewed as a genetic abnormality at the multipotent stem cell level with defective cell production. At present however, details of the defect await further investigation possibly including an evaluation of the role of primitive stem cells and their regulators.