SummaryThe existence of different stem cell populations has been reported. Different sub-populations of committed-secondary stem cells arise from a less differentiated pluripotent-primary-stem cell. Primary stem cells have an extensive capacity. for self-renewal. The secondary stem cells have a limited self-renewal capacity and are committed tot the formation of red cells, granulocytes and monocytes, or platelets. Hormonal substancef are needed for their proliferation and differentiation. The circulating blood cells and functional demands are of major importance in the , different feedback systems which govern haemoPoiesis.