[Humoral factors in the regulation of cell proliferation in haematopoiesis. II. Erythropoiesis, megakaryopoiesis, multipotent stem cell (author's transl)].

Diffusible factors probably play an important role in the control of cell proliferation during haematopoiesis. Several stimulators of erythropoiesis including erythropoietin are to be found in sera. Inhibitors are found in erythrocyte supernatants, in the urin of newborn children, and in plasma of patients with renal insufficiency. Stimulators of megakaryopoiesis have been shown to be present in conditioned media of lymphoid cells and bone marrow cells. Inhibitors have been detected in various sera. Proliferation of multipotent stem cells is probably also regulated by a humoral mechanism. Stimulators and inhibitors are present in bone marrow extracts. Clinical relevance of these factors is discussed.