Effect of parathyroid hormone PTH (1-34) on hemopoietic and stromal stem cells

Long-term administration of parathyroid hormone causing activation and proliferation of osteoblasts to mice increases the concentration of primitive hemopoietic precursor cells (cobblestone area-forming cells) in long-living bone marrow culture after 28-35 days. The concentrations of later precursors forming colonies in the spleen and the concentration of cobblestone-area forming cells in long-living bone marrow culture after 7 days decrease, while the concentration of more differentiated cells forming colonies in the culture does not change. Transplantation of the bone marrow from mice treated with parathyroid hormone under the renal capsule of syngeneic recipients results in the formation of a focus of ectopic hemopoiesis not differing by size from the control. Injection of parathyroid hormone to mice during the growth of the ectopic focus did not modulate its size. These foci tolerate retransplantation procedure similarly as controls. Hence, parathyroid hormone has no effect on mesenchymal stem cells responsible for transfer of the stromal microenvironment. Therefore, the number of stem hemopoietic cells in the body is regulated by not stromal stem cells, but their better differentiated descendants.