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Hematopoietic stem cells isolated from human bone marrow, coloured scanning electron micrograph (SEM). These stem cells are part of the hematopoietic and reticuloendothelial systems and give rise to specialized blood cells (macrophages, histiocytes). This is the macrophage system that includes all histiocytes, macrophages, reticular cells, monocytes and microglia. It does not include the granular white blood cells (granulocytes). Magnification: x2,200 when shortest axis printed at 25 millimetres. (KEYSTONE/SCIENCE PHOTO LIBRARY/DENNIS KUNKEL MICROSCOPY/SCIENCE PHOTO LIBRARY)