Coloured scanning electron micrograph (SEM) of mammal leg bone marrow packed with hemopoietic cells, reticular cells and adipose cells. They occupy all the extravascular spaces around an extensive system of channels called venous sinuses. Blood cells develop outside of the extravascular system and must pass through the walls of the sinuses to enter the circulation system. The renewal of all types of blood cells is necessitated by their relatively short life span. The common stem cells developing in bone marrow differentiate along various lines to form red and white blood cells (granular leukocytes or monocytes). Active or red marrow is dominated by the production of red blood cells. With increasing age the marrow becomes less active and is progressively inhabited by adipocytes (fat cells). Osteocyte cell bodies also reside in other smaller cavities (lacunae) of the surrounding compact bone. Magnification: x240 when shortest axis printed at 25 millimetres.