Compact bone and bone marrow cavity (lacuna), animated coloured scanning electron micrograph (SEM). The bone marrow consists of packed hemopoietic cells, reticular cells and adipose cells. They occupy all the extravascular spaces around an extensive system of channels called venous sinuses (green). Blood cells develop outside of the extravascular system and must pass through the walls of the sinuses to enter the circulation system. 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.