Artwork illustrating the development of immature blood cells in human bone marrow. The renewal of all types of blood cells is necessitated by their relatively short lifespan. The common stem cells developing in bone marrow differentiate to become either red blood cells, granular leukocytes or monocytes. Active bone marrow, also known as red marrow, is dominated by the production of red blood cells. With increasing age the marrow becomes less active & is progressively inhabited by adipocytes (fat). Mature marrow is known as yellow marrow, due to the preponderance of these yellow fat cells.