Because of the inability of erythrocytes (red blood cells) to divide to replenish their numbers, the old ruptured cells must be replaced by totally new cells. This short life span necessitates the process erythropoiesis, which is the formation of red blood cells. All blood cells are formed in the bone marrow, the erythrocyte factory, which is soft, highly cellar tissue that fills the internal cavities of bones. The following are the major steps in erythropoiesis. Erythrocytes are derived in the red bone marrow from pluripotent stem cells that give rise to all types of blood cells. Myeloid stem cells are partially differentiated cells that give rise to erythrocyte and several other types of blood cells.