Bone marrow. Coloured Scanning Electron Micrograph (SEM) of bone marrow stem cells. These cells are known as multipotential stem cells because they form the precursors to every type of blood cell. During blood cell development the multipotential stem cell develops by a process known as haemo- poiesis into either red blood cells (erythrocytes), or one of three types of white blood cell (granulocytes, lymphocytes or monocytes). Blood cells are produced constantly by bone marrow as their lifespan is so short - red blood cells, for example, last on average only 120 days before being broken down. Magnification x4,000 at 6x6cm size.