Red blood cell formation or erythropoiesis. Light micrograph of tissue in the bone marrow showing cells which will develop into red blood cells. The bone marrow is the site where all of the red blood cells, the plateletes and most of the white blood cells are produced. Cells in the bone marrow, also known as stem cells, are stimulated to form blood cells by erythropoietin, a hormone originating in the kidney. The tiny dark areas are the nuclei of the stem cells; at the end of the maturation process the nuclei are extruded and the red blood cell is almost entirely formed by haemoglobin. Magnification: x70 at 35mm size.