Reticulocyte. Coloured Transmission Electron Micr- ograph of a forming red blood cell (erythrocyte) from the human bone marrow. This is the final stage in the development of the cell, when it is known as a reticulocyte. A large red cell mass is seen where the nucleus has been removed. At this stage the reticulocyte will spend several days synthesising haemoglobin, the pigment which allows the red blood cells to hold and transport oxygen around the body. After this it will become the characteristic concave shape of a red blood cell. Erythrocyte formation is called erythropoie- sis. Magnification: unknown.