Blood cells come in several kinds, including red cells that carry oxygen from the lungs to the tissues and carbon dioxide back to the lungs, white cells of many types that help defend against disease, and platelets that promote blood coagulation and wound healing. All descend from a relatively few large cells in the bone marrow, called pluripotential stem cells.

Red cell precursors, known as erythroblasts, pass through several stages of differentiation as they develop to maturity. Early in the process they are crowded with ribosomes, which manufacture proteins including the oxygen-carrying hemoglobin that will eventually take up almost all the space inside the cell.

Meanwhile the cells continue shrinking; their nuclei are finally ejected shortly before the cells leave the bone marrow. At this stage the red cells still contain a network of ribosomes and are called reticulocytes (reticule is the Latin word for net), but this network soon dissolves, leaving the familiar supple and slightly concave disk-shaped red cells that constitute half the volume of the blood.

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