Slightly immature, non-nucleated red cells (reticulocyte stage) appear blue-gray on Wright-stained smears due to the presence of residual ribonucleic acid (RNA). These cells are referred to as polychromatophilic cells. Polychromatophilic cells are frequently larger in size than mature red cells and can be distinguished from macrocytes by their distinctive blue-gray color.
The cells that are indicated by the arrows on this slide are slightly blue-gray and are examples of polychromatophilic red cells. Increased numbers of these cells (averaging 2 or more per oil immersion field) indicate increased red cell output by the bone marrow. Under normal conditions, these young red cells remain in the bone marrow one or two days before release into the bloodstream. However, when the bone marrow is stressed due to blood loss or other conditions, these cells are prematurely released into the blood. If stained with a supravital stain, they would be identified as reticulocytes.