Scanning electron micrographs of reticulocytes, showing the irregular forms these membrane replete cells can assume. Reproduced from: "Erythrocytic series, Figure 4, pg 47" in Blood Smears Reinterpreted by M. Bessis, 1976; with kind permission from Springer Science and Business media. Reticulocytes are slightly immature, anucleate red cells that contain RNA. The RNA, which binds basic dyes, may be sufficient to impart a blue-gray tint, called polychromatophilia, to young red cells in blood smears stained with polychrome stains (e.g., Wright's, Hemacolor). In many species, once the cell reaches the reticulocyte stage, it remains in the marrow for about 2 days, then is released to complete its maturation by losing its RNA and some of its surface membrane while it circulates. This is usually accomplished in the spleen. Reticulocytes are not released from the marrow in normal horses and ruminants. A more sensitive method to show the presence of RNA, however, is to incubate blood with a 0.5% solution of new methylene blue (NMB). NMB precipitates the RNA and stains it blue. Reticulocytes in smears of NMB stained blood are seen as erythrocytes with blue granules. The reticulocyte count is done by counting a large number of red cells in a smear of new methylene blue stained blood and recording the number of cells that contain blue granules, then calculating the percentage of total red cells that are reticulocytes. This result is used to help determine if an anemic patient has a healthy marrow response to the anemia as would be indicated by increased numbers of young red cells produced to replace lost red cells. As discussed more fully later, the interpretation of the reticulocyte count differs with species. There is often confusion as to the meaning of the terms, reticulocytes and polychromatophils. Reticulocytes are immature erythrocytes that contain reticulin as observed in a new-methylene blue-stained blood smear. Polychromatophils are immature erythrocytes that stain blue-purple in Wright's stained blood smears. All polychromatophils are reticulocytes, however not all reticulocytes are polychromatophils on a Wright's stained blood smear. For example, punctate reticulocytes in cats are readily visualized in new methylene blue-stained blood smears, but they do not contain sufficient RNA to appear blue-purple in Wright's stained smears. Therefore, they actually appear red (the color of normal erythrocytes), although they are often macrocytic as they are still larger than normal erythrocytes.