Polychromasia is a term used to describe a variation in the color of red blood cells (RBCs) when a sample is examined under a microscope. This visual observation is made on a peripheral blood smear, a common diagnostic tool used to assess blood cell health. The presence of cells with differing colors signals that red blood cells of various maturity levels are circulating in the bloodstream. This finding indicates that the body is producing and releasing immature red blood cells from the bone marrow.
The Visual Definition of Polychromasia
When a blood smear is prepared and stained, mature red blood cells exhibit a uniform, salmon-pink color. This consistent color is due to the high concentration of hemoglobin within the cell cytoplasm. Polychromasia refers to the simultaneous presence of cells that stain with a different, often bluish-gray or purplish hue, scattered among the normal pink cells.
These unusual cells frequently appear slightly larger than the surrounding mature red blood cells. The varying shades of color are caused by the mixture of staining properties within the cell’s cytoplasm. A pathologist observes the overall color distribution to determine the degree of polychromasia, which is a measure of red blood cell immaturity in the circulating blood.
The Cellular Mechanism: Reticulocytes
The bluish-gray cells seen in polychromasia are known as reticulocytes, which represent the immediate precursors to mature red blood cells. These cells have been released prematurely from the bone marrow, the primary site of red blood cell production. The distinct color difference stems from the internal composition of the reticulocyte compared to the fully developed erythrocyte.
Mature red blood cells have shed their nucleus and most other organelles during maturation. Reticulocytes, however, still contain residual amounts of ribosomal ribonucleic acid (RNA), which are remnants from the cell’s protein-making machinery. Ribosomal RNA is highly acidic, and therefore attracts the basic component of the Wright-Giemsa stain, which is blue.
The resulting color of the reticulocyte is a blend of the pink-staining hemoglobin and the blue-staining residual RNA. This combination produces the bluish-gray or purplish tone observed microscopically. As the reticulocyte spends its final day or two maturing in the bloodstream, it degrades the remaining RNA, and the cell transitions to the uniform pink of a mature red blood cell.
Clinical Significance and Associated Conditions
Observing polychromasia on a blood smear indicates an increased rate of erythropoiesis, which is the body’s process of making red blood cells. This finding suggests that the bone marrow is actively compensating for a shortage of mature red blood cells in the circulation. The body is pushing out young cells early to maintain oxygen-carrying capacity.
One of the most common causes is hemolytic anemia, where red blood cells are destroyed prematurely faster than the bone marrow can keep up. This rapid destruction signals the bone marrow to accelerate production, leading to a surge of reticulocytes and, consequently, polychromasia. Acute blood loss, such as from trauma or internal bleeding, also triggers this compensatory mechanism, causing a temporary increase in polychromatic cells.
Polychromasia can also be a positive sign of recovery in certain types of anemia. For example, when a patient with a severe nutritional deficiency, such as vitamin B12 or folate deficiency, begins treatment, the bone marrow starts a robust new wave of red blood cell production. This surge of new, immature cells into the blood results in marked polychromasia. In rare cases, the finding can be linked to bone marrow disorders like myelodysplastic syndromes, where abnormal production leads to the premature release of cells.