There are various types of erythrocyte inclusions that can be found when performing a peripheral smear. While some inclusions can be seen using a Wright’s stain, other inclusions can only be viewed with special staining. Each type of inclusion has its own unique properties and associated disease states. It is important for the purposes of a diagnosis that inclusions are noted when found while performing a differential.
Reticulocytes are the most common type of erythrocytes containing inclusions. A reticulocyte is a young erythrocyte that has extruded its nucleus leaving behind reticulum. This is a normal part of the maturation cycle of erythrocytes. This reticulum can appear an even bluish color or a patchy bluish-orange color under a Wright’s stain. This is referred to a polychromatophilia. In order to visualize the inclusions in reticulocytes a peripheral smear can be stained with new methylene blue. Under this stain the inclusions will appear either as filaments or as granules. It is normal for adults to have 0.5 to 1.5% reticulocytes. In general the amount of reticulocytes present is proportional to the production of erythrocytes. In cases of hemorrhage or severe red cell destruction the percentage of reticulocytes present will increase. In the case of chronic anemia or other diseases of defective erythrocyte production the percentage of reticulocytes will be decreased. Reticulocyte counts can be performed manually using new methylene blue stain and counting the cells on a hemocytometer. Most laboratories have instrumentation that will perform this count.
Basophilic stippling, Howell-Jolly bodies, and siderotic granules are common types of erythrocyte inclusions that can be visualized using Wright’s stain. Basophilic stippling is the aggregation of ribosomal material within the erythrocyte. The cell appears basophilic or blue to purple in color, with the presence of dark bluish granules spread throughout the cell. These granules are seen in cases of abnormal heme synthesis and in lead intoxication. Howell-Jolly bodies are composed of nuclear material that has remained inside the erythrocyte after the nucleus has been extruded. The granules are large, round, and are not refractile. These inclusions are very dark in color. Howell-Jolly bodies are usually found in cases of splenectomy and in some hemolytic anemias. They are not usually found in iron-deficiency anemia. Siderotic granules, also known as Pappenheimer bodies, are inclusions that contain iron. These inclusions stain a faint blue color. Siderotic granules are smaller than Howell-Jolly bodies and are usually found in clusters of two or more, often to the side of the cell. These granules are often found after a splenectomy, or whenever hemoglobin synthesis is impaired. They are absent in iron-deficiency anemia. While it is normal to see an occasional Howell-Jolly body in a peripheral smear, the presence of basophilic stippling and siderotic granules is abnormal.
It is important to take note of any type of erythrocytic inclusions when performing a peripheral smear. Each type of inclusion is composed of its own unique cellular material and can be associated with various disease states. While some inclusions may present with similar characteristics closer inspection of the entire peripheral smear can help clarify which inclusions if any are present.
• Miale, J. B., Laboratory Medicine: Hematology. St. Louis, Missouri: The C.V. Mosby Company (1982). p. 486-489.
• Carr, J. H., Rodak, B.F., Clinical Hematology Atlas. St. Louis, Missouri: Elsevier Saunders (2004). p. 111-115.