Microscopical assessment of erythrocyte morphology is an important clinical tool for diagnosing the cause of anemia and also some other disorders. Normal erythrocytes are circular discs with central pallor, which is much less prominent in cats than in dogs. Observing them under microscope, one should pay attention to their size, color, shape, structures in or on them, stage of their development and arrangement of cells on blood films. The interpretation should be made in connection to quantitative data from CBC. Important for relevant blood-cell evaluation is adequate preparation of the blood smear.
Any variation in the size or volume of erythrocytes is called ANISOCYTOSIS. The size of erythrocytes should be interpreted in connection with MCV.
Macrocytes are large erythrocytes with increased MCV. Increased numbers of large immature polychromatophilic cells are most commonly seen with regeneration. Macrocytosis without polychromasia or other evidence of an appropriate regeneration is a common finding with myelodysplasia and myeloproliferative disease and is associated with feline leukemia virus (FeLV) infection. Infrequent causes of normochromic macrocytosis are macrocytosis of Poodles and hereditary stomatocytosis (Alaskan malamute, miniature schnauzer).
MCV is more valuable than blood film in assessing the size of erythrocytes, because the cells must be markedly small before that can be visually detected. The most common cause of microcytosis is iron deficiency. Iron-deficient cells can have normal diameter and appear normal on blood film despite decreased MCV and only in case of severe iron deficiency, microcytosis may be observed on the film. Other causes of microcytic anemia are portosystemic shunt and anemia of chronic disease. Akita, Shiba Inu, Sharpei and Chow Chow are breeds that can naturally have smaller erythrocytes.
Polychromatophilic cells are early released young erythrocytes. They are larger and more blue than mature erythrocytes (blue color results from organelles, still present in immature cells). They are sign of bone marrow response, so pointing toward regenerative anemias.
Hypochromic cells have increased central pallor, usually as a result of decreased hemoglobin concentration in iron deficiency. This is more obvious in dogs, erythrocytes of iron-deficient cats are usually not hypochromic. Immature polychromatophilic erythrocytes in regeneration may also appear hypochromic, because their hemoglobin concentration is less than normal due to increased cell volume. Hypochromic cells should be differentiated from torocytes, which have sharply defined central clear area and thicker rim of hemoglobin and are insignificant.
STRUCTURES IN OR ON ERYTHROCYTES
- HEINZ BODIES
Heinz bodies are small, eccentric, pale structures within the erythrocytes and can seem to protrude from the cell margin. They are difficult to see on Wright-stained smears, especially in dogs, where they appear small and multiple. In feline erythrocytes they are usually single and large structures. Heinz bodies are formed by precipitation of denatured hemoglobin after exposure to oxidative insult (garlic, onion, zinc, copper, acetaminophen, propofol …). They reduce deformability of the cells, making them more susceptible to hemolysis. Cats may have increased concentration of Heinz bodies without exposure to oxidant chemicals in case of diabetes mellitus, lymphoma or hyperthyroidism.
- HOWELL-JOLLY BODIES
Howell-Jolly bodies are remnants of incompletely extruded nucleus in erythrocytes, visible as small, round, dark-blue, usually eccentric inclusions. Increased concentration is associated with regeneration, splenectomy, suppressed splenic function and bone marrow disease.
- BASOPHILIC STIPPLING
Basophilic stippling is aggregation of ribosomes into small granules inside erythrocytes or their precursors. It may be seen in dogs and cats with intensely regenerative anemia. Without severe anemia and polychromasia it may suggest lead poisoning.
NUCLEATED RED BLOOD CELLS
Increased number of nucleated erythrocytes (particularly metarubicytes) is associated with regenerative anemias and early release of cells in response to hypoxia. It may also be seen with extramedullary hematopoiesis, hemangiosarcoma, splenectomy, increased levels of corticosteroids (endo- or exogenous), lead poisoning, myelodysplasia or myeloproliferative disease … The presence of earlier erythrocyte precursors suggests severe bone marrow or reticuloendothelial disease.
ERYTHROCYTE ARRANGEMENT ON BLOOD FILMS
- ROULEAUX FORMATION
Rouleaux formation is spontaneous association of erythrocytes into linear stacks. Slight amount of it is normal in cats and dogs, but it enhances with increased concentration of plasma protein (fibrinogen, immunoglobulins). It is often suggestive of gammopathy (multiple myeloma).
Agglutination is irregular clumping of cells, which is formed in presence of antibodies on the erythrocyte wall. Agglutination is very suggestive of immune-mediated hemolytic anemia but can also be seen after a mismatched blood transfusion.
More about changes in shape of erythrocytes … next time 😊
Thrall M. A., Weiser G., Allison R. W., Campbell T. W.: Veterinary Hematology and Clinical Chemistry, 2nd Ed. Wiley-Blackwell, 2012
Day M. J., Kohn B.: BSAVA Manual of Canine and Feline Haematology and Transfusion Medicine, 2nd Ed. BSAVA, 2012