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In the differential leukocyte count, a determination is made as to the different types of cells present, and the percentage distribution of these various leukocyte types in the peripheral blood. It is very important to scan the slide before beginning counts, to get an idea of what each cell looks like. After learning to identify normal avian cells, it would beneficial for the student to obtain a comparative avian hematology textbook such as the Color Atlas of Comparative Veterinary Hematology by C.M. Hawkey and T.B. Dennett. This book illustrates abnormal cell morphology and species differences to further their study. Following is a description of normal avian blood cells. The main characteristics are bolded to help in identifying the cells. A PowerPoint presentation will further assist in the differentiation of the cells.
Erythrocyte
Students of mammalian hematology will initially be surprised to find that the red blood cells of avians are nucleated. These cells are oval and contain a centrally located nucleus. The cytoplasm is light orange-pink. The nucleus becomes more condensed with age. In young red blood cells (polychromatic red blood cells) the cytoplasm is generally more blue-gray or basophilic. The presence of 1-5% polychromatic, young erythrocytes is normal in avian samples. The youngest red blood cells seen in the peripheral blood are usually larger, rounder and much more basophilic. The nucleus is larger and less organized with the chromatin being more open than an adult cell. Large numbers of young cells can be suggestive of anemia. poikilocytosis and anisocytosis are all indicative of underlying pathology. Smudge cells are often seen, and are usually the result of slide preparation technique.
Heterophil
The heterophil is a granulocyte and is usually the most common of the white blood cells observed in many avian species. The cell is round and contains round or rod-shaped brick red granules. The nucleus is purple and is bi-lobed or segmented. The cytoplasm is colorless.
Eosinophil
The eosinophil is also a granulocyte. The cell is round with a greater tendency toward irregularity than heterophils. There are round to rod-shaped red granules in the cytoplasm. The cytoplasm is a blue-gray color, in contrast to the colorless cytoplasm of heterophils. The nucleus is purple. These cells usually stain more noticeably than the heterophils. Novices sometimes confuse these cells with heterophils. Statistically, the heterophil will be the most prevalent granulocyte present.
Basophil
The basophil is a granulocyte as well. The cell is round and contains a nucleus that is generally round, eccentrically placed, and light blue in color. It often contains a large number of deeply basophilic -staining granules that in some instances completely cover the nucleus. Basophils may make up 20% of the number of cells in the 100 cells counted in the differential.
Lymphocyte
Lymphocytes can be classified into three groups according to cell size (small, medium, and large). These cells are typically round but often show irregularity due to molding around adjacent cells in the smear. The nucleus is usually round, though occasionally indented slightly. The nuclear chromatin is very coarse. There is usually a high nuclear to cytoplasmic ratio that is typical in mammal blood as well. The cytoplasm is weakly basophilic . Occasionally lymphocytes have irregular cytoplasmic projections, known as blebs.
Monocyte
The monocyte is the largest leukocyte found in peripheral blood films. It is a mononuclear cell and has an abundant amount of cytoplasm compared to lymphocytes. The nucleus may be indented or irregular and stains a homogenous blue. It occasionally has light and dark-staining zones in the cytoplasm. The cytoplasm is gray, may contain vacuoles and tiny eosinophilic granules and may be up to 50% larger than the nucleus of the cell.
Thrombocyte
The hemostatic cell in avians that is the counterpart to platelets in mammals is known as the thrombocyte. In contrast to mammalian platelets, avian thrombocytes are larger and are nucleated. Mature thrombocytes are small round to oval cells. They possess very course and tightly clumped nuclear chromatin. The cytoplasm is clear, and often has a reticulated appearance. These cells frequently contain red granules at the poles of the cells in the cytoplasm. Thrombocyte activation leads to cytoplasmic vaculoation, and clumping. Their detection is often aided by the fact that they form clumps in peripheral blood smears. Due to their tendency to clump, a thrombocyte count can only be done subjectively. Normal counts would be 1 or 2 thrombocytes per oil immersion field. An increase would be reported if counts were greater than 2 thrombocytes per oil immersion field. A decrease would be reported if counts were less than 1 per oil immersion field. If clumps are observed, thrombocyte counts are probably adequate in number.