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Leukocytes


The five major categories of leukocytes in mammals are neutrophils, eosinophils, basophils, lymphocytes, and monocytes. The first three collectively are called granulocytes, because they contain granules in their cytoplasm, and the latter two collectively are called mononuclear cells in reference to their round nuclei. These collective terms are somewhat misleading since some granulocytes have barely perceptible granules and most monocytes do not have round nuclei. Granulocytes of most mammals are classed as neutrophil, eosinophil, and basophil based on the reaction of their specific granules with the dyes in polychrome blood stains, called Romanowsky stains.

The prototype species for granulocyte classification is human, in which neutrophil granules are small and pink with weak affinity for azure complexes, basophil granules are deep purple because of strong affinity for the azure complexes, and eosinophils are intensely orange because of strong binding of eosin. Classification of granulocytes in animal species is based in part on homology of appearance to human granulocytes and in part on demonstration of homology of enzyme content and function.

There are many differences, some subtle and some obvious, among species and within species. This portion of the module presents the identifying features of leukocytes in several animal species and illustrates the species differences. Continue by working through the following topics







1)



(polymorphonuclear leukocytes, PMNs, polys)




The neutrophil, or the heterophil in birds, reptiles and some mammals, is the predominant granulocyte. In many species it is the predominant leukocyte in health. In health, only mature neutrophils are released from marrow to circulate in blood. Since mature neutrophils in prototypical human blood have nuclei that are segmented into lobes of condensed chromatin connected by filaments, mature neutrophils are called segmented neutrophils, or segs. The mature neutrophils of many animal species do not



have a distinct lobe and filament arrangement, nevertheless, they also are called segmented neutrophils. The primary criterion for classification of neutrophils as either segmented (mature) or non- segmented (younger stages including band neutrophils, metamyelocytes, and myelocytes) is the shape of the nucleus. A neutrophil should be counted in the segmented category if the nucleus has regions that are distinctly constricted or has lateral protrusions resulting in irregular nuclear margins. A cell whose nuclear margins are smooth and parallel (or nearly parallel) should be counted as a band neutrophil. The presence of non-segmented neutrophils in blood is called a left shift. Inflammation is the most common cause of a left shift, but some other conditions, such as hemolytic anemia and primary diseases of marrow, also can cause release of immature neutrophils from marrow.

Normal mature neutrophils of several species are shown at the right. Canine neutrophils have white cytoplasm that contains small pink granules. Feline neutrophils have cytoplasm that is white and lacks visible granules. Equine neutrophils have white or slightly pink cytoplasm with no visible granules. The nuclei of equine neutrophils typically are long, thin and "knobby" with clumps of condensed chromatin projecting from the sides. Ruminant neutrophils have white cytoplasm with small pink granules; these impart an overall pink tint compared to the other species.

Rabbits, birds, amphibians and reptiles have heterophils. In contrast to the inconspicuous granules of neutrophils, granules in heterophils are large and stained deep orange to red. A typical avian heterophil is shown in "Blood Cells by Species".

A common and important morphologic abnormality of neutrophils is so-called "toxic change

2)
Lymphocytes

Most of the lymphocytes that circulate in healthy dogs, cats, and horses are small (mature) cells that have round nuclei with smooth, dense chromatin and

a small rim of blue cytoplasm. These are sometimes confused with nucleated RBCs. Some lymphocytes, as illustrated by the equine lymphocyte, are slightly larger and have a finer, looser chromatin pattern. The lymphocytes found in blood of normal ruminants are quite variable in appearance. Many are large lymphocytes, as shown in the right middle panel, with loosely clumped chromatin and fairly abundant cytoplasm.

So-called "reactive" lymphocytes are larger cells with coarse (mature) chromatin, and deep blue cytoplasm as shown in the bottom left panel. Lymphocytes with these characteristics are sometimes called immunocytes and are associated with an immune response. These are fairly common in blood of young animals, especially ones that have been recently vaccinated.

Another type of lymphocyte that is found in low number in healthy animals is the granular lymphocyte, shown in the bottom right panel. Granular lymphocytes are distinguished by the presence of small pink granules collected into one area of the cytoplasm. Not all lymphocytes in blood smears are round. Some are distorted into other shapes by the mechanical forces applied to them during smearing and some are molded by contact with red cells.

3)
mono cytes.

Monocytes are extremely variable in appearance but there are no consistent species differences. Each of the four monocytes shown here as examples are from different species but all could be found in blood of any individual animal of any species. For example, the canine monocyte shown here would feel equally at home in a smear of horse blood and the cat monocyte could have been in dog blood.

Some monocytes resemble band or segmented neutrophils, as illustrated by the canine moncyte, while others, like this equine monocyte, may be mistaken for lymphocytes. In general, monocytes are recognized as cells that have abundant light to deep blue cytoplasm and nuclei that are not round. Chromatin is reticular or only slightly condensed. Most are larger than neighboring neutrophils and lymphocytes. Small pink granules are visible in some monocytes and many monocytes have several small discrete vacuoles. Nuclear shape amongst monocytes ranges from ameboid to bean-shaped to band-like to almost segmented. The texture of cytoplasm in a monocyte is slightly grainy or coarse compared to lymphocyte cytoplasm, which is very smooth and glassy.

4)
Eosinophils

Eosinophil granules in most animals are orange but there are always exceptions. The granules in eosinophils of iguanas and some birds are actually pale blue. In general, nuclei of mature eosinophils are shorter and less segmented than neutrophil nuclei and the cytoplasm, if visible, is pale blue. Marked species variation exists regarding the number, size, and shape of eosinophil granules.

The most intraspecies variation is found in dog eosinophils. Marked variation in granule size, number and shape occurs within and between individuals of any breed. A specific and predictable morphologic variant of the eosinophil is seen in greyhounds and other sighthounds (eg, whippets, deerhounds). Eosinophils in these breeds lack visible granules and appear as cells with slightly segmented nuclei, gray cytoplasm, and vacuoles. They are sometimes mistaken for toxic neutrophils or monocytes.

Eosinophils of the cat have small rod-shaped orange granules that fill the cytoplasm, while those of the horse have very large globular orange granules. Ruminant eosinophils have many small very round orange granules.


5)
Basophils

Prototypical basophils of human blood are filled with small, round, dark purple granules. Similar basophils are found in some species but not in others.

Canine






basophils are difficult to recognize since many do not have readily apparent granules. The major identifying features of canine basophils are the long and folded nucleus, described as ribbon-like, and the unusual gray to lavender hue of the cytoplasm. Some basophils contain a few purple granules. Basophils are rare in blood of healthy dogs.

Feline basophils are packed with small, slightly oval granules that are pale lavender rather than deep purple. The nucleus in many appears to have vacuoles, which are actually granules lying on top of the chromatin. Basophils are rare in blood of healthy cats.

Basophils of horses and ruminants are similar to each other and to human basophils. These cells contain many small deep purple granules that obscure the nucleus in many cells. Arrows point to nuclear lobes in the picture of a cow basophil. Some basophils have few granules, which probably is the result of degranulation in the sample. Low numbers of basophils are commonly found in blood of healthy cattle and horses