CHAPTER 4

Platelets

• NORMAL PLATELET MORPHOLOGY

Blood platelets (thrombocytes) in mammals are small, round-to-oval anucleatedcell fragments (thin discs when unstimulated) that form from cylinders ofmegakaryocyte cytoplasm. Platelet cytoplasm appears light blue with many smallreddish purple granules when visualized using routine blood stains (Figs. 28A,28B). Equine platelets often stain poorly with Wright-Giemsa stain (Fig. 28C)but generally stain well with Diff-Quik (Fig. 28D). Platelets typically stainuniformly purple with the new methylene blue wet preparation (Fig. 29A).

The diameter of platelets varies depending on the species, with cats havinglarger platelets than other domestic animals (Fig. 28E). Cat platelets appearespecially sensitive to activation during blood sample collection and handling,resulting in degranulated platelet aggregates, which may be overlooked by aninexperienced observer (Fig. 28F). Some of the precipitated cryoglobulin recog­nized in blood from a cat with a monoclonal cryoglobulinemia (Fig. 28G) alsoresembled aggregates of degranulated platelets.

Newly formed platelets have higher RNA content and have been termedreticulated platelets. They cannot be quantified by morphology but can becounted using flow cytometry following labeling of RNA with a fluorescentdye.245.246

Platelet morphology is typically normal in animals with thrombocythemia,a chronic myeloproliferative disorder in which platelet counts typically exceed1 X 106/ILL (Fig. 29B). However, increased mean platelet volumes (MPV) havebeen reported in two dogs believed to have thrombocythemia.v"

• ABNORMAL PLATELET MORPHOLOGY

MacroplateletsPlatelets that are as large, or larger, in diameter as erythrocytes are calledmacroplatelets, megaplatelets, or macrothrombocytes (Figs. 28H-28L). Theymay be seen in low numbers in normal cats. The presence of frequent macro-

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76 SECTION I / BLOOD

FIGURE 28. Normal and abnormal morphology of platelets. A. An aggregate of platelets inblood from a dog. Wright-Giemsa stain. B. An aggregate of platelets in blood from a cow. Wright­Giemsa stain. C. Three pale-staining platelets in blood from a horse. An erythrocyte contains aHowell-Jolly body. Wright-Giemsa stain. D. Five platelets in blood from a horse. Diff-Quik. stain.E. An aggregate of platelets in blood from a cat, demonstrating the presence of large plateletscharacteristicof this species. Wright-Giemsa stain. F. An aggregate of activated and degranulated platelets in blood

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CHAPTER 4 I PLATELETS 77

platelets in a thrombocytopenic animal suggests that enhanced thrombopoiesisis present,' but macroplatelets may also be present in thrombocytopenic animalswith myelodysplastic or myeloproliferative disorders.!" Macroplatelets may bepresent in nonthrombocytopenic animals that have recently recovered fromthrombocytopenia (Figs. 29C, 29D). A population of macroplatelets may beseen in healthy King Charles spaniel dogs248 and in otter hound dogs with ahereditary platelet function defect,249

Activated PlateletsPartially activated platelets are no longer discs but have thin cytoplasmic pro­cesses extending from a spherical cell body. When platelets are more fullyactivated, their granules are crushed together by a surrounding web of microtu­bules and microfilaments. This central aggregate of platelet granules may bemistaken for a nucleus (Fig. 28n. Platelet aggregates form following plateletactivation in vitro. If degranulation occurs, aggregates may be difficult to recog­nize, appearing as light-blue material on stained blood films (Figs. 28F, 29E).The presence of platelet aggregates should be recorded because the plateletcount may be erroneously decreased.

Hypogranular PlateletsHypogranular platelets may result from platelet activation and secretion, butthey have also been seen in animals with myeloproliferative disorders (Figs.

from a cat. Only a single platelet in the upper left of the aggregate still contains visible granules. Wright­Giemsa stain. G. Blue-staining homogenous globules of cryoglobulin in blood from an Americandomestic shorthair cat with multiple myeloma and an IgG monoclonal cryoglobulinemia. The precipitatedglobules sometimes mimic the appearance of an aggregate of degranulated platelets. A single platelet ispresent at the bottom between two erythrocytes. Photograph of a stained blood film from a 1999 ASVCPslide review case submitted by T. Stokol, J. Blue, F. Hickford, Y. von Gessel, and J. Billings. Wright stain.H. Macroplatelet in blood from a dog with immune-mediated thrombocytopenia. Wright-Giemsa stain.I. Macroplatelet in blood from a dog with Ehrlichia platys infection and thrombocytopenia.Wright-Giemsastain. J. Macroplatelet with aggregated granules that may be mistaken for a nucleus in blood from acat with an abdominal abscess and toxic left shift in the blood. Wright-Giemsa stain. K. Macroplateletin blood from a dog with CML. Wright-Giemsa stain. L. Macroplateletwith centrally located granulesin blood from a cat with myelodysplastic syndrome. Wright-Giemsa stain. M. Platelet with granules(top left) and a hypogranular platelet (right) in blood from a dog with CML. Both platelets exhibit thincytoplasmic processes. Wright-Giemsa stain. N. Hypogranular platelet in blood from a dog with CML,exhibiting thin cytoplasmic processes. Wright-Giemsa stain. O. Hypogranular platelet in blood from adog with erythroleukemia (AML-M6Er). Wright-Giemsa stain. P. Hypogranular macroplatelet in bloodfrom a dog with CML. Wright-Giemsa stain. Q. Cytoplasmic fragment in blood from a cow withleukemic lymphoma. The fragment might be confused with a hypogranular platelet. Wright-Giemsa stain.R. Two platelets containing Ehrlichia platys morulae, which stain dark-blue in contrast to the normalmagenta-staining granules, in blood from a dog. Wright-Giemsa stain. S. Platelet containing anEhrlichia platys morula, which stains dark-blue in contrast to the normal magenta-staining granules, inblood from a dog. Wright-Giemsa stain. T. Large platelet containing what appears to be two Ehrlichiaplatys morulae, each with multiple subunits, in blood from a dog. New methylene blue wet mountpreparation.

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FIGURE 29. See legend on opposite page.

CHAPTER 4 / PLATELETS 79

28M-28P).13l·250 Hypogranular platelets must be differentiated from cytoplasmicfragments from other cells, as has been reported in ruminants with leukemiclymphomas (Fig. 28Q).251

Ehrlichia Platys Infection

Ehrlichia platys is a rickettsial parasite that specifically infects platelets in dogs.Morulae appear as tightly packed basophilic clusters of organisms within thecytoplasm of platelets (Figs. 28R-28T).252 Similar-appearing inclusions havebeen seen in platelets from a cat.m

FIGURE 29. Abnormalities of platelets and a dwarf megakaryocyte. A. Mild thrombocytosisfollowing splenectomy in blood from a dog. Platelets appear as small purple cells and erythrocytesappear asunstained "ghosts." Single small Heinz bodies in many erythrocytes appear as basophilic "dots." A reticulo­cyte is present at center right. New methylene blue wet mount staining procedure. B. Markedlyincreased platelet numbers in blood from a dog with thrombocythemia. A basophil (bottom) and neutrophil(top right) are also present. Photograph of a stained blood film from a 1987 ASVCP slide review casesubmitted by C.P. Mandell, N.C. Jain, J.G. Zinkl. Wright stain. C. Lack of platelets in blood from adog with immune-mediated thrombocytopenia (platelets = 20 X 103/ p.L) and regenerative anemia. Poly­chromatophilic erythrocytes, a metarubricyte (bottom), and two neutrophils are present. Wright-Giemsastain. D. Thrombocytosis (platelets = 950 X 103/ p.L) with several macroplatelets in blood from thesame dog as shown in Figure 29C a week after beginning prednisone therapy. Wright-Giemsa stain. E.A large platelet aggregate in blood from a cat with thrombocytosis. The presence of many platelet aggregatesin the blood sample resulted in an erroneously high total leukocyte count measured electronically usingimpedance technology. Wright-Giemsa stain. F. Dwarf megakaryocyte and many platelets in the buffycoat of a cat with myelodysplastic syndrome. Wright-Giemsastain.