From Dog
Fine needle aspirate from benign mixed mammary tumor, showing the presence of a megakaryocyte (arrow)[1]

Megakaryocytosis is defined as an increase in the number of large thrombocytes (megakaryoblasts), cells primarily found in bone marrow, spleen and liver.

The opposite condition is megakaryocytopenia, usually associated with thrombocytopenia[2] and reported in the Norfolk Terrier[3].

Megakaryoblasts are derived from hematopoietic stem cell precursor cells and transform into platelets. Their production has been shown to be stimulated by use of lithium carbonate[4].

Megakaryocytes appear as large, atypical cells with blue vacuolated cytoplasm, cytoplasmic blebs, round to oval central nuclei, and elevated numbers of cytoplasmic fragment resembling macroplatelets[5].

In dogs this is usually associated with:

Megakaryocytic leukemia has been reported with a predisposition in the German Shepherd[9] and is characterized by megakaryocytosis with neoplastic megakaryoblasts identified in spleen, liver, mesenteric lymph nodes and pulmonary vasculature[10]


  1. 1.0 1.1 Grandi F et al (2010) Extramedullary hematopoiesis in a case of benign mixed mammary tumor in a female dog: cytological and histopathological assessment. BMC Vet Res 6:45
  2. Silva LF et al (2012) Measurement of thrombopoietic activity through the quantification of megakaryocytes in bone marrow cytology and reticulated platelets. Res Vet Sci 93(1):313-317
  3. Gelain ME et al (2012) Macrothrombocytopenia in a group of related Norfolk terriers. Vet Rec 167(13):493-494
  4. Leclerc A et al (2010) Effects of lithium carbonate on carboplatin-induced thrombocytopenia in dogs. Am J Vet Res 71(5):555-563
  5. Comazzi S et al (2010) Acute megakaryoblastic leukemia in dogs: a report of three cases and review of the literature. J Am Anim Hosp Assoc 46(5):327-335
  6. Ferreira HM et al (2011) Myeloperoxidase-positive acute megakaryoblastic leukemia in a dog. Vet Clin Pathol 40(4):530-537
  7. Auler P et al (2011) Myeloid metaplasia in canine mixed mammary tumors: occurrence and characterization. Vet Q 31(4):173-177
  8. Paim CB et al (2012) Thrombocytopenia and platelet activity in dogs experimentally infected with Rangelia vitalii. Vet Parasitol 185(2-4):131-137
  9. Barger AM et al (2012) Cytologic identification of erythrophagocytic neoplasms in dogs. Vet Clin Pathol 41(4):587-589
  10. Ameri M et al (2010) Acute megakaryoblastic leukemia in a German Shepherd dog. Vet Clin Pathol 39(1):39-45