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Myeloid cells arise from a common stem cell whose development is regulated by stimulatory and inhibitory growth factors. Pluripotential hematopoietic stem cells are most influenced by IL-3, GM-CSF, and stem cell factor while committed progenitor cells are regulated by variable concentrations of GM-CSF, G-CSF, M-CSF, IL-5, Epo, and Tpo. As a result of their common origin, a key point to remember about myeloproliferative disorders is the involvement of multiple cell lines in dysplastic and neoplastic conditions. Dysplastic changes may signal early neoplastic changes with cases progressing to acute leukemia. Myelodysplastic syndrome (MDS) is associated with anemia or multiple cytopenias, normal to hypercellular bone marrow, ineffective hematopoiesis, and less than 30% blast cells of all nucleated cells in the bone marrow. Chronic myeloid leukemias also have less than 30% blast cells of all nucleated cells in the bone marrow and are distinguished from MDS by elevated cell counts of one or more cell lines with mature forms predominating. Acute myeloid leukemias, often the end result of all myeloproliferative disorders, are recognized by equal or greater 30% blast cells of all nucleated cells in the bone marrow. Additional diagnostic information from cytochemical stains, immunohistochemical staining, and cytogenetic analysis can influence the final diagnosis when morphology alone is equivocal. In conclusion, prognosis and response to treatment are best determined by application of a uniform set of standards in evaluating hematolymphatic neoplasia. Critical to diagnosis are complete blood and bone marrow evaluations including observation for dysplastic changes and blast cell quantitation. In addition, evidence for tissue infiltration identified through cytologic or histologic evaluations of lymph node, spleen, or liver is recommended.

This article contains an overview of the classification of neoplasia of the hematopoietic system in the dog. The objective of this article is to convey an understanding of currently used classification schemes and to correlate this classification with modern concepts of hematopoietic system biology. This article will be concerned with those neoplastic disease processes that are derived from the mesodermally derived pluripotent hematopoietic stem cells and their progeny.

Myeloproliferative disorders are uncommon in the dog and may be classified as chronic or acute. Excessive proliferation of mature cells leads to an overproduction of terminally differentiated blood cells (chronic MPD). Inability of cells to mature results in the accumulation of poorly differentiated blast cells in the peripheral blood and bone marrow (acute MPD). Because the lesion appears to be at the level of the hematopoietic stem cell, all cell lines in the bone marrow may be affected. Diagnosis depends upon the accurate identification of neoplastic cells and the absence of other diseases associated with bone marrow hyperplasia. The prognosis for chronic MPD is guarded, whereas for acute MPD it is grave. Accurate identification of these disorders in animals is important. Investigation and greater understanding of the pathophysiologic mechanisms may lead to more lasting therapeutic successes in the future.