drug-related myelosuppression. Selective megakaryocytic hypoplasia is extremely uncommon and relates usually to drug treatments. Hyperplasia The bone marrow under conditions of strong regeneration is characteristically hyperplastic. The regeneration may be anticipated from examination of the peripheral blood with the presence of macrocytic polychromatic erythrocytes or reticulocytes, left-shifted granulocytes, or megaplatelets. With a hyperplastic marrow, a reduced M:E would suggest erythroid hyperplasia and suggest a response to erythropoietin (Epo) stimulation leading to the proliferation of late state forms and increased released of reticulocytes. This will include many immune-mediated and non- immune-mediated hemolytic anemias or blood loss conditions. The level of erythroid response is in proportion to the degree of anemia. Excess Epo production in response to chronic hypoxia such as higher elevations produces increased and complete erythropoiesis. A non-physiologic and inappropriate release of Epo occurs with certain renal tumors and infections that will result in erythrocytosis. An inappropriate erythropoiesis is recognized by the lack of increased reticulocyte counts and a maturation arrest at one stage of development. Such conditions may include immune-mediated hemolytic anemia where destruction is aimed at a metarubricyte stage or earlier. Other disorders where the maturation is not complete although there is excessive erythroid response include nutritional deficiencies such as iron deficiency anemia. An elevated M:E could suggest granulocytic hyperplasia. During early inflammation, there is depletion of the storage pool and a subsequent neutrophilia. With continued inflammation and a constant need for neutrophils, neutropenia may result but the marrow remains hyperplastic and left-shifted. Therefore the peripheral cell count will reflect the severity and duration of the inflammatory response. Conditions causing granulocytic hyperplasia include infections, neoplasms both through direct tissue damage and indirectly related to a paraneoplastic condition, tissue necrosis, generalized inflammatory diseases, immune-mediated neutropenia, hypersensitivity states, hypersplenism, and rebound from suppression by drugs, toxins, or infections such as feline panleukopenia virus. The bone marrow may be hyperplastic relative to megakaryocyte numbers in consumptive or destructive platelet disorders such as subacute to chronic disseminated intravascular coagulopathy or immune-mediated thrombocytopenia. Other non-neoplastic conditions include acute hemorrhage or hemolysis, infections, and hypersplenism. Increased numbers of megakaryocytes are often associated with myelofibrosis related to cytokines such as PDGF and TGF- .
Myelodysplasia The term dysplasia refers to abnormal growth of cells and myelodysplasia occurs in the bone marrow. Cytologically, these abnormalities include asynchronous maturation of the nucleus and cytoplasm resulting in megaloblastic erythroid precursors, abnormally segmented neutrophils, or dwarf megakaryocytes.
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- Fall '19