Thrombocytosis: Increased numbers of platelets in the peripheral blood. (Dorland, 27th ed)Platelet Count: The number of PLATELETS per unit volume in a sample of venous BLOOD.Thrombocythemia, Essential: A clinical syndrome characterized by repeated spontaneous hemorrhages and a remarkable increase in the number of circulating platelets.Anemia, Refractory: A severe sometimes chronic anemia, usually macrocytic in type, that does not respond to ordinary antianemic therapy.Myeloproliferative Disorders: Conditions which cause proliferation of hemopoietically active tissue or of tissue which has embryonic hemopoietic potential. They all involve dysregulation of multipotent MYELOID PROGENITOR CELLS, most often caused by a mutation in the JAK2 PROTEIN TYROSINE KINASE.Thrombopoietin: A humoral factor that stimulates the production of thrombocytes (BLOOD PLATELETS). Thrombopoietin stimulates the proliferation of bone marrow MEGAKARYOCYTES and their release of blood platelets. The process is called THROMBOPOIESIS.Anemia, Sideroblastic: Anemia characterized by the presence of erythroblasts containing excessive deposits of iron in the marrow.Polycythemia Vera: A myeloproliferative disorder of unknown etiology, characterized by abnormal proliferation of all hematopoietic bone marrow elements and an absolute increase in red cell mass and total blood volume, associated frequently with splenomegaly, leukocytosis, and thrombocythemia. Hematopoiesis is also reactive in extramedullary sites (liver and spleen). In time myelofibrosis occurs.Receptors, Thrombopoietin: Cell surface receptors that are specific for THROMBOPOIETIN. They signal through interaction with JANUS KINASES such as JANUS KINASE 2.Megakaryocytes: Very large BONE MARROW CELLS which release mature BLOOD PLATELETS.Primary Myelofibrosis: A de novo myeloproliferation arising from an abnormal stem cell. It is characterized by the replacement of bone marrow by fibrous tissue, a process that is mediated by CYTOKINES arising from the abnormal clone.Thrombopoiesis: The process of generating thrombocytes (BLOOD PLATELETS) from the pluripotent HEMATOPOIETIC STEM CELLS in the BONE MARROW via the MEGAKARYOCYTES. The humoral factor with thrombopoiesis-stimulating activity is designated THROMBOPOIETIN.Leukocytosis: A transient increase in the number of leukocytes in a body fluid.Janus Kinase 2: A Janus kinase subtype that is involved in signaling from GROWTH HORMONE RECEPTORS; PROLACTIN RECEPTORS; and a variety of CYTOKINE RECEPTORS such as ERYTHROPOIETIN RECEPTORS and INTERLEUKIN RECEPTORS. Dysregulation of Janus kinase 2 due to GENETIC TRANSLOCATIONS have been associated with a variety of MYELOPROLIFERATIVE DISORDERS.Paraneoplastic Syndromes: In patients with neoplastic diseases a wide variety of clinical pictures which are indirect and usually remote effects produced by tumor cell metabolites or other products.Blood Platelets: Non-nucleated disk-shaped cells formed in the megakaryocyte and found in the blood of all mammals. They are mainly involved in blood coagulation.Splenomegaly: Enlargement of the spleen.Erythrocyte Inclusions: Pathologic inclusions occurring in erythrocytes.Tuberculoma: A tumor-like mass resulting from the enlargement of a tuberculous lesion.Selenium Radioisotopes: Unstable isotopes of selenium that decay or disintegrate emitting radiation. Se atoms with atomic weights 70-73, 75, 79, 81, and 83-85 are radioactive selenium isotopes.Thrombocytopenia: A subnormal level of BLOOD PLATELETS.Splenectomy: Surgical procedure involving either partial or entire removal of the spleen.Plateletpheresis: The preparation of platelet concentrates with the return of red cells and platelet-poor plasma to the donor.Eye Manifestations: Ocular disorders attendant upon non-ocular disease or injury.Pyoderma: Any purulent skin disease (Dorland, 27th ed).