Isolated thrombocytosis and bone marrow fibrosis in the absence of clinical features to support a diagnosis of primary myelofibrosis. (A) Bone marrow samples obtained at diagnosis from a 54-year-old female patient with thrombocytosis in the absence of additional features of PMF: left, bone marrow aspirate stained with Wright-Giemsa showing a large megakryocyte with a hyperlobated nucleus; middle, bone marrow trephine biopsy stained with hematoxylin and eosin showing loose clusters of megakaryocytes containing both large megakaryoctes with hyperlobated nuclei and smaller forms with cloud-like or hyperchromatic nuclei; right, silver-stained bone marrow section showing grade 4 reticulin fibrosis (on 0-4 scale). (B) Blood counts during the course of 54 months from diagnosis showing stabilization of the platelet count and no change in hemoglobin level. Red and blue shaded areas represent the normal range for hemoglobin and platelet count, respectively.