Color Atlas of Hematology - Practical Microscopic and Clinical ...

50Normal Cells of the Blood and Hematopoietic OrgansMegakaryocytes and ThrombocytesMegakaryocytes can enter the bloodstream only in highly pathologicalmyeloproliferative disease or acute leukemia. They are shown here inorder to demonstrate thrombocyte differentiation. Megakaryocytes residein the bone marrow and have giant, extremely hyperploid nuclei (16times the normal number of chromosome sets on average), which build upby endomitosis. Humoral factors regulate the increase of megakaryocytesand the release of thrombocytes when more are needed (e.g., bleeding orincreased thrombocyte degradation). Cytoplasm with granules is pinchedoff from megakaryocytes to form thrombocytes. The residual naked megakaryocytenuclei are phagocytosed.Only mature thrombocytes occur in blood. About 1–4 µm in size andanuclear, their light blue stained cytoplasm and its processes give them astar-like appearance, with fine reddish blue granules near the center.Young thrombocytes are larger and more “spread out;” older ones looklike pyknotic dots.Diagnostic Implications. In a blood smear, there are normally 8–15 thrombocytesper view field using a 100 objective; they may appear dispersedor in groups. To someone quickly screening a smear, they will give a goodindication of any increase or strong decrease in the count, which can beuseful for early diagnosis of acute thrombocytopenias (p. 164 f.).Small megakaryocyte nuclei are found in the bloodstream only insevere myeloproliferative disorders (p. 171).