megakaryocytum

Megakaryocytum is not a widely used term in modern hematology. When encountered, it may refer to megakaryocytes collectively, to the process of megakaryocytopoiesis, or, less commonly, to megakaryocytosis. In practice, more precise terms are megakaryocytes, megakaryocytopoiesis, or thrombopoiesis.

Megakaryocytes are large, polyploid cells of the bone marrow derived from the myeloid lineage of hematopoietic stem cells. They reside in the marrow and extend cytoplasmic processes called proplatelets into sinusoidal vessels, from which platelets are released into the circulation. The production of platelets from megakaryocytes is a key aspect of thrombopoiesis.

Megakaryocytopoiesis involves endomitosis, a process in which DNA replicates without cell division to yield polyploid cells. Development progresses from megakaryoblasts to promegakaryocytes and then to mature megakaryocytes. Thrombopoietin (TPO) is the principal regulator, signaling through the c-Mpl receptor; additional cytokines such as IL-3, IL-6, and transcription factors including GATA-1 and NF-E2 contribute to maturation. Platelet generation results from fragmentation of the megakaryocyte cytoplasm into platelets.

Clinically, increased numbers of megakaryocytes can be described as megakaryocytosis and may occur in reactive states or in myeloproliferative neoplasms such as essential thrombocythemia. Abnormal megakaryopoiesis can contribute to either thrombocytosis or thrombocytopenia, and assessment often involves bone marrow examination and platelet counts. Therapeutic considerations frequently target thrombopoietin signaling or the underlying condition driving altered megakaryocyte production.

See also: thrombopoiesis, megakaryocyte, thrombopoietin, hematopoiesis.