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Bone marrow nonneoplastic

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Megakaryocytes

Author: Dragos C. Luca, M.D.

Last author update: 1 August 2012

Last staff update: 18 April 2024 (update in progress)

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PubMed Search: Bone marrow megakaryocytes [title]

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Table of Contents

Cite this page: Luca DC. Megakaryocytes. PathologyOutlines.com website. https://www.pathologyoutlines.com/topic/bonemarrowmegakaryocytematuration.html. Accessed April 20th, 2024.

Definition / general

Progresses from myeloid stem cell (pluripotential hematopoietic stem cell) to megakaryoblast to promegakaryocyte to megakaryocyte to proplatelets (released into circulation) to platelets (J Thromb Haemost 2003;1:1580, Front Biosci 2007;12:2050)
Two groups of factors influencing megakaryopoiesis are early acting multilineage cytokines (stem cell factor, GM-CSF, IL3, IL6) and cytokines that are more megakaryocyte specific (thrombopoietin)
Thrombopoietin: produced by bone marrow stromal cells, hepatocytes and proximal renal tubular epithelial cells; poorly understood regulation
Maturation is characterized by an increase in size and lobulation of nuclei and is controlled by thrombopoietin (J Clin Invest 2005;115:3339)
Endomitosis: unique property of megakaryocytes characterized by ongoing DNA synthesis, without mitosis resulting in progressive nuclear lobulation with increasing DNA content (nuclear lobes remain connected)
Megakaryocytes form demarcation membrane within cytosol, which leads to production of platelets
Maturation is associated with acquisition of surface GP Ib, GP IIb and von Willebrand factor
Four types of cytoplasmic granules are alpha, delta, gamma and peroxisomes
Megakaryocytes reside next to the bone marrow sinuses and project pseudopodia through the sinus walls, releasing platelets directly into the circulation

Diagrams / tables

Images hosted on other servers:

Megakaryopoiesis pathways

Proplatelet formation

Clinical features

Megakaryocytes are smaller in fetuses and infants and show less nuclear segmentation when compared to older children
Platelet production appears to be dependent on megakaryocyte ploidy
Platelet production rate under homeostatic conditions: 2 - 4 x 10⁹/kg/day; may increase 8 - 10x if needed; 10 day survival time in peripheral blood
Emperipolesis: other cells, especially neutrophils, pass through megakaryoctye tubular system (due to its location) to enter the circulation
Thrombocytopenia with normal / increased megakaryocytes: congenital (Bernard-Soulier, May-Hegglin, gray platelet syndrome) or acquired (ITP, DIC, TTP, HUS, hypersplenism, megaloblastic anemia)
Thrombocytopenia with decreased megakaryocytes: congenital (Fanconi, thrombocytopenia with absent radii, X linked amegakaryocytic thrombocytopenia, dyskeratosis congenita, Shwachman-Diamond) or acquired (aplastic anemia, bone marrow infiltration, infections, toxins, immune disorders)
Thrombocytosis: congenital (familial essential thrombocythemia, other familial myeloproliferative disorders) or acquired (reactive or neoplastic)

Microscopic (histologic) description

Megakaryoblast: variable size (7 - 35 microns); may be designated micromegakaryoblast if < 15 microns; round / ovoid cells with scanty blue agranular cytoplasm that often forms a rim around nucleus and may have a few small budding protrusions at periphery; nuclei are round / oval with coarse granular chromatin, one or more nucleoli
Megakaryocyte: randomly disbursed throughout bone marrow; 50 - 150 microns (largest normal nucleated cell in marrow); micromegakaryocytes measure 15 - 30 microns; abundant light blue to pink cytoplasm with numerous purple red or pink granules; nucleus has 8, 16 or 32 overlapping lobes; no nucleolus; megakaryocytes producing platelets may have demarcated granular clumps of platelets streaming from the margins

Microscopic (histologic) images

Contributed by Dragos C. Luca, M.D. and AFIP images