HEMA PRELIMS

• In megakaryopoiesis, as the cell matures, the cell size increases
• Hematopoiesis

  1. Controlled and regulated process of producing blood cells

  2. Starts from undifferentiated pluripotential hematopoietic stem cells giving rise to myeloid and lymphoid progenitor cells

  3. Lymphoid progenitor gives rise to B cells and T cells

  4. Myeloid stem cell gives rise to granulocytes, monocytes, red cells, and megakaryocytes

  5. Stimulation of progenitors to be committed in producing specific cells using growth factors

  6. Example: erythropoietin (EPO) stimulates myeloid progenitor to produce RBCs
• Platelets are considered the most significant for hemostasis as they contain substances essential for maintaining hemostasis
• Hemostasis is a series of complex processes by which the body stops bleeding and maintains blood in its fluid state within the blood vessel compartment
• Megakaryocyte Lineage Progenitors

  • From PHSC to myeloid stem cell, forming units (BFU or CFU) are formed
  • Morphologically, forming units look the same
  • Proliferative phase increases in number through cell division or mitosis
  • Terminal differentiation phase matures every stage to differentiate one cell to another
• Megakaryopoiesis

  1. Maturation series of a hematological cell committed to platelet production

  2. Production of megakaryocytes which shed out platelets

  3. Responds to growth factor Thrombopoietin to stimulate stem cells to produce megakaryocytes

  4. Other stimulating factors include Colony Stimulating Factors (CSF-GEMM) and CSF-Meg
• CFU-Meg


  Colony Forming Unit Megakaryocyte
• Cells with a large nucleus due to endomitosis will not form platelets
• DMS (Demarcating Membrane System) is present in all stages and is crucial for platelet formation
• Maturation time from the earliest recognizable stage to megakaryocyte phase takes around 5-7 days
• Terminal differentiation phase

  Cells differentiate until the end stage for platelet release
• LD-CFU-Meg

  Light Density Colony Forming Unit Megakaryocyte
• Mature megakaryocytes are the largest cells in the bone marrow, needing more nuclei for higher platelet production
• Cell division process

  Mother cell gives rise to two identical daughter cells
• BFU-Meg

  Burst Forming Unit Megakaryocyte
• Endomitosis is unique in megakaryopoiesis where DNA replication and maturation of cytoplasm occur without cell division, resulting in multiple nuclei
• BFU-Meg and CFU-Meg undergo mitosis to generate multiple progenitors, ensuring a continuous supply for megakaryocyte production
• At least 4 nuclei are needed for platelets to be released from the cytoplasm
• Megakaryoblast (MK I) is the least differentiated megakaryocyte precursor with specific characteristics
• Characteristics of Megakaryopoiesis: Cell size increases, N:C ratio decreases, Number of nuclei increases
• BLEBS or TAGS disappear as cells mature and expand
• Identification of cell stage is important for understanding the platelet production process
• Features of the Three Terminal Megakaryocyte Differentiation Stages

  • MK-I
  • MK-II
  • MK-III
• Thrombopoietin circulates as a hormone in plasma. Plasma concentration is inversely proportional to platelet and megakaryocyte mass
• Promegakaryocyte (MK II)

  1. Nuclear lobularity first becomes apparent

  2. DMS becomes clearly visible

  3. 20-80 um in diameter

  4. The cytoplasm has visible bluish-stained granules

  5. There is still an ongoing endomitosis
• 1% of megakaryocytes could be present in the lungs
• IL-6 and IL-11 enhance endomitosis and maturation
• Megakaryocyte (MK III)


  1. Easily recognized at 10X magnification

  2. Mature/Immature

  3. At full maturation, platelet shedding proceeds (it is called thrombocytopoiesis)

  4. Shedding - coming out of the platelets from the cytoplasm

  5. 30-100 um in diameter

  6. Once platelets are out, they punch through the endothelial cells so that platelets will be present in the blood

  7. 70% of platelets will be circulated and 30% will be sequestered by the spleen

  8. Every megakaryocyte can release 2000-4000 platelets

  9. Once all the platelets are released, the megakaryocytes will be removed by the bone marrow macrophages
• IL-3 acts in synergy with TPO
• MEGAKARYOCYTE MEMBRANE RECEPTORS AND MARKERS

  • MPL (Murine Myeloproliferative Leukemia) - TPO receptor site
  • CD34 - Stem cell and common myeloid progenitor marker
  • Gp IIb/IIIa - First appears on megakaryocyte progenitors and remains present throughout maturation
  • Other markers: CD36, CD42, CD61, CD62
  • Other substances: Factor VIII, VWF (Von Willebrand Factor), Fibrinogen - seen on megakaryocyte and platelets
• There should always be a check and balance mechanism in cell production
• Thrombopoietin


  1. Circulates as a hormone in plasma

  2. Plasma concentration is inversely proportional to platelet and megakaryocyte mass
• Other factors involved in platelet production

  • kit-ligand
  • mast cell growth factor
  • GM-CSF
  • G-CSF
  • acetylcholinesterase-derived megakaryocyte growth stimulating peptide
• IL-6 and IL-11


  Enhance condomitosis and maturation
• Platelets could generate their own energy unlike red cells that require sugar for them to have energy (Embden-Meyerhof pathway)
• Stress platelets appear in compensation or as a reaction to thrombocytopenia
• As we age, our platelet count will also decrease
• To further identify stress platelets, nucleic acid dyes (e.g. thiazole orange) can be used
• The peripheral zone of platelets is the outer layer known as the 'transmitter region' or stimulus area for platelets
• After releasing platelets, the megakaryocyte becomes "naked" and is removed by macrophages in the bone marrow to control platelet count