PLATELET

• HEMOSTASIS


  1. Series of complex processes by which the body stops bleeding and maintains blood in its fluid state within the blood vessel compartment

  2. Achieves balance/equilibrium through the interaction of different components of blood essential for hemostasis

  3. Platelets are considered the most significant for hemostasis as they contain substances essential for maintaining it
• Morphologically, forming units all look the same
• TOPIC OUTLINE

  1. Review of Hematopoiesis

  2. Megakaryopoiesis - General and Specific Features, Maturation Series

  3. Platelets - Structural components, Functions
• MEGAKARYOPOIESIS

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

  2. Production of megakaryocytes which give rise to platelets

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

  4. Other stimulating factors include Colony stimulating factors granulocyte, erythrocyte, monocyte, megakaryocyte (CSF-GEMM) and CSF-Meg

  5. In megakaryopoiesis, as the cell matures, the cell size increases
• Megakaryocytes are just cytoplasmic fragments of megakaryocytes
• REVIEW OF HEMATOPOIESIS

  1. Hematopoiesis is the controlled and regulated process of producing blood cells

  2. Starts from an 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 like erythropoietin (EPO)
• 2nd SEMESTER

  • MEGAKARYOPOIESIS AND PLATELETS
• Production of Megakaryocyte

  Production of platelets by megakaryocytes
• Megakaryocyte


  Capable of cell division or mitosis
• If the CFU matures, it will become a megakaryocyte
• LD-CFU-Meg

  • No longer undergoes mitosis, but the cell will continuously mature
• CFU-Meg


  • Can undergo mitosis
• In their mitosis, you will start from a mother cell that will give rise to two identical daughter cells
• Mature megakaryocytes are the largest cells in the bone marrow and need more nucleus for higher chances of platelets to come out
• The DMS is the individual network system of the platelet, forming platelets. All stages have DMS
• Maturation time from the earliest recognizable stage up until reaching the megakaryocyte phase takes around 5-7 days
• Terminal differentiation phase

  You can differentiate cell until the end stage which will further release the platelets
• Megakaryoblast (MK I)


  • Least differentiated megakaryocyte precursor, Presence of plasma membrane blebs, 15-20 um in diameter, Granules are absent, Round or oval kidney-shaped at times, Chromatin pattern is fine
• BLEBS or TAGS disappear as cells mature/expand
• Endomitosis


  Unique process in megakaryopoiesis where there is DNA replication and maturation of cytoplasm, but no cell division
• BFU-Meg


  • Can undergo mitosis
• Once EPO stimulated the progenitors, you will have BFU and CFU. Both of them are capable of undergoing cell division through mitosis
• Megakaryocyte

  Megakaryocyte
• Terminal differentiation phase

  Maturation of every stage to differentiate one cell to another
• BFU-Meg and CFU-Meg undergo mitosis to generate multiple progenitors, ensuring a continuous supply for megakaryocyte production
• After LD-CFU-Meg, you can now enter the recognizable stage Megakaryoblast (MK I)
• At least 4 nucleus are needed for the platelet to come out
• One cell with one nucleus will produce one cell with more nucleus
• Characteristics of Megakaryopoiesis

  • Cell size increases, N:C ratio decreases, Number of nucleus increases
• There is a presence of DMS (demarcating membrane system) in the earliest recognizable stage
• Every 100013M cell undergoes endomitosis, leading to the formation of megakaryocyte blebs or tags
• Without DMS within the cytoplasm of the cell, it will not form platelets
• Once platelets are out, they punch through endothelial cells to enter the blood circulation
• All stages have DMS
• 1% of megakaryocytes could be present in the lungs
• Features of the Three Terminal Megakaryocyte Differentiation Stages are provided in Table 1
• Every megakaryocyte can release 2000-4000 platelets
• CD34 is a stem cell and common myeloid progenitor marker
• CD34 is a marker for myeloid progenitor cells, disappearing in the terminal differentiation phase
• Protrusion on cell


  Outward bulging or protrusion on a cell