Platelets are small cell fragments without nuclei, but with a variety of organelles. Platelets are involved in primary hemostasis by adhering to damaged blood vessels and aggregating with one another (platelet plug).
- Diameter: 2–3 µm
- Lens-shaped biconvex discoid
- Normal count: 150,000–450,000 platelets/µL
- Lifespan: up to 10 days
- Outer membrane: Receptors facilitate platelet aggregation and adhesion to endothelial surfaces.
- Open canalicular system: tunnels from membranes to platelet interior:
- For entry of exterior elements
- For granule release
- For storage of glycoproteins
- In the cytoplasm:
- α granules (most abundant) contain:
- Proteins for primary hemostasis (e.g., integrin)
- Proteins such as von Willebrand Factor (vWF), fibrinogen, factor V, and factor XI (needed for secondary hemostasis)
- May have antiangiogenic-protein populations
- May have proangiogenic-protein populations (e.g., vascular endothelial growth factor (VEGF))
- δ granules contain:
- Mediators of vascular tone: serotonin, calcium
- ADP and ATP
- Lysosomes with hydrolytic enzymes:
- Digest cytosolic contents
- Participate in fibrinolysis and destruction of the extracellular matrix
- α granules (most abundant) contain:
- The cytoskeleton (including actin, spectrin, tubulin, and filamin) is involved in:
- Changing shape
- Motility towards the site of injury
- Granule release
- Contains: the Golgi complex, elements of the rough endoplasmic reticulum (RER), mitochondria, and glycogen granules
- Hematopoiesis location:
- 1st–2nd month in utero: mesoderm of the yolk sac
- 2nd month: liver and spleen
- 5th month: bone marrow, which becomes the predominant source of blood cells
- As with other blood cells, thrombopoiesis starts with multipotent hematopoietic stem cells (HSCs).
- HSCs → multipotent progenitor (MPP) cells → common myeloid progenitor (CMP) or colony-forming unit–granulocyte, erythrocyte, monocyte, megakaryocyte (CFU-GEMM) → fragmentation of megakaryocyte (platelets)
Stages of platelet development
Development takes 1 week on average:
- Basophilic cytoplasm with a large, ovoid-, or kidney-shaped nucleus
- May have several nucleoli
- Giant cells (up to 150 μm in diameter)
- Less basophilic
- Long, branching cytoplasmic projections/extensions (proplatelets)
- Results from megakaryocyte-cytoplasm fragmentation
- Fragmented-cell remains become apoptotic and removed by macrophages.
|Stem cell factor (SCF)||Stimulates all hematopoietic progenitor cells||Bone-marrow stromal cells|
|Granulocyte-macrophage colony-stimulating factor (GM-CSF)||Stimulates myeloid progenitor cells||Endothelial cells, T cells|
|Thrombopoietin (TPO)||Stimulates thrombopoiesis||Kidney, liver|
|Interleukin-3 (IL-3)||Mitogen for all granulocyte and megakaryocyte-erythrocyte progenitor cells||T helper cells|
- Thrombocytopenia: The patient presents with circulating platelets in the bloodstream below normal levels (< 150,000 platelets/µL).
- Von Willebrand disease (vWD): The most frequently inherited bleeding disorder among humans. Von Willebrand factor (vWF) allows platelet aggregation and contributes to the formation of fibrin clots. Different types of vWD (types 1, 2, and 3) are based on qualitative or quantitative defects in vWF. The condition may also be acquired (e.g., lupus and myeloproliferative disorders).
- Essential thrombocythemia (ET): a myeloproliferative neoplasm characterized by the clonal thrombocytosis linked to somatic mutations involving JAK2, CALR, and MPL oncogene. The presentation can be complicated by thrombohemorrhagic events, and progression to myelofibrosis and acute myeloid leukemia. The diagnosis is by laboratory finding of thrombocytosis, bone-marrow biopsy, and genetic studies. Treatment aims to reduce platelet counts with cytoreductive agents (hydroxyurea) and decrease thrombosis with aspirin and systemic anticoagulation.
- Italiano Jr., J.E., Whiteheart, S.W., Bray, P.F., Li, Z., Coller, B.S., Smyth, S.S. (2021). Platelet morphology, biochemistry, and function. Kaushansky K., Prchal J.T., Burns L.J., Lichtman M.A., Levi M., Linch D.C. (Eds.), Williams Hematology, 10e. McGraw-Hill. https://accessmedicine.mhmedical.com/content.aspx?bookid=2962§ionid=252535907
- Kaushansky K. (2021). Megakaryopoiesis and thrombopoiesis. Kaushansky K., Prchal J.T., Burns L.J., Lichtman M.A., Levi M., Linch, D.C. (Eds.), Williams Hematology, 10e. McGraw-Hill. https://accessmedicine.mhmedical.com/content.aspx?bookid=2962§ionid=252535799
- Mescher, A.L. (Ed.), (2021). Hemopoiesis. Junqueira’s Basic Histology Text and Atlas, 16e. McGraw-Hill. https://accessmedicine.mhmedical.com/content.aspx?bookid=3047§ionid=255121548
- Shin, E.K., Park, H., Noh, J.Y., Lim, K.M., Chung, J.H. (2017). Platelet Shape Changes and Cytoskeleton Dynamics as Novel Therapeutic Targets for Anti-Thrombotic Drugs. Biomolecules & Therapeutics, 25(3), 223–230. https://doi.org/10.4062/biomolther.2016.138
- Twomey, L., Wallace, R., Cummins, P., Degryse, B., Sheridan, S., Harrison, M., Moyna, N., Meade-Murphy, G., Navasiolava, N., Custaud, M., Murphy, R. (2018). Platelets: From Formation to Function, Homeostasis-An Integrated Vision, Fernanda Lasakosvitsch and Sergio Dos Anjos Garnes, IntechOpen. https://www.intechopen.com/books/homeostasis-an-integrated-vision/platelets-from-formation-to-function