Platelets

Abstract

Platelets are small anucleate blood cells that are produced by megakaryocytes in the bone marrow, and also likely in the lungs. Due to their small size relative to other circulating blood cells, platelets are displaced from the center of vessels and circulate close to the vessel wall. This position near the vessel wall allows platelets to rapidly respond to vascular injuries and contribute to hemostasis, the physiological process that stops bleeding. Following vascular damage, exposure of subendothelial matrix proteins to the blood initiates the first wave of hemostasis: platelet adhesion, activation, and aggregation. Activated platelets also play important roles in secondary hemostasis by binding coagulation factors and enhancing cell‐based thrombin generation at sites of vessel injury. Beyond hemostasis, platelets have been demonstrated by recent studies to play critical roles in a range of biological processes including inflammation, immune regulation, atherosclerosis, lymphatic vessel development, angiogenesis, liver regeneration, and tumor metastasis.

Key Concepts

  • Platelets are small, versatile blood cells that are produced by megakaryocytes in the bone marrow.
  • Platelets circulate in the blood for 7–10 days before being cleared.
  • The most well‐appreciated role of platelets is their role in haemostasis.
  • Abnormalities in platelet number and function can cause either bleeding or thrombotic disorders.
  • Platelets have been implicated in a variety of physiological and pathological processes including inflammation, immune regulation, atherosclerosis, angiogenesis and lymphatic vessel development, liver regeneration and tumour metastasis.

Keywords: platelets; receptors; haemostasis; thrombocytopenia; bleeding; thrombosis

Figure 1. (a) Electron micrograph of a quiescent platelet (magnification × 30 000) showing platelet constituents. DB, dense body; G, glycogen; MT, mitochondria; CM, cell membrane; OCS, open canalicular system; DTS, dense tubular system. (b) An activated platelet demonstrating the collagen‐induced shape changes (magnification × 30 000). (c) Collagen‐induced platelet aggregation (magnification × 30 000).
Figure 2. Platelets play important roles in haemostasis and thrombosis. Following vessel injury, plasma and possibly platelet proteins are deposited onto the exposed extracellular matrix. Platelet adhesion and aggregation on the damaged vessel wall is considered the ‘first wave of haemostasis’. The GPIb‐IX‐V complex and its ligand adherent VWF mediate platelet rolling along the vessel wall. Following GPVI‐collagen stimulation, stable adhesion requires several integrin–ligand interactions. During platelet adhesion and subsequent activation, the αIIbβ3 integrin changes conformation from a low‐ to high‐affinity ligand‐binding state for fibrinogen, VWF and unidentified ‘X‐ligand(s)’. This leads to platelet aggregation and thrombus formation. Platelet activation and phosphatidylserine exposure contribute to cell‐based thrombin generation, enhancing platelet aggregation and blood coagulation.
close

Further Reading

Bussel JB and Cines DB (1995) Immune thrombocytopenic purpura, neonatal autoimmune thrombocytopenia, and post‐transfusion purpura. In: Hoffman R , Benz EJ , Shattil SJ , et al. (eds) Hematology: Basic Principles and Practice, 2nd edn, pp. 1849–1869. New York: Churchill Livingstone.

Cines DB , Bussel JB , McMillan RB and Zehnder JL (2004) Congenital and acquired thrombocytopedia. Hematology/the Education Program of the American Society of Hematology. American Society of Hematology. Education Program 1: 390–406.

Diz‐Kucukkaya R , Gushiken FC and Lopez JA (2007) Thrombocytopenia. In: Lichtman MA , Beutler E , Kipps TJ , et al. (eds) Williams' Hematology, 7th edn, pp. 1749–1783. New York: McGraw‐Hill Medical Publishing Division.

Gewirtz AM and Schick B (1994) Megakaryocytopoiesis. In: Colman RW , Hirsh J , Marder VJ and Salzman EW (eds) Hemostasis and Thrombosis: Basic Principles of Clinical Practice, 3rd edn, pp. 353–396. Philadelphia, PA: Lippincott.

Kaplan C , Ni H and Freedman J (2013) Alloimmune thrombocytopenia. In: Michelson AD (ed) Platelets, 3rd edn, pp. 953–970. London: Elsevier.

Parise LV , Smyth SS , Shet AS and Coller BS (2007) Platelet morphology, biochemistry, and function. In: Lichtman MA , Beutler E , Kipps TJ , et al. (eds) Williams' Hematology, 7th edn, pp. 1587–1663. New York: McGraw‐Hill Medical Publishing Division.

Plow EF and Ginsberg MH (1995) Molecular basis of platelet function. In: Hoffman R , Benz EJ , Shatill SJ , et al. (eds) Hematology: Basic Principles and Practice, 2nd edn, pp. 1524–1535. New York: Churchill Livingstone.

Wang Y , Gallant RC and Ni H (2016) Extracellular matrix proteins in the regulation of thrombus formation. Current Opinion in Hematology 23 (3): 280–287. Philadelphia, PA: Wolters Kluwer Health.

Sullivan KA and Kipps TJ (2007) Human leukocyte and platelet antigens. In: Lichtman MA , Beutler E , Kipps TJ , et al. (eds) Williams' Hematology, 7th edn, pp. 2137–2150. New York: McGraw‐Hill Medical Publishing Division.

Xu XR , Zhang D , Oswald BE , et al. (2016) Platelets are versatile cells: new discoveries in hemostasis, thrombosis, immune responses, tumour metastasis and beyond. Critical Reviews in Clinical Laboratory Sciences 53 (6): 409–430. New York: Taylor and Francis Group.

Contact Editor close
Submit a note to the editor about this article by filling in the form below.

* Required Field

How to Cite close
Gallant, Reid C, Wang, Yiming, and Ni, Heyu(Apr 2018) Platelets. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0001221.pub3]