Platelets (Thrombocytes) – Definition, Structure, Function

Platelets (Thrombocytes) Definition

Platelets or thrombocytes are anucleated cells formed from megakaryocytic cells in the bone marrow that are involved in the development of non-hemostatic immunological functions.

• Platelets, also known as thrombocytes, are small, colorless cell fragments that circulate in the blood and play an important role in hemostasis, the process by which bleeding is stopped.
• Platelets are produced in the bone marrow from precursor cells called megakaryocytes, and are essential for the formation of blood clots that prevent excessive bleeding after injury.
• When a blood vessel is damaged, platelets adhere to the site of injury and form a plug, which is then reinforced by a network of fibrin strands.
• In addition to their role in hemostasis, platelets are also involved in inflammation, wound healing, and the immune response.
• Abnormalities in platelet function or count can lead to bleeding disorders or thrombotic disorders, which increase the risk of blood clots.
• Platelet transfusions can be used to treat bleeding disorders, while medications that inhibit platelet aggregation can be used to prevent blood clots.
• There are three types of granules in platelets: alpha granules, dense granules, and lysosomes. Granule secretion produces several proteolytic enzymes, coagulation factors, growth factors, cytokines, and a variety of cytokines. Platelets include a number of platelet agonist receptors.
• Platelets are approximately 20% smaller in diameter than red blood cells. Normal platelet counts range from 150,000 to 350,000 per microliter of blood, however because platelets are so minute, they represent a negligible portion of blood volume. Platelets’ primary job is to prevent bleeding.

Characteristics of Platelets (Thrombocytes)

Platelets, also known as thrombocytes, are small, disc-shaped, non-nucleated cell fragments that circulate in the blood. Here are some of their key characteristics:

1. Size: Platelets are much smaller than other blood cells, with an average diameter of 2-3 micrometers.
2. Shape: Platelets are disc-shaped, with a flattened appearance. However, their shape can change rapidly in response to external stimuli, allowing them to form plugs and clots.
3. Origin: Platelets are produced in the bone marrow from precursor cells called megakaryocytes.
4. Lifespan: Platelets have a relatively short lifespan, typically surviving in the circulation for around 7-10 days.
5. Role in hemostasis: Platelets are essential for the process of hemostasis, which is the way that the body stops bleeding after injury. They adhere to damaged blood vessel walls and aggregate to form a plug that helps to stop bleeding.
6. Role in inflammation: Platelets are also involved in the inflammatory response, releasing cytokines and other molecules that help to attract immune cells to the site of injury.
7. Role in wound healing: Platelets release growth factors that promote the growth of new blood vessels and help to repair damaged tissue.
8. Response to stimuli: Platelets can change their shape and behavior in response to a range of external stimuli, including changes in blood flow, exposure to certain drugs, and the presence of microorganisms.
9. Abnormalities: Abnormalities in platelet function or count can lead to bleeding disorders or thrombotic disorders, which increase the risk of blood clots.
10. Medical applications: Platelet transfusions can be used to treat bleeding disorders, while medications that inhibit platelet aggregation can be used to prevent blood clots.

Structure of Platelets

Platelet structure can be split into four zones, from periphery to centre:

• Peripheral zone – Rich in glycoproteins necessary for adhesion, activation, and aggregation of platelets. For example, GPIb/IX/V; GPVI; GPIIb/IIIa.
• Sol-gel zone – is abundant in microtubules and microfilaments, which enables the platelets to keep their discoid shape.
• Organelle zone – is abundant in platelet granules. Alpha granules are composed of coagulation mediators including factor V, factor VIII, fibrinogen, fibronectin, platelet-derived growth factor, and chemotactic molecules. Delta granules or dense bodies contain platelet-activating mediators ADP, calcium, and serotonin.
• Membranous zone – includes membranes generated from the smooth endoplasmic reticulum of megakaryocytes arranged into a dense tubular structure responsible for thromboxane A2 production. This thick tubular structure is linked to the surface platelet membrane to facilitate the release of thromboxane A2.

Shape of Platelets

• Inactive platelets in circulation are biconvex discoid (lens-shaped) structures, 117–18 2–3 µm in diameter. On the surface of activated platelets are cell membrane projections.
• In a first approximation, the shape of platelets resembles oblate spheroids with a semiaxis ratio between 2 and 8.
• This approximation is frequently employed to describe the hydrodynamic and optical features of a platelet population and to recover the geometric parameters of individual platelets obtained by flow cytometry.
• Increasingly precise biophysical models of the platelet surface morphology, which represent its form from basic principles, make it possible to generate a more realistic platelet geometry in both a relaxed and activated state.

Production of Platelets

• The development of megakaryocytes and platelets is controlled by thrombopoietin, a hormone produced by the kidneys and liver.
• Over its lifetime, each megakaryocyte produces between 1,000 and 3,000 platelets.
• Healthy adults create an average of 10¹¹ platelets every day.
• Reserve platelets are kept in the spleen and are released when necessary via sympathetically induced splenic contraction.
• The average platelet lifespan is between 8 and 9 days.
• Each platelet lifespan is governed by the internal apoptotic regulatory system, which contains a Bcl-xL timer.
• In the spleen and liver, old platelets are eliminated through phagocytosis.

How do Platelets work against pathogens? (Immunity)

• Every platelets must undergo a sequence of transformations in response to various stimuli. Platelet activation induces distinct morphological alterations in the cell.
• The cell loses its round, spherical appearance and takes on an irregular form with pseudopodia.
• The cytoplasmic storage granules fuse with the cell membrane, which results in the release of cytokines and the expression of surface molecules.
• In addition, activation increases the affinity of adhesion molecules such as GPIb-V-X complexes, resulting in increased platelet adherence and aggregation.
• Platelet surface receptors include TLR receptors, which are critical for the activation of various immune cells.
• It has also been demonstrated that the activation of platelets by TLR4 is due to an increase in PMN cell adhesion. This increases neutrophil activity and the development of neutrophil extracellular traps, hence preventing the spread of germs.
• Platelets also express CD154 receptors, which directly activate endothelial cells to produce inflammation in blood vessel wall.
• Platelets also contain FcγRIIA, glycoprotein VI, and C-type lectin receptor; however, the presence of these receptors relies on the unique characteristics of each cell.
• The activation of platelets and the release of cytoplasmic granules are brought about by the binding of these receptors to fibrinogen.
• Platelet activation results in the release of several chemicals into the circulation. These chemicals can provoke inflammation or other immunological reactions in the body.

Disorders Related to High and Low Number of Platelets

Abnormalities in platelet count or function can lead to bleeding disorders or thrombotic disorders, which increase the risk of blood clots. Here are some examples of disorders related to high and low numbers of platelets:

1. Thrombocytopenia: This is a condition in which the number of platelets in the blood is lower than normal. It can be caused by a range of factors, including bone marrow disorders, autoimmune diseases, infections, and certain medications. Thrombocytopenia can lead to excessive bleeding or bruising, and in severe cases, can be life-threatening.
2. Idiopathic thrombocytopenic purpura (ITP): This is a type of autoimmune disorder in which the body produces antibodies that attack and destroy platelets. It can result in low platelet counts and can cause excessive bruising and bleeding.
3. Thrombocytosis: This is a condition in which the number of platelets in the blood is higher than normal. It can be caused by a range of factors, including cancer, infections, and certain medications. Thrombocytosis can increase the risk of blood clots, which can lead to serious health problems such as stroke or heart attack.
4. Hemophilia: This is a genetic disorder in which the blood does not clot properly, leading to excessive bleeding. It is caused by a deficiency of certain clotting factors, which can include platelets.
5. Von Willebrand disease: This is a genetic disorder in which the body does not produce enough of a protein called von Willebrand factor, which is needed for proper blood clotting. It can lead to excessive bleeding or bruising.
6. Disseminated intravascular coagulation (DIC): This is a serious medical condition in which the body’s clotting system becomes overactive, leading to the formation of small blood clots throughout the body. This can result in a depletion of platelets, leading to bleeding disorders.

Treatment for platelet disorders may involve medications to stimulate or suppress platelet production, blood transfusions, or other medical interventions to manage the underlying cause of the disorder.

Platelet receptors

Platelets, also known as thrombocytes, have several different types of receptors on their surface that play a critical role in the process of hemostasis. These receptors allow platelets to sense changes in the local environment and to respond appropriately to injuries or other stimuli. Here are some of the main types of platelet receptors:

1. Adhesion receptors: Platelets use adhesion receptors, such as integrins and glycoproteins, to adhere to the damaged blood vessel wall and to other platelets, forming a platelet plug.
2. Activation receptors: Platelets also have activation receptors, such as P2Y12 and thromboxane A2 receptor, that respond to signals from other cells or from the damaged tissue, triggering a series of intracellular signaling pathways that lead to platelet activation and aggregation.
3. Fc receptors: Platelets have Fc receptors, which allow them to bind to and phagocytose antibody-coated cells and other particles, removing them from the circulation.
4. Toll-like receptors (TLRs): Platelets also have Toll-like receptors, which recognize and respond to pathogen-associated molecular patterns (PAMPs) and danger-associated molecular patterns (DAMPs), activating the platelets and contributing to the immune response.
5. Clearance receptors: Platelets have clearance receptors, such as the thrombopoietin receptor, that regulate platelet production and clearance from the circulation.
6. Protease-activated receptors (PARs): Platelets also have protease-activated receptors, which are activated by thrombin and other proteases that are generated during the coagulation cascade. PARs play a key role in platelet activation and aggregation.
7. C-type lectin receptors: Platelets have C-type lectin receptors, which recognize and bind to carbohydrates on the surface of pathogens, contributing to the immune response.
8. CD40 ligand (CD40L) receptors: Platelets express CD40L, which binds to CD40 on the surface of immune cells, activating them and contributing to the immune response.

The different types of platelet receptors allow platelets to respond to a wide range of signals and to coordinate their responses with other cells in the body. Dysregulation of platelet receptors can lead to bleeding disorders or thrombotic disorders, increasing the risk of bleeding or blood clots. Understanding the role of platelet receptors is important for developing new therapies for platelet-related disorders.

Functions of Platelets

Platelets, also known as thrombocytes, play a crucial role in the process of hemostasis, which is the body’s way of stopping bleeding after an injury. Here are some of the key functions of platelets:

1. Formation of platelet plug: Platelets are the first cells to arrive at the site of a blood vessel injury, where they adhere to the damaged blood vessel wall and aggregate to form a platelet plug. The platelet plug helps to stop bleeding by sealing the damaged blood vessel.
2. Activation of coagulation: Platelets activate the coagulation cascade, which involves a series of chemical reactions that result in the formation of fibrin, a protein that reinforces the platelet plug and stabilizes the blood clot.
3. Secretion of growth factors: Platelets contain a range of growth factors, such as platelet-derived growth factor (PDGF) and vascular endothelial growth factor (VEGF), which promote the growth of new blood vessels and help to repair damaged tissue.
4. Regulation of inflammation: Platelets also play a role in the immune response by releasing cytokines and other molecules that attract immune cells to the site of injury and regulate the inflammatory response.
5. Removal of damaged cells: Platelets help to remove damaged cells from the circulation by phagocytosing them and delivering them to the liver and spleen for breakdown.
6. Prevention of infection: Platelets contain antimicrobial peptides that help to prevent the growth of bacteria and other microorganisms in the blood.
7. Maintenance of blood vessel integrity: Platelets secrete molecules that help to maintain the integrity of the blood vessel wall, preventing the leakage of fluid into surrounding tissues.
8. Formation of thrombi: In certain medical conditions, such as deep vein thrombosis, platelets can contribute to the formation of blood clots, which can lead to serious complications such as pulmonary embolism.

Platelets play a critical role in maintaining the integrity of the circulatory system and preventing excessive bleeding after injury. Abnormalities in platelet function or count can lead to bleeding disorders or thrombotic disorders, which increase the risk of blood clots.

FAQ

References

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