Platelet

Platelets, also called thrombocytes, are tiny parts of blood that help stop bleeding when a blood vessel gets hurt. They are not like normal cells because they do not have a nucleus. Instead, they are pieces of bigger cells called megakaryocytes that live in bone marrow or lung tissue before they enter the bloodstream. Platelets are found only in mammals.

When there is an injury, platelets rush to the spot and stick together to form a plug. This helps stop the bleeding. They do this in several steps: first, they stick to the area outside the damaged vessel lining, then they change shape and release chemicals, and finally they connect to one another to make a clot. This clot helps stop the bleeding.

Besides helping with clotting, platelets also carry special substances that can help wounds heal and tissues grow back healthier. This makes platelets very important for keeping our bodies safe and repairing damage quickly.

Term

The word thrombocyte (meaning "clot cell") started being used in the early 1900s and sometimes means the same as platelet. It is used in some scientific words related to platelets, like thrombocytopenia, which means having too few platelets.

In some cases, the word thrombus is used to mean the same as clot, no matter what the clot is made of. Sometimes, thrombus is used to talk about normal clots that help stop bleeding.

Structure

Platelets have a special structure that helps them do their job. They can be divided into four zones:

• The peripheral zone is full of proteins that help platelets stick together. Examples include GPIb/IX/V, GPVI, and GPIIb/IIIa.
• The sol-gel zone has tiny structures that help platelets keep their shape.
• The organelle zone contains small packets with important substances needed for clotting, like factor VIII and fibrinogen. There are also tiny bodies that hold mediators that activate platelets, such as ADP, calcium, and serotonin.
• The membranous zone has membranes that help make a substance called thromboxane A2, which aids in platelet activation. This zone connects to the platelet’s surface to release thromboxane A2.

When platelets are not active, they look like small, flat discs. But when they become active, they grow little projections on their surface.

Hemostasis

Main article: Hemostasis

Platelets are tiny pieces in our blood that help stop bleeding when we get a cut or injury. They clump together to form a plug, which helps prevent blood from flowing out of the damaged blood vessel. This process is called hemostasis.

First, platelets stick to the spot where the blood vessel is injured. This happens because of special proteins in the blood and on the surface of the platelets. Then, the platelets become active and change shape, which helps them stick together more tightly. Finally, the platelets link up with each other and with other proteins in the blood to form a stable clot, stopping the bleeding. Platelets are important for stopping blood loss and helping wounds heal.

Immune function

Platelets help the body fight infections. They can stick to and remove harmful germs. When someone has a serious bacterial or viral infection, they often have fewer platelets.

When there is an injury or infection, platelets move to the area and help manage the body's response. They work with other immune cells and release important signals.

Platelets also help stop infections by forming blood clots that can trap bacteria. This process is called immunothrombosis. It involves platelets working with other immune cells like neutrophils and monocytes. If this process is not managed well, it can cause problems such as unwanted blood clots. Platelets can also help start the body's immune system by working with antibodies and releasing substances that kill harmful bacteria.

Measurement and testing

Platelet concentration in the blood, called the platelet count, can be measured using tools like a hemocytometer or machines such as a Coulter counter. Common blood testing methods often include checking the platelet count, which is usually reported as PLT.

Platelet numbers can vary between people and change over time. The average range is between 250,000 and 260,000 cells per mm³. Normal levels in labs are usually between 150,000 and 400,000 cells per mm³ or 150–400 billion per liter. When looking at a stained blood smear, platelets look like dark purple spots, about one-fifth the size of red blood cells. A healthy adult usually has many more red blood cells than platelets.

Clinical significance

Platelet disorders can cause bleeding problems. If there are too few or not well-working platelets, a person might bleed more easily from cuts. They may see small purple spots on the skin, called petechiae, purpura, and ecchymoses. Bleeding can also happen in places like the gums, nose, or stomach. Having too many platelets can sometimes cause dangerous clots in the veins or arteries, known as venous thrombosis and arterial thrombosis. The effects depend on where these clots form in the body.

Disorders

Platelet disorders happen when there are too few or too many platelets, or when platelets don’t work right. When there are not enough platelets, it is called thrombocytopenia. This can happen because the body makes fewer platelets, destroys them too quickly, or they get stuck somewhere else in the body. The opposite, having too many platelets, is called thrombocytosis. This can be because of genetics, reactions to the body’s signals, or because of certain diseases.

There are also problems when platelets act incorrectly. They might stick to blood vessel walls when they shouldn’t, forming clots inside healthy vessels. This can reduce blood flow to parts of the body or even block it completely. Some conditions that affect platelet function are inherited, while others happen later in life due to diseases or medications.

Pharmacology

Some medicines that help reduce swelling and pain can also change how platelets work in the blood. Non-steroidal anti-inflammatory drugs (NSAIDs), like aspirin and ibuprofen, can stop platelets from doing their job for a little while. Aspirin lasts longer, but ibuprofen’s effect goes away faster.

Doctors sometimes use special drugs to stop platelets from clumping together or to help the body make more platelets. These include medicines like clopidogrel, ticagrelor, and desmopressin.

Therapies

Transfusion

Platelet transfusion helps people with very low platelet counts. It can prevent bleeding or help before surgeries where some bleeding might happen. It is also used when platelets are not working well, like with certain medicines. But it is not helpful for everyone and can sometimes cause problems.

Platelets for transfusion come from two main places. They can be taken from whole blood donations or collected using a special machine called apheresis. This machine separates platelets from the donor’s blood and returns the rest. Both methods work to keep the platelets safe and effective for patients.

Wound repair

Main article: Wound repair

After a blood clot stops bleeding, the body needs to fix the wound. Platelets help with this by releasing special chemicals. These chemicals encourage new tissue to grow and help the wound heal. They can also be used in treatments to make wounds heal faster.

Non-mammals

Non-mammalian vertebrates, such as birds and reptiles, do not have platelets like mammals. Instead, they have nucleated thrombocytes. These look similar to some immune cells. Thrombocytes clump together when they meet a substance called thrombin, but they do not react to other signals that affect platelets in mammals.

History

Scientists have studied platelets for a long time. In 1841, George Gulliver used a special microscope to draw pictures of platelets. Other scientists, like William Addison, Lionel Beale, and Max Schultze, also looked at platelets.

One important scientist, Giulio Bizzozero, studied platelets in 1882 and named them "piastrine," which means little plates. Later, in 1906, James Wright used a special stain to study platelets and called them "plates." He changed the name to "platelets" in 1910.