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 fragments of bigger cells called megakaryocytes that live in bone marrow or lung tissue before they enter the bloodstream. Platelets are found only in mammals, while other animals like birds and amphibians have similar cells that stay whole instead of breaking into pieces.

When there is an injury, platelets rush to the spot and stick together to form a plug, which is an important part of stopping the bleed. 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 and also starts other processes in the body that involve proteins called coagulation factors, which create a strong network to seal the wound.

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, while thrombosis talks about when too many clots form and cause problems. In another way, thrombus is the clot itself, and thrombosis is the process that makes it.

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 and involves several steps.

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. This process involves many proteins working together, but the main idea is that platelets are essential for stopping blood loss and helping wounds heal.

Immune function

Platelets play an important role in helping the body fight infections. They can bind to and help destroy harmful germs, and they are often fewer in number when someone has serious bacterial or viral infections. When there is an injury or infection, platelets move quickly to the area and help control the body's response by interacting with other immune cells and releasing important signaling molecules.

Platelets also help control the spread of infections by forming blood clots that can trap bacteria. This process, known as immunothrombosis, involves platelets working with other immune cells like neutrophils and monocytes. However, if this process is not regulated properly, it can lead to problems such as unwanted blood clots. Platelets can also help activate the body's larger immune system by interacting with antibodies and releasing substances that kill harmful bacteria directly.

Measurement and testing

Platelet concentration in the blood, known as the platelet count, can be measured using tools like a hemocytometer or automated machines such as a Coulter counter. Common blood testing methods usually include checking the platelet count, which is often 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, but 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 appear as dark purple spots, about one-fifth the size of red blood cells. A healthy adult typically has many more red blood cells than platelets.

Clinical significance

Platelet disorders can lead to bleeding problems. When there are too few or poorly functioning platelets, a person might bleed more easily from cuts, experience small purple spots on the skin called petechiae, purpura, and ecchymoses, or have bleeding in places like the gums, nose, or stomach. Sometimes, having too many platelets can cause dangerous clots in the veins or arteries, known as venous thrombosis and arterial thrombosis, and 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 affect how platelets work in the blood. Non-steroidal anti-inflammatory drugs (NSAIDs), like aspirin and ibuprofen, can temporarily stop platelets from doing their job. Aspirin, in particular, has a longer effect, while ibuprofen’s impact is shorter.

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

Therapies

Transfusion

Platelet transfusion is used to help people with very low platelet counts, either to prevent bleeding or before surgeries where some bleeding is expected. It can also be used if platelets aren’t working properly, like when someone is taking certain medicines. However, it’s not helpful for everyone and can sometimes make things worse.

Platelets used for transfusion come from two main sources. They can be taken from whole blood donations or collected using a special machine called apheresis, which separates platelets from the donor’s blood and returns the rest. Both methods aim to make sure the platelets are safe and effective for patients.

Wound repair

Main article: Wound repair

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

Non-mammals

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

History

Scientists have been studying platelets for a long time. In 1841, George Gulliver used a special microscope to draw pictures of platelets for the first time. Other scientists, like William Addison, Lionel Beale, and Max Schultze, also observed and described platelets in the years that followed.

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