Plaquettes, commonly known as platelets, are tiny, specialized cell fragments circulating in the bloodstream. They play a significant part in the body’s immediate response to injury. These small components are not complete cells but are nevertheless fundamental for maintaining the integrity of blood vessels and initiating repair processes.
The Primary Function of Platelets
Platelets perform their primary function by stopping bleeding, a process called hemostasis. When a blood vessel sustains damage, platelets are quickly drawn to the injury site. They adhere to the exposed collagen and other substances in the vessel wall, a step known as adhesion. This initial attachment is mediated by von Willebrand factor, a protein that acts as a bridge between the platelet and the damaged surface.
Upon adhesion, platelets undergo activation, changing their shape from a disc to a sphere and extending projections, which increases their surface area. During this activation, they release various chemical signals from their internal granules, including adenosine diphosphate (ADP) and thromboxane A2 (TXA2). These released substances amplify the response by attracting more platelets to the site and enhancing their stickiness.
The recruited platelets then clump together, a process called aggregation, forming a temporary plug that seals the breach in the blood vessel wall. This platelet plug provides a surface for the coagulation cascade, a series of protein interactions, to occur. The cascade ultimately leads to the formation of a stable fibrin mesh, which reinforces the platelet plug, creating a more durable blood clot. Beyond clot formation, platelets also contribute to wound healing by releasing growth factors, such as platelet-derived growth factor (PDGF) and transforming growth factor-beta (TGF-β), which promote tissue repair.
Platelet Formation and Lifespan
Platelets originate in the bone marrow from large cells called megakaryocytes. These megakaryocytes undergo a maturation process where they develop cytoplasmic extensions known as proplatelets. Platelets then fragment from the tips of these proplatelets, entering the bloodstream.
This continuous production ensures a steady supply of platelets, as they have a short lifespan. A typical platelet circulates for 7 to 10 days before it is removed from the body. Old or damaged platelets are cleared from circulation by specialized cells in the spleen and liver. The spleen acts as a filter, removing aged platelets.
Interpreting Platelet Counts
A standard blood test, often part of a complete blood count, measures the number of platelets circulating in a microliter of blood. A normal platelet count ranges from 150,000 to 450,000 platelets per microliter, though variations may occur between laboratories. Maintaining a count within this range supports proper blood clotting and prevents excessive bleeding or clotting.
A platelet count below 150,000 per microliter is termed thrombocytopenia. Symptoms associated with thrombocytopenia can include easy bruising, the appearance of tiny red or purplish spots on the skin called petechiae, prolonged bleeding from minor cuts, and frequent nosebleeds or bleeding gums. Below 50,000 per microliter, the risk of serious bleeding, including internal bleeding, increases.
Conversely, a platelet count above 450,000 per microliter is known as thrombocytosis. While some individuals with high counts may not experience symptoms, the primary concern with thrombocytosis is an increased risk of blood clot formation. These clots can block blood flow to organs, potentially leading to a stroke or heart attack. Less commonly, very high platelet levels can paradoxically cause bleeding due to platelet dysfunction.
Causes of Abnormal Platelet Levels
Abnormal platelet counts arise from underlying conditions affecting their production or their removal from the bloodstream. Low platelet levels (thrombocytopenia) can result from decreased production in the bone marrow. This can occur with cancers like leukemia or lymphoma, chemotherapy or radiation, heavy alcohol consumption, or nutritional deficiencies (iron, folate, or vitamin B12).
Another cause of low platelet counts is increased destruction or consumption of platelets. Autoimmune disorders like immune thrombocytopenia (ITP), lupus, or rheumatoid arthritis can cause the immune system to attack and destroy platelets. Bacterial and viral infections can also increase platelet destruction or usage. An enlarged spleen can also contribute to thrombocytopenia by trapping excessive platelets, reducing their circulation.
High platelet counts (thrombocytosis) are categorized into two types. Reactive thrombocytosis, the more common form, occurs as a temporary response to other medical issues. Causes include blood loss, infections, chronic inflammatory conditions (e.g., rheumatoid arthritis), iron deficiency, or spleen removal. Essential thrombocythemia is a primary bone marrow disorder where the bone marrow produces too many platelets. This condition is linked to genetic mutations in genes like JAK2, CALR, or MPL, which regulate blood cell production.