#Platelets

#Biology

#BloodComponents

Introduction

• Platelets, also called thrombocytes, are membrane-bound cell fragments derived from the fragmentation of larger precursor cells called megakaryocytes, which are derived from stem cells in the bone marrow.

• Platelets are important for the blood clotting process, making them essential for wound healing.

• Platelets circulate in blood plasma and are primarily involved in hemostasis (stopping the flow of blood during injury), by causing the formation of blood clots, also known as coagulation.

• The adhesive surface proteins of platelets allow them to accumulate on the fibrin mesh at an injury site to form a platelet plug that clots the blood.

• The complex process of wound repair can only begin once the clot has stopped bleeding.

Structure

• Platelets are irregularly shaped, have no nucleus, and typically measure only 2–3 micrometers in diameter. Platelets are not true cells, but are instead classified as cell fragments produced by megakaryocytes. Because they lack a nucleus, they do not contain nuclear DNA.

• However, they do contain mitochondria and mitochondrial DNA, as well as endoplasmic reticulum fragments and granules from the megakaryocyte parent cells.

• Platelets also contain adhesive proteins that allow them to adhere to fibrin mesh and the vascular endothelium, as well as to a microtubule and microfilament skeleton that extends into filaments during platelet activation.

• Less than 1% of whole blood consists of platelets. They are about 1/10th to 1/20th as abundant as white blood cells.

Thrombopoiesis

• Platelets are produced during hematopoiesis in a sub-process called thromopoiesis, or production of thrombocytes. Thrombopoiesis occurs from common myeloid progenitor cells in the bone marrow, which differentiate into promegakaryocytes and then into megakaryocytes.

• Megakaryocytes stay in the bone marrow and are thought to produce protoplatelets within their cytoplasm, which are released in cytoplasmic extensions upon cytokine stimulus.

• The protoplatelets then break up into hundreds of platelets that circulate throughout the bloodstream, while the remaining nucleus of the ruptured megakaryocyte is consumed by macrophages.

• Megakaryocyte and platelet production is regulated by thrombopoietin, a hormone produced by the liver and kidneys.

• Thrombopoietin stimulates differentiation of myeloid progenitor cells into megakaryocytes and causes the release of platelets.

• Thrombopoietin is regulated by a negative feedback mechanism based on platelet levels in the body so that high levels of platelets result in lower levels of thrombopoietin, while low levels of platelets result in higher levels ofthrombopoietin.