Platelets – differentiation, structure and function

Platelets

Thrombocytes, or platelets, are non-nucleated blood cells that play an essential role in stopping bleeding.

Megakaryocyte development and platelet formation

Physiological values

The normal platelet count in the blood is around 150–400 thousand per microliter. This value is independent of age or gender. A decrease in the platelet count below the physiological limit is referred to as thrombocytopenia, an increase as thrombocytosis. Approximately 2/3 of platelets are found in the blood circulation and 1/3 in the spleen.

Differentiation

Platelets are produced from megakaryocytes in the bone marrow. Megakaryocytes are large cells with multiple nucleoli. When they mature, they extend their processes into the sinusoids of the bone marrow. When they pass through the capillary wall, the most massive cleavage of fragments occurs, which are released into the blood. Platelets are therefore not real cells, but only fragments of the cytoplasm of megakaryocytes. Up to 5000 platelets are produced from one megakaryocyte.

The development of megakaryocytes in the marrow (megakaryocytopoiesis) is stimulated by thrombopoietin and influenced by several other substances. Platelets live for 9–12 days, and most of them die by being absorbed by the vascular endothelium.

Structure

Platelets are the smallest blood elements, they are shaped like discs with a diameter of 2–4 μm, a thickness of 0.5–1 μm, and a volume of 4–8 fl. They are nuclear-free (therefore, protein synthesis does not occur in them). They contain mitochondria (production of ATP and ADP) and residues of the Golgi apparatus and endoplasmic reticulum (storage of calcium ions). In the cytoplasm there is a system of interconnected channels, connected to the surface of the platelet (important for secretion). The dense tubular system in the cytoplasm contains enzymes for the synthesis of arachidonic acid derivatives.

In platelets, we find 3 types of granules:

• Alpha granules: contain von Willebrand factor (vWF), platelet factor 4, PDGF, fibrinogen, thrombospondin, factor V (proaccelerin),
• Dense granules: contain ADP, ATP, Ca2+, serotonin,
• Lysosomes: contain lysosomal enzymes.

Substances from platelet granules are used in vasoconstriction at the site of injury, hemocoagulation and subsequent repair of the injured vessel.

The cytoplasm contains actin and myosin (platelet constriction is important for the release of granule contents). The shape of platelets is maintained by marginal microtubules under the membrane (the formation of pseudopodia during the formation of a provisional plug). The contractile protein thrombostenin also actively changes the shape of the platelet.

The platelet membrane contains phospholipids. Platelet phospholipids represent the so-called platelet factor 3. Phospholipids in the platelet membrane are necessary for the activation of some coagulation factors. In addition to phospholipids, the membrane also contains glycoproteins that prevent platelets from attaching to the undamaged vascular wall and, conversely, allow platelets to attach to the wall that has been damaged.

Function

Platelets play an important role in hemostasis.

Platelet

1. Adhesion - platelets adhere to the subendothelial collagen exposed by injury. In adhesion, von Willebrand factor is used.
2. Aggregation - platelets aggregate via fibrinogen, for which they express receptors.
3. Constriction - due to contractile proteins, the shape of platelets changes, and the release of active substances occurs.
4. Thrombus formation - a white or platelet thrombus is formed (a provisional hemostatic plug). Its further transformations are part of the hemocoagulation process.
5. Healing - platelets contain substances such as PDGF (Platelet Derived Growth Factor), which have proliferative effects and are used in the regeneration of injured tissue.

References

KITTNAR, Otomar a ET AL.. Lékařská fyziologie. 1. vydání. Praha : Grada, 2011. 790 s. s. 145. ISBN 978-80-247-3068-4.

TROJAN, Stanislav, et al. Lékařská fyziologie. 4. vydání. Praha : Grada, 2003. 771 s. ISBN 80-247-0512-5.

KITTNAR, Otomar, et al. Lékařská fyziologie. 1. vydání. Praha : Grada, 2003. 790 s. ISBN 978-80-247-3068-4.

ŠVÍGLEROVÁ, Jitka. Trombocyt [online]. Poslední revize 2009-02-18, [cit. 2010-11-13].  <https://web.archive.org/web/20141108073650/http://wiki.lfp-studium.cz:80/index.php/Trombocyt>.