University of Virginia Health System

Platelet
(Thrombocyte)

 
Peripheral blood smear, Wright-Giemsa stain, 1000x 

Development and Description:

When platelets, formed by fragmentation of the megakaryocyte cytoplasm, are released into the circulation, they usually are larger than the more mature platelets, and often are referred to as megathrombocytes. The size of the more mature, resting platelets is variable (1-3 μm in diameter). Like erythrocytes, platelets have no nucleus, but unlike erythrocytes have granules and mitochrondria. The cytoplasmic borders are irregular. The cytoplasm is grey to light blue in color, and contains a variable number of azurophilic granules of variable size that tend to concentrate in the center of the cell. From ultrastructural studies, the granules have been classified into different subtypes that include:

• Dense bodies (δ granules) containing ATP, ADP, serotonin, calcium, and other substances.
• α granules containing platelet factor 4, platelet-derived growth factor (PDGF), fibrinogen and other substances.

Function:

Platelets play an important role in hemostasis by adhering to damaged endothelium, the initial step in the formation of the hemostatic thrombus. Adherence to the exposed subendothelial connective tissue, mostly the collagen-von Willebrand complex, is mediated through several glycoprotein (GP) structures on the platelet membrane (e.g. GPIb complex, GPIIb-IIIa). Platelet activation followed by the release reaction (e.g. release of ADP, thromboxane A-2, fibrinogen) leads to recruitment of other platelets and the formation of the platelet plug. The platelets are held together by cross-linked fibrin generated by local thrombin formation. Final thrombus formation involves the interactions of multiple other factors.

Other platelet functions:

• Participation in the inflammatory response (e.g. release of vascular permeability and chemotactic factors).
• Participation in the vascular repair process (e.g. release of factors that aid endothelial repair and stimulate fibroblast proliferation).

Trafficking:

Platelets not utilized in response to vascular injury die of senescence, mostly in the spleen, after a lifespan of about 9 days.

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