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Quick Facts

A platelet is a disk-shaped structure, 2 to 4 μm in diameter, found in the blood of all mammals and chiefly known for its role in blood coagulation; platelets, which are formed in the megakaryocyte and released from its cytoplasm in clusters, lack a nucleus and DNA but contain active enzymes and mitochondria (Dorland, 2011).

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Cell Morphology

Platelets are small colorless discoid cell fragments that lack nuclei. They have a diameter of 2–4 μm, making them the smallest blood cell type (Silverthorn et al., 2013; Marieb, Wilhelm and Mallatt, 2012). The cytoplasm of each platelet contains mitochondria, smooth endoplasmic reticulum, and granules. These granules contain growth factors and cytokines.

The concentration of platelets is around 150,000–500,000 per microliter of blood and they generally live for 5–10 days (Marieb, Wilhelm and Mallatt, 2012). Platelets are located towards the edge of a blood vessel in an inactivated form, which enables them to act fast once they become activated.


Platelets are produced in the red bone marrow from pluripotent hematopoietic stem cells. Pluripotent hematopoietic stem cells become myeloid progenitor cells and differentiate into megakaryocytes, and further into megakaryoblasts. Megakaryoblasts undergoes fragmentation, which results in the production of over 1000 platelets per cell. Thrombopoietin, produced by the kidneys and liver, regulates the production of platelets by regulating the growth and maturation of the megakaryocytes (Pawlina, 2016). Development of platelets takes 4–5 days (Marieb, Wilhelm and Mallatt, 2012).

When the endothelium of a blood vessel becomes injured, collagen from connective tissue is exposed to blood. Platelets become activated by signaling molecules and adhere to collagen fibers, thus forming an initial seal on the damaged blood vessel. Clotting factors from platelets, blood plasma, and damaged endothelial cells trigger a series of enzymatic reactions. That results in prothrombin to transform into thrombin, which causes fibrinogen to transform to fibrin. Fibrin ultimately forms a clot and seals the injured blood vessel (Reece et al., 2014).


Platelets are involved in coagulation, by monitoring the endothelium of blood vessels and sealing small tears in injured blood vessels.

List of Clinical Correlates



—Bernard-Soulier Syndrome


Dorland, W. (2011) Dorland's Illustrated Medical Dictionary. 32nd edn. Philadelphia, USA: Elsevier Saunders.

Marieb, E. N., Wilhelm, P. B. and Mallatt, J. (2012) Human Anatomy. 14th edn.: Benjamin Cummings.

Pawlina, W. 2016. Histology: A text and atlas with correlated cell and molecular biology. 7th ed. Philadelphia: Wolters Kluwer.

Silverthorn, D. U., Johnson, B. R., Ober, W. C., Garrison, C. W. and Silverthorn, A. C. (2013) Human Physiology: An Integrated Approach. Pearson Education.

Complete Anatomy

The world's most advanced 3D anatomy platform

Complete Anatomy