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Platelet Granular Content – Lysosomes, Dense granules, and Alpha granules

Platelets are very complex elements of the blood that actively participate in multiple processes, ranging from stopping the bleeding of injured vessels to releasing growth factors that signal intracellular pathways to induce tissue proliferation.

They play an essential part in all these processes thanks to its granule content. Platelets synthesize and store a myriad of molecular mediators such as coagulation factors, chemokines, adhesion molecules, immunologic molecules, regulators of growth and angiogenesis, ATP, ADP, Ca++ enzymes, neurotransmitters and sugars.

To store these molecules that are so different from one another, platelets have three main types of granules called lysosomes, dense granules and alpha granules.

Lysosomes store more than 60 different molecules. Its main characteristic is that it mainly contains digestive enzymes. Among its most characteristic contents are glycohydrolases, which are able to degrade glycoproteins, glycolipids and glycosaminoglycans; and proteolytic enzymes that degrade and break down virtually all kinds of biomolecules.

On the other hand, the dense granules of human platelets contain neurotransmitters and ions required for molecule and cell signalling such as adenosine diphosphate (ADP), adenosine triphosphate (ATP), ionized calcium (which is necessary for several steps of the coagulation cascade), and serotonin1.

The more complex granules in the platelet cytoplasm are the alpha granules. Some components in these granules are related to growth and angiogenesis. Their content includes insulin-like growth factor 1, platelet-derived growth factors, TGFβ, platelet factor 4 (which is a heparin-binding chemokine) and other clotting proteins such as thrombospondin, fibronectin, factor V,2 and von Willebrand factor.3 Most of these molecules stimulate cellular growth, proliferation and differentiation.

The alpha granules also express the adhesion molecule P-selectin1 and CD63,3 that are transferred to the membrane after synthesis.

Understanding the vital role of platelets in tissue regeneration will inevitably open new pathways in regenerative medicine to treat osteoarthritic joints and damaged tissues.

References:

  1. http://www.bloodjournal.org/content/118/5/1190
  2. Nurden AT (2011). “Platelets, inflammation and tissue regeneration” (PDF). THROMBOSIS AND HAEMOSTASIS. 105 (Suppl 1): S13–S33. doi:10.1160/THS10-11-0720. PMID 21479340.
  3. William B. Coleman; Gregory J. Tsongalis (2009). Molecular pathology: the molecular basis of human disease. Academic Press. pp. 258–. ISBN 978-0-12-374419-7. Retrieved 2 November 2010.