Alpha granules, (α-granules) also known as platelet alpha-granules are a cellular component of
platelets
Platelets or thrombocytes () are a part of blood whose function (along with the coagulation factors) is to react to bleeding from blood vessel injury by clumping to form a blood clot. Platelets have no cell nucleus; they are fragments of cyto ...
.
Platelet
Platelets or thrombocytes () are a part of blood whose function (along with the coagulation#Coagulation factors, coagulation factors) is to react to bleeding from blood vessel injury by clumping to form a thrombus, blood clot. Platelets have no ...
s contain different types of
granules that perform different functions, and include alpha granules,
dense granules, and
lysosome
A lysosome () is a membrane-bound organelle that is found in all mammalian cells, with the exception of red blood cells (erythrocytes). There are normally hundreds of lysosomes in the cytosol, where they function as the cell’s degradation cent ...
s.
Of these, alpha granules are the most common,
making up 50% to 80% of the secretory granules.
Alpha granules contain several
growth factor
A growth factor is a naturally occurring substance capable of stimulating cell proliferation, wound healing, and occasionally cellular differentiation. Usually it is a secreted protein or a steroid hormone. Growth factors are important for ...
s.
Contents
Contents include
insulin-like growth factor 1
Insulin-like growth factor 1 (IGF-1), also called somatomedin C, is a hormone similar in tertiary structure, molecular structure to insulin which plays an important role in childhood growth, and has Anabolism, anabolic effects in adults. In the ...
,
platelet-derived growth factor
Platelet-derived growth factor (PDGF) is one among numerous growth factors that regulate cell growth and division. In particular, PDGF plays a significant role in blood vessel formation, the growth of blood vessels from already-existing bloo ...
s,
TGF beta
Transforming growth factor beta (TGF-β) is a multifunctional cytokine belonging to the Transforming growth factor beta family, transforming growth factor superfamily that includes three different mammalian isoforms (TGF-β 1 to 3, HUGO Gene Nomen ...
,
platelet factor 4
Platelet factor 4 (PF4) is a small cytokine belonging to the CXC chemokine family that is also known as chemokine (C-X-C motif) ligand 4 (CXCL4) . This chemokine is released from alpha-granules of activated platelets during platelet aggregation ...
(which is a heparin-binding chemokine) and other
clotting
Coagulation, also known as clotting, is the process by which blood changes from a liquid to a gel, forming a blood clot. It results in hemostasis, the cessation of blood loss from a damaged vessel, followed by repair. The process of coagulation ...
proteins (such as
thrombospondin
Thrombospondins (TSPs) are a family of secreted glycoproteins with antiangiogenic functions. Due to their dynamic role within the extracellular matrix they are considered matricellular proteins. The first member of the family, thrombospondin 1 ...
,
fibronectin
Fibronectin is a high- molecular weight (~500-~600 kDa) glycoprotein of the extracellular matrix that binds to membrane-spanning receptor proteins called integrins. Fibronectin also binds to other extracellular matrix proteins such as col ...
,
factor V
Coagulation factor V (Factor V), also less commonly known as proaccelerin or labile factor, is a protein involved in coagulation, encoded, in humans, by ''F5'' gene. In contrast to most other coagulation factors, it is not enzymatically active b ...
,
and
von Willebrand factor
Von Willebrand factor (VWF) () is a blood glycoprotein that promotes primary hemostasis, specifically, platelet adhesion. It is deficient and/or defective in von Willebrand disease and is involved in many other diseases, including thrombotic thro ...
).
The alpha granules express the adhesion molecule
P-selectin
P-selectin is a type-1 transmembrane protein that in humans is encoded by the SELP gene.
P-selectin functions as a cell adhesion molecule (CAM) on the surfaces of activated endothelial cells, which line the inner surface of blood vessels, and a ...
and
CD63
CD63 antigen is a protein that, in humans, is encoded by the ''CD63'' gene. CD63 is mainly associated with membranes of intracellular vesicles, although cell surface expression may be induced.
Function
The protein encoded by this gene is a me ...
.
These are transferred to the membrane after synthesis.
The other type of granules within platelets are called dense granules.
Clinical significance
A deficiency of alpha granules is known as
gray platelet syndrome
Gray platelet syndrome (GPS), or platelet alpha-granule deficiency, is a rare congenital autosomal recessive bleeding disorder caused by a reduction or absence of alpha-granules in blood platelets, and the release of proteins normally contained in ...
.
See also
*
Platelet rich fibrin
Platelet-rich fibrin (PRF) or leukocyte- and platelet-rich fibrin (L-PRF) is a derivative of PRP where autologous platelets and leukocytes are present in a complex fibrin matrix to accelerate the healing of soft and hard tissue and is used as a ti ...
References
Growth factors
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