Abstract
Blood platelets have important roles in haemostasis, where they quickly stop bleeding in response to vascular damage. They have also recognised functions in thrombosis, immunity, antimicrobal defense, cancer growth and metastasis, tumour angiogenesis, lymphangiogenesis, inflammatory diseases, wound healing, liver regeneration and neurodegeneration. Their brief life span in circulation is strictly controlled by intrinsic apoptosis, where the prosurvival Bcl-2 family protein, Bcl-xL, has a major role. Blood platelets are produced by large polyploid precursor cells, megakaryocytes, residing mainly in the bone marrow. Together with Mcl-1, Bcl-xL regulates megakaryocyte survival. This review describes megakaryocyte maturation and survival, platelet production, platelet life span and diseases of abnormal platelet number with a focus on the role of Bcl-xL during these processes.
Highlights
Platelets are small (2–4 μm in diameter), anucleate blood cells with a characteristic discoid shape
Not all platelets are circulating in the blood flow, but approximately 30% of the total platelet mass exists as an exchangeable pool residing in the spleen
Platelets are a unique model in terms of life span regulation because they mostly depend on only two antagonistic Bcl-2 family members, i.e., Bcl-xL for their survival and Bak for their death [19,21], when in circulation at steady-state
Summary
Platelets are small (2–4 μm in diameter), anucleate blood cells with a characteristic discoid shape They have multiple functions and a life span of 7–10 days in human. The main function of platelets is to participate in the physiological process of haemostasis, allowing bleeding to stop at the site of an injury, while maintaining normal blood flow elsewhere in the circulation. The binding of platelet receptors with their ligands leads to platelet activation, i.e., secretion of the content of their granules, production of thromboxane A2 and subsequent activation of other local platelets, exposure of membrane phospholipids and induction of conformational changes of the αIIbβ (CD41/CD61) receptors These receptors, when activated, can bind the fibrinogen that physiologically circulates in the blood. This explains why increased spleen mass (splenomegaly) can be associated with lower platelet count and why absence of spleen (asplenia) is associated with increased platelet counts
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