Platelets Contain Mitochondria, But Lack a Nucleus GalNAc transferase When megakaryocytes, the progenitors of red blood cells, are exposed to thrombopoietin they may fragment and form platelets. Like red blood cells, platelets lack a nucleus, but unlike erythrocytes they possess mitochondria, lysozymes, and a tubular network that forms an open canalicular system. This honeycomb of channels increases the sur face area of the platelets, which are spheroidal at rest, thereby facilitating the secretion of various endocrine and coagulation factors. These factors are stored inside the platelets within densely packed secretory vesicles, called granules. Dense granules contain Ca2+, ADP, and serotonin, while α-granules contain fibrinogen, fibronectin, platelet derived growth factor, von Willebrand factor, or other coagulation factors ready for release on receipt of an appropriate stimulus. Under normal circumstances, these small (2-μm diameter), enucleated cells circulate at a density of 2 to 4 × 105 platelets per milliliter of blood. While platelets derive the majority of their energy from metabolizing glucose, their mitochondria enables them to generate ATP via the β-oxidation of fatty acids.

Platelet Disorders Compromise Hemostasis

Abnormalities in platelet number or function can have serious physiologic consequences. For example, in acute coronary syndrome, the formation of enlarged, hyperreactive platelets results in an increased risk of thrombosis, the unregulated formation of blood clots that obstruct the circulation. The presence of larger than normal platelets also correlates with an increased frequency of myocardial infarction.

Immune thrombocytopenic purpura is caused by the formation of antibodies against ones own platelets. This autoimmune disorder is marked by depressed platelet counts (thrombocytopenia). When a platelet’s surface becomes decorated with antibodies, it is subject to clearance by splenic macrophages. In some instances, platelet autoantibodies will bind to differentiating megakaryocytes, depressing platelet production. Thrombocytopenia also can occur when persons who are homozygous for a mutant variant of glycoprotein IIb/IIIa in which the leucine 33 is replaced by proline receive blood from a donor that is homo- or heterozygous for the wild-type form of this major platelet antigen. Exposure to the donor’s platelets triggers the production of alloantibodies that attack not only the donated platelets, but the patient’s endogenous platelets as well. In neonatal alloimmune thrombocytopenia, which affects roughly 1 in 200 term pregnancies, antibodies from the mother’s circulation cross the placental barrier and attack platelets in the fetus’ circulatory system. Thrombocytopenia also can be induced by drugs such as tamoxifen, ibuprofen, vancomycin, and many sulfonamides.

The symptoms of hemolytic-uremic syndrome, a disease of infants characterized by progressive kidney failure, include both thrombocytopenia and hemolytic anemia. By contrast, the abnormal bleeding associated with von Willebrand disease is caused by a genetic defect that compromises the ability of platelets to adhere to the endothelium, rather than a deficit in platelet number. Other bleeding disorders resulting from defects in platelet adherence include Bernard-Soulier syn drome (genetically inherited deficiency in glycoprotein 1b), and Glanzmann thrombasthenia (genetically inherited deficiency in the glycoprotein IIb/IIIa complex).

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مواضيع ذات صلة

The Biochemical Basis of The ABO System

The Red Blood Cell Membrane

Oxidation of Heme Iron Compromises oxygen Transport

Hypervariable Regions Confer Binding Specificity

Immunoglobulins Are Comprised of Multiple Polypeptide Chains

Serum Inhibitors Prevent Indiscriminate Proteolysis

Intracellular Iron Homeostasis is Tightly Regulated

Clinical Biochemistry of Exercise: Biochemical Adaptation

Iron is Strictly Conserved

Haptoglobin Protects the Kidneys

The Levels of Certain Plasma Proteins Increase During Inflammation or Following Tissue Damage

Albumin is the Most Abundant Protein in Human Plasma