Hémostase Primaire

Hémostase Primaire

• Thrombocytes (Plaquettes): Small, anucleated, disc-shaped cells (2-4µm diameter, 7-10fL volume). Shape changes to spherical with pseudopods upon activation.
• Morphology (MGG): Contain granules (α and dense granules) and hyalomere within the cytoplasm. α granules store proteins for hemostasis. Dense granules hold ADP, Ca2+, ATP, and serotonin. Also have an intracellular tubule system (STD) for calcium storage and prostaglandin synthesis.
• Membrane: Composed of an asymmetrical phospholipid bilayer and internal projections of the open canalicular system (SCOE). Membrane proteins are anchored to the cytoskeleton and can move. Glycoproteins are also present on the membrane.
• GP Ib-IX-V Complex: Receptor for von Willebrand factor (vWF), crucial for platelet adhesion in conditions of high shear stress. GP Ib binds to vWF, linking platelets to the vessel wall. GP V is cleaved by thrombin after activation.
• GP IIb-IIIa Complex (Integrin α2β3): Binds to fibrinogen, mediating platelet aggregation. In the resting state, it has a different structure compared to the active state, which is calcium dependent.
• Other Receptors: Receptors for ADP, thrombin, TXA2, serotonin, and others. Other receptors exist for collagen (Gp la/lla and GpVI), thrombospondin (Gp IV).
• Granules (α): Contain proteins crucial to hemostasis, like clotting factors, and adhesive molecules (e.g., fibrinogen, von Willebrand factor, thrombospondin).
• Granules (Dense): Store agonists that activate other platelets and initiate hemostasis.

Thrombocytopoïese

• Mégacaryocytes: Large cells (20-60µm) with a multi-lobed nucleus, playing a key role in platelet production in the bone marrow. Cytoplasm matures from basophilic to acidophilic, displaying compartmentalization into platelet-producing territories.
• Different stages: The maturation of megakaryocytes has four distinct stages (MGC I-IV). Platelets (thrombocytes) are released from mature megakaryocytes.
• Factors driving maturation: Thrombopoietin (TPO) is the primary regulator, with its receptor, c-Mpl, being essential. SCF, IL-3, GM-CSF contribute by inducing megakaryocyte proliferation and maturation, but are not specific to megakaryocytes.
• Regulation: TPO circulating levels are influenced by the number of megakaryocytes, reflecting platelet production in the bone marrow. These levels correlate with megakaryocyte mass.

Physiologie Plaquettaire

• Activation Mechanisms: Activation is triggered by agonists that bind to specific receptors on the platelet surface.
• Internal Signal Transduction: Binding initiates signal transduction and amplification through intracellular cascades.
• External Changes: Shape change, pseudopods formation, and cytoskeletal reorganization are hallmarks of platelet activation.
• Metabolic Response: Phospholipid rearrangements enable increased procoagulant activity by platelets. Secretion is a major consequence of platelet activation.

Interactions Platelet-Vascular System

• Endothelial Cells Role: Endothelial cells play a primary role in maintaining vascular homeostasis by producing molecules like prostacyclin (PGI2), which inhibits platelet activation and aggregation while also mediating vasodilation.
• Plaque Response: Injury to a blood vessel exposes collagen and other proteins. Activated platelets adhere to these components. Then, they secrete substances and undergo aggregation, forming a primary platelet plug.