Platelet Ultrastructure (Part 1) PDF MT 64 2024-2025

MT 64 | A.Y. 2024-2025 INTRODUCTION ❖ Light Microscopy Dense to purple particles with granules (Romanowsky stain preparation) 1) Diameter: 2-4 μm 2) Nucleus: none (anucl...

MT 64 | A.Y. 2024-2025 INTRODUCTION ❖ Light Microscopy Dense to purple particles with granules (Romanowsky stain preparation) 1) Diameter: 2-4 μm 2) Nucleus: none (anucleate) 3) Cytoplasm: light blue (with evenly dispersed, fine red-purple granules) 4) Shape:  Discoid (inactive or unstimulated)  Irregular sphere with spiny pseudopods (stimulated) ❖ Electron Microscopy Four Major Areas: 1) Peripheral Zone  Consists of platelet’s outer membrane and related structures 2) Submembrane Area  Links the membrane and the inner cell body 3) Sol-Gel Zone  Constitutes the matrix or muscle and skeletal portion of the platelet 4) Organelle Zone  Consists of granules, dense bodies, lysosomes and mitochondria  Metabolic center RESTING PLATELET PLASMA MEMBRANE ❖ Glycocalyx Fluffy coat surrounding the plasma membrane Absorbs and transports proteins thru endocytosis Thickness: 20-30 nm | adhesive Layer of plasma proteins and carbohydrates molecules (with a net negative charge) ❖ Plasma membrane (bilayer): protein + lipids | selectively permeable Predominant lipids: phospholipids and cholesterol Phospholipids  Main function: support platelet activation (internally) and plasma coagulation (externally)  Predominate in the outer, plasma layer: phosphatidylcholine and sphingomyelin  Predominate in the inner, cytoplasmic layer: phosphatidylinositol, phosphatidylethanolamine and phosphatidylserine Cholesterol  Functions: membrane stabilization, maintains fluidity, regulates or control transmembranous passage of materials  Esterified cholesterol + unesterified cholesterol Glycoproteins and Proteoglycans  Function: support surface glycosaminoglycans, oligosaccharides, glycolipids & plasma surface-oriented glycosylated receptors PREPARED BY: KAREN B. ROSETE, RMT 1  MT 64 | A.Y. 2024-2025 SUBMEMBRANE AREA ❖ underlies the plasma membrane ❖ contains an organized system of filaments ❖ region that separates organelles and cell membrane ❖ Functions: 1) contribute to the regulation of normal platelet discoid shape 2) act as a base for pseudopod formation 3) modulate platelet adhesion and clot retraction after activation SURFACE-CONNECTED CANALICULAR SYSTEM (SCCS) ❖ Sponge-like appearance ❖ Intracellular canalicular system that opens to the platelet’s external environment ❖ Functions: 1) Storage of additional quantities of hemostatic proteins 2) Allows enhanced interaction of platelet with its environment 3) Increases access to platelet interior (delivery routes for substances ingested by platelets) 4) Increases egress of platelet products (route for endocytosis and secretion of α-granule contents) DENSE TUBULAR SYSTEM (DTS) ❖ Condensed remnant of rough endoplasmic reticulum ❖ “Control center” for platelet activation ❖ Functions: Sequesters Ca2+ Consists of enzymes that supports platelet activation ▪ Phospholipase A2 ▪ Cyclooxygenase ▪ Thromboxane synthetase CYTOSKELETON: MICROFILAMENTS AND MICROTUBULES ❖ Circumferential microtubules Function: maintain platelet’s discoid shape | control platelet shape change | facilitate expression of alpha-granule contents | provide rigidity to pseudopods Cylindrical | Diameter: 25 nm | made up of 8-20 tubules (composed of multiple subunits of tubulin) Lower temperature: platelets become round | 37°C: platelets recover their original discoid shape ❖ Microfilaments Composed of actin Function: maintain platelet’s discoid shape | control platelet shape change | control extension of pseudopods | control secretion of granule contents ACTIN  anchors the plasma membrane glycoproteins and proteoglycans  Platelet cytoplasm: 20 % to 30 %  Resting platelet: actin is globular and amorphous | activated platelet: as calcium ↑, actin becomes filamentous and contractile PREPARED BY: KAREN B. ROSETE, RMT 2  MT 64 | A.Y. 2024-2025 ❖ Intermediate Filaments: Desmin and Vimentin Rope-like polymers | diameter: 8-12 nm Function: maintain platelet shape | control platelet shape change | control extension of pseudopods | control secretion of granule contents PLATELET GRANULES: α-Granules, Dense Granules and Lysosomes Platelet Alpha-granule Proteins: 50-80 alpha granules/platelet Coagulation Proteins Noncoagulation Proteins Proteins Present in Platelet Cytoplasm and α-Granules Endocytosed Fibronectin Albumin Fibrinogen Immunoglobulins Megakaryocyte synthesized Factor V Thrombospondin VWF Proteins Present in α-Granules but not in Cytoplasm Megakaryocyte synthesized β-thromboglobulin EGF HMWK Multimerin PAI-1 PDC1 Plasminogen PDGF PF4 TGF-β Protein C inhibitor VEGF/VPF Platelet Membrane-Bound Proteins Restricted to α-granule P-selectin GMP33 membrane Osteonectin In α-granule and plasma GP IIb/IIIa cap1 membrane GP IV CD9 GP Ib/IX/V PECAM-1 Platelet Dense Granule Proteins: 2-7 dense granules/platelet Small Molecule Property ADP Nonmetabolic; supports neighboring platelet aggregation by binding to the P2Y1 and P2Y12 receptors ATP Function unknown; ATP release is detectable on platelet activation Serotonin Vasoconstrictor that binds endothelial cells and platelet membranes Ca and Mg 2+ 2+ Divalent cations support platelet activation and coagulation Lysosomes ❖ Diameter: 300 nm ❖ Contains: arylsulfatase, β-glucuronidase, acid phosphatase, catalase ❖ Functions: digest vessel wall matrix components (during in vivo aggregation) | digest debris MAIN REFERENCES: ❖ Keohane, E.M., Otto, C.N., Walenga, J.M. (2020). Rodak’s Hematology: Clinical Principles and Applications. (6th Ed.). Singapore: Elsevier ❖ Turgeon, M.L. (2012). Clinical Hematology: Theory and Procedures. (5th Ed.). Philadelphia: Lippincott Williams & Wilkins ❖ Steininger, C.A., Martin, E. S., Koepke, J.A. (1992). Clinical Hematology: Principles, Procedures, Correlations. Philadelphia: J.B. Lippincott Company PREPARED BY: KAREN B. ROSETE, RMT 3