Platelets (Thrombocytes) Platelets are produced by the bone marrow. They are not whole cells, but are fragments of extraordinarily large bone marrow-bound megakaryocytes. The hormone THROMBOPOIETIN (produced by liver) increases the # of megakaryocytes in the bone marrow, stimulating them to produce more platelets * Cell fragments bound to megakaryocytes * “Bud Off” and are released into the blood

In Bone Marrow

Thrombocyte Development

Thrombocytes Shape: Minute round or oval discs (2-4 ) non-nucleated Number: 150.000 – 400.000/mm3 Life span: 8-12 days, removed by tissue macrophage as spleen so, the number increases after spleenectomy.

Platelet structure 1- Surface coat: mucopolysaccharide 2- Plasma membrane: is formed of 3 layers and invaginates inside to form open membrane system and contain phospholipids. This membrane is the basis structure of platelet factor 3 (PF3) 3- Beneath the membrane there are: a- Microfilaments b- Microtubules

4- Granules : 3 types of granules - Dense granules contain (ADP) serotonin and calcium. - Specific (Alpha) granules contain platelet growth factor, beta thromboglobulin, fibrinogen, clotting factors as V, XIII, platelet factor 4 (PF4) which is heparin antagonist factor. - Dense tubular system contains calcium and may be site of synthesis of prostaglandin and thromboxane A2 5-mitochondria, glycogen & lysosomes which contain hydrolytic enzymes

Platelet structure

Von Willebrand factor (vWF) An adhesive protein formed by the vascular endothelial cells and secreted in the plasma. It is formed by megakaryocytes and stored in the platelets. Blood clotting factor VIII circulated in plasma bound to vWF

Hemostasis It is the stoppage of bleeding in case of small vascular injury. It depends on: the vascular wall, platelets and the clotting factors

[I] Vascular factor (local vascular spasm) Causes a- Direct effect of serotonin and thromboxane A2 released from platelets  V.C. b- Injury of endothelium   endothelial derived relaxing substance  V.C. c- Pain  nervous V.C. reflex

[II] Formation of platelet plug 1) Platelet adhesion: Following blood vessel injury, platelets adhere to the exposed subendothelial collagen. Adhesion depends on a. A surface membrane glycoportein b. Von-Willebrand factor

3) Platelet aggregation 2) Platelets release: a- Release of platelets contents such as ADP, serotonin, VWF, fibrinogen and platelet factor b- Platelets enzymes form thromboxane A2  release of platelets contents . 3) Platelet aggregation Released ADP and thromboxaneA2 platelet aggregation  further release  more aggregation and so on.

Activated platelets adhere to other activated platelets. This is mediated by adhesive receptors on the platelet surface that bind to vWF and fibrinogen. Thus, fibrinogen forms a bridge between activated platelets  plug formation

Platelet Activation Pathways ADP Adrenaline COLLAGEN THROMBIN ADP GpIIb/IIIa Aggregation GpIIb/IIIa Aggregation GpIIb/IIIa Aggregation GpIIb/IIIa Adhesion Platelet GpIb Adrenaline Adhesion vWF Exposed Collagen Endothelium

4) Platelet plug At first, the plug is loose then becomes firm and tight (by contraction of contractile proteins and by fibrin threads). The normal endothelial cells around the injury release prostacyclin. Prostacyclin causes limitation of the plug to site of the injury by inhibition of platelet aggregation.

Importance of the plug A- It stops blood loss from small vessels B- It closes the minute ruptures of very small vessels, which occur hundreds of times daily C- The chemicals released from the platelets help other mechanisms of hemostasis: - Serotonin and thromboxane A2 V.C. - platelet factor 3 and clotting factors (factor V and fibrinogen and factor XIII) help clotting release thrombasthenin & ATP  clot retraction - Release of platelet growth factor  repair of the damaged vessels.

Steps of formation of platelets plug

[III] Blood coagulation It needs the following clotting factors I (Fibrinogen) II (prothrombin) III (thromboplastin) tissue factor IV (Calcium) V (Labile factor) VII (stable factor) IX (christmas factor) VIII (anti-haemophilia globulin) X (Stuart prower factor) XI (plasma thromboplastin antecadent) XII (Hageman factor) XIII (fibrin stabilizing factor) Platelet factor-3 (phospholipid) vWF High molecular weight kininogen and prekallikrein.

The clotting factors are formed in The liver Some clotting factors require vit.K in their synthesis from the liver (factor II,VII,XI and X) Megakaryocytes Share in formation of fibrinogen, Factor V, XIII and vWF. Macrophages Share in synthesis of factors V, VII, IX and X. Endothelial cells form vWF

Mechanism of blood coagulation (Coagulation cascade) (1) Intrinsic pathway by surface activator Collagen of injured blood vessel (inside the body) (2) Extrinsic pathway by tissue factor (thromboplastin) from tissue damage

Steps of blood clotting A- Extrinsic pathway: extrinsic because it depends on external factors from the tissue “tissue thromboplastin” (lipoprotein mixture)

Activation of the Blood Clotting Cascade via Tissue Factor Extrinsic pathway Activation of the Blood Clotting Cascade via Tissue Factor

B- Intrinsic pathway: 1- When the blood comes in contact with subendothelial collagen in injured blood vessel. 2- Also, subendothelial collagen activate the platelet to release platelets phospholipids

Intrinsic pathway Xa Sub-endothelial collagen XII XIIa XI XIa IX IXa X Prekallikrein IX IXa VIII VIIIa X Xa Va V XIII Prothrombin Thrombin XIIIa Fibrinogen Fibrin (soluble) Fibrin (insoluble)

Intrinsic Pathway Extrinsic Pathway Blood Vessel Injury (collagen) Tissue Injury Tissue Factor XII XIIa Thromboplastin XI XIa IX IXa VIIa VII X Xa X Thrombin Prothrombin Fribrin monomer Fibrinogen Fibrin polymer XIII

Interaction between the extrinsic and intrinsic mechanisms 1- Both pathways begins at the same time but the extrinsic mechanism takes 15-20 seconds whereas the extrinsic mechanism takes 2-10 minutes. 2- The intrinsic mechanism clots the blood in the injured vessel, whereas the extrinsic mechanism clots the blood that escapes into the tissue 3- Both pathways are complementary as factor VIIa activate IX also XIIa, X1, Xa and thrombin activate VII