MLAB 1227: C OAGULATION K ERI B ROPHY -M ARTINEZ Overview of Hemostasis: Part Two

F UNCTION OF P LATELETS Surveillance of blood vessel continuity Checks endothelial lining for gaps and breaks Fill-in small gaps caused by separation of endothelial cells Formation of primary hemostatic plug Surface for coagulation factors to make secondary hemostatic plug Aid in healing injured tissue

F ORMATION OF P RIMARY H EMOSTATIC P LUG Once the platelets “normal” environment is changed, they become activated or adhesive Stages of plug formation Adhesion Secretion Aggregation Stabilization

S TAGE 1: P LATELET A DHESION Platelets attach to non-platelet surfaces, such as collagen fibers in the subendothelium Platelets move from the blood vessels and into the tissues. Exposure to surfaces in the tissues causes them to bind to collagen with the presence of von Willebrand factor ( vWF) through the Glycoprotein Ib/IX receptor Results in a bridge formation, which triggers a shape change in the platelets Reversible No energy required

P LATELET A CTIVATION Platelets undergo a shape change from disc to spiny sphere with projections Activation required for 1 O hemostatic plug formation Activation continues until Ca ++ threshold met Platelets remain localized in injured area

A CTIVATION Platelet shape change after exposure to agonist. Agonists include: ADP, Serotonin, collagen, thrombin

S TAGE 2: P LATELET S ECRETION / P LATELET RELEASE REACTION Secretion Requires ATP Provides a positive feedback by releasing more agonists to stimulate more plt receptors Release of Granule contents Causing vasoconstriction Release of ADP(agonist)= increased Ca +, plt release, increase in fibrinogen receptors Trigger a secondary aggregation which is irreversible

W HAT ’ S IN A G RANULE ? Granules consist mainly of: Alpha granules: Factor V, vWF, Fibrinogen, platelet factor 4, β-thromboglobulin Factor V: receptor on platelet surface for factor Xa & prothrombin PF4: heparin neutralizing factor Dense bodies: ATP, ADP, serotonin, Ca

S IDE NOTE Heparin is used on patients who clot excessively. Endothelial cells make heparin-like molecules and expose them on their surface. PF4 binds these substances. Heparin can complex with bound PF4 and heparin will be neutralized.

S TAGE 3: P LATELET AGGREGATION Chemical changes cause platelets to aggregate and stick to one another Newly arriving platelets become activated by agonists Exposure of GPIIb/IIIa sites bind fibrinogen Fibrinogen + activated platelets serves as a bridge between two platelets Calcium must be present

A DHESION & A GGREGATION

F INAL S TAGE : S TABILIZATION OF C LOT AKA: primary hemostatic plug formation Thrombus formation Platelets release Factor V Expose factor III (TF) Accelerating coagulation cascade Promote activation of clotting factors

S TAGES OF 1 O PLUG FORMATION

B LOOD CLOT

C OAGULATION S YSTEM  Composed of 14 coagulation factors (serine proteases) which are interdependent (Factors I through XIII – there is no Factor VI – and PK and HMWK) ◦ Inactive form of each is an enzyme precursor which is usually designated by a Roman numeral but also given a name – Ex. Factor I fibrinogen. Numbers correspond to order of discovery NOT order in cascade. ◦ Active forms are usually designated by the letter “a” after the Roman numeral and may also have a different name – Ex. Ia Fibrin ◦ Cofactors are needed for many reactions in the cascade – Ex. Calcium, platelet factor 3 (PF 3 ) ◦ Each molecule must be present in sufficient quantity as well as functioning normally  Final product is fibrin mesh or clot which completely stops bleeding ◦ Secondary hemostasis  Slow contraction or retraction and lysis of the clot occurs

F IBRINOLYTIC S YSTEM Plasminogen is converted to plasmin Plasmin enzymatically attacks the fibrin molecule producing fibrin degradation products (FDPs, sometimes called FSPs) that are cleared from the circulation by macrophages Fibrin is a product formed during hemostasis, tissue repair or inflammation Fibrin plays a temporary role Once injury heals, the fibrin clot is lysed

C OAGULATION I NHIBITION S YSTEM Provides balance and control of clotting mechanisms Natural inhibitors and anticoagulants circulate in the plasma to: Prevent clotting when it’s not needed Limit or localize the clotting that is needed Examples: Protein C and S, antithrombin III

R EFERENCES McKenzie, Shirlyn B., and J. Lynne. Williams. "Chapter 29." Clinical Laboratory Hematology. Boston: Pearson, Print. Platelet Research Laboratories. Platelet Function. Retrieved from research.org/1/function_hemo.htm.