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MODULE MTP09/HEMA02 – HEMATOLOGY 2 CHAPTER 1: INTRODUCTION OBJECTIVES: 1. Describe and compare the histological features of the tissues of the arteries and veins. 2. Define the term vasoconstriction. 3. Explain how vasoconstriction participates in hem...

MODULE MTP09/HEMA02 – HEMATOLOGY 2 CHAPTER 1: INTRODUCTION OBJECTIVES: 1. Describe and compare the histological features of the tissues of the arteries and veins. 2. Define the term vasoconstriction. 3. Explain how vasoconstriction participates in hemostasis. 4. Outline the general process of hemostasis in small vessels that contributes to the maintenance of vascular integrity. 5. Describe the metabolic activity of the endothelium and its role in Hemostasis. 6. Give examples and describe conditions that contribute to the defective production of blood coagulation factors. 7. Describe the clinical findings of bleeding disorders. 8. Describe the morphological features of the mature stages of development in the megakaryocyte series. 9. List the ultrastructural components and cytoplasmic constituents of a mature platelet and describe the overall function of each. 10. Define generally the terms platelet adhesion and platelet aggregation. TOPIC 1: Blood Vasculature: Structure and Function OVERVIEW OF VASCULAR BIOLOGY P a g e 1 | 21  MODULE MTP09/HEMA02 – HEMATOLOGY 2 The coagulation system has evolved to prevent excessive blood loss at sites of vascular injury. As much as the fluid phase of clotting is readily amenable to detailed molecular and biochemical dissection, thrombus formation is typically localized at the blood vessel wall. Indeed, the vascular wall plays a central role in orchestrating the hemostatic response. Thus, a full appreciation of the mechanisms of coagulation requires a sound understanding of vascular biology. Arteries and Veins Arteries are the distributing vessels that leave the heart, and veins are the collecting vessels that return to the heart. Arteries have the thickest walls of the vascular system. Although variations in the size (Fig. 23.1A) and type of vessel exist, the tissue (Fig. 23.1B) in a vessel wall is divided into three coats or tunics. These coats are the tunica intima, tunica media, and tunica adventitia. 1. The tunica intima forms the smooth glistening surface of endothelium that lines the lumen (inner tubular cavity) of all blood and lymphatic vessels and the heart. The tunica intima consists of a single layer of endothelial cells thickened by a subendothelial connective tissue layer containing elastic fibers. 2. The tunica media, the thickest coat, is composed of smooth muscle and elastic fibers. 3. The tunica adventitia consists of fibrous connective tissue that contains autonomic nerve endings. P a g e 2 | 21  MODULE MTP09/HEMA02 – HEMATOLOGY 2 Arterioles and Venules Arteries branch extensively to form a tree of ever-smaller vessels. Arterioles are the microscopic continuation of arteries that give off branches called metarterioles, which in turn join the capillaries. The walls become thinner as the arterioles approach the capillaries, with the wall of a very small arteriole consisting only of an endothelial lining and some smooth muscle surrounded by a small amount of connective tissue. The microscopically sized veins are referred to as venules. Venules connect the capillaries to the veins. Capillaries The capillaries, arterioles, and venules constitute the major vessels of the microcirculation. As a unit, the microcirculation functions as the link between the arterial and venous circulation. Blood passes from the arterial to the venous system via the capillaries. Capillaries are the thinnest walled and most numerous of the blood vessels. Capillaries are small structures consisting of a supportive basement membrane to which a single layer of endothelium is tightly anchored. VASCULATURE PHYSIOLOGY The Role of Vasoconstriction in Hemostasis Vasoconstriction is a vascular injury to a large or medium-size artery or vein requires rapid surgical intervention to prevent exsanguination. When a smaller vessel, such as an arteriole, venule, or capillary, is injured, contraction occurs to control bleeding. Vasoconstriction is a short-lived reflex reaction of the smooth muscle in the vessel wall produced by the sympathetic branches of the autonomic nervous system. This narrowing, or stenosis, of the lumen of the blood vessel decreases the flow of blood in the injured vessel and surrounding vascular bed and may be sufficient to close severed capillaries. The Role of the Endothelium The endothelium contains connective tissues such as collagen and elastin. This connective tissue matrix regulates the permeability of the inner vessel wall and provides the principal stimulus to thrombosis following injury to a P a g e 3 | 21  MODULE MTP09/HEMA02 – HEMATOLOGY 2 blood vessel. The endothelium is highly active metabolically and is involved in the clotting process by producing or storing clotting components The endothelium forms a biological interface between circulating blood elements and all the various tissues of the body. It is strategically situated to monitor systemic as well as locally generated stimuli and to adaptively alter its functional state. This adaptive process typically proceeds without notice, contributing to normal homeostasis. The endothelium is involved in the metabolism and clearance of molecules such as serotonin, angiotensin, and bradykinin that affect blood pressure regulation, the movement of fluid across the endothelium, and inflammation. Maintenance of Vascular Immunity Vascular integrity or the resistance to vessel disruption requires three essential factors. These factors are circulating functional platelets, adrenocorticosteroids, and ascorbic acid. A lack of these factors produces fragility of the vessels, which makes them prone to disruption. Maintenance of vascular integrity through the hemostatic process depends on the events previously described. The importance of these reactions varies with vessel size (e.g., capillaries seal easily because of vasoconstriction). The integrity of arterioles and venules depends on vasoconstriction, the formation of a plug of fused platelets over the injury, and the formation of a fibrin clot. Arteries, because of their thick walls, are the most resistant to bleeding; however, hemorrhage from these vessels is the most dangerous. Vasoconstriction is of ultimate importance in damaged arteries. Veins, which P a g e 4 | 21  MODULE MTP09/HEMA02 – HEMATOLOGY 2 contain 70% of the blood volume, may rupture with a slight increase in hydrostatic pressure. REFERENCE: Turgeon, Louisse.,Clinical Hematology: Theory and Principles, Chapter 23 TOPIC 2: Basic Terminology for Clinical Findings in Bleeding Disorders The hematologist is often faced with the question of whether a patient has an underlying bleeding disorder. This may be in the setting of unexplained bleeding, either spontaneous or with minor or major trauma or procedures. Or, the hematologist may be asked to evaluate a person's risk of bleeding with an upcoming procedure, particularly in cases of a personal or family history of bleeding or screening laboratory results that suggest a bleeding risk. This chapter reviews key points to help determine whether an underlying bleeding disorder is present, first focusing on the history and physical examination. Basic screening laboratory tests usually available at the time of evaluation are reviewed, including the platelet count, prothrombin time (PT), also expressed as the international normalized ratio (INR), and activated partial thromboplastin time (aPTT). Laboratory tests useful in pinpointing specific hemostatic abnormalities are then discussed. Many of the disorders highlighted here are presented in more detail in other chapters in this text. Bleeding disorders are classically divided into those of primary or secondary hemostasis: Disorders of Primary Hemostasis - Include those that affect initial platelet plug formation and typically result in mucosal bleeding, usually the result of a platelet or Von Willebrand factor (VWF) disorder. Disorders of Secondary Hemostasis - Involve disorders of fibrin formation and often manifest as soft tissue or joint bleeding. [This is commonly due to specific coagulation factor deficiencies, notably factor VIII (FVIII) and factor IX (FIX) (hemophilia A and B, respectively)]. P a g e 5 | 21  MODULE MTP09/HEMA02 – HEMATOLOGY 2 THE BLEEDING HISTORY From a detailed personal and family history, the clinician should have a good idea of whether - a bleeding disorder is present; - the disorder is inherited or acquired; - the pattern of bleeding suggests a primary or secondary hemostatic defect (or combination); and - a person's propensity to bleed has been enhanced by an underlying medical condition or exposure to medications or dietary supplements. MUCOSAL BLEEDING 1. Epistaxis - Epistaxis is a common symptom, particularly in children and in dry climates, and may not reflect an underlying bleeding disorder. Clues that epistaxis is a symptom of an underlying bleeding disorder include lack of seasonal variation and bleeding of prolonged duration (> 10 minutes), bleeding which requires medical attention, or frequent bleeding episodes. 2. Oral Mucosal Bleeding - Excessive bleeding or swelling after episodes of minor oral trauma may be a clue to an underlying bleeding disorder. Bleeding with eruption of primary teeth is seen in children with more severe bleeding disorders, such as moderate and severe hemophilia, but is uncommon in children with mild bleeding disorders. 3. Gynecologic and Obstetrical Bleeding - Gynecologic and obstetrical bleeding are common presentations in women with underlying bleeding disorders. Menorrhagia is a common symptom, but the history can be difficult as a complaint of heavy menses is subjective and has a poor correlation with excessive blood loss. 4. Other Mucosa[ Bleeding - Gastrointestinal (GI) bleeding is usually due to underlying anatomic pathology and not a hemostasis defect. However, von Willebrand disease (VWD), particularly acquired VWD and types 2 and 3, can be associated with angiodysplasia of the bowel resulting in GI bleeding. 5. Easy Bruising - The development of bruises (ecchymoses) after trauma is normal; however, an exaggerated response to trauma may be an indication of an underlying bleeding disorder. P a g e 6 | 21  MODULE MTP09/HEMA02 – HEMATOLOGY 2 6. Minor Trauma - Bleeding after minor trauma is also normal. However a persistent history of excessive or prolonged bleeding after minor wounds, such as small sharp cuts from razors, paper, or knives, particularly bleeding which lasts >5 minutes or cannot be managed by the patient themselves, may suggest an underlying bleeding disorder. 7. joint and Muscle Bleeds - Hemarthroses and spontaneous muscle or soft tissue hematomas are characteristic of moderate or severe congenital factor deficiencies, most commonly FVIII or FIX deficiency. 8. Excessive Traumatic or Surgical Bleeding - Hemorrhage in the setting of severe trauma can be due to a combination of the trauma injury itself, surgical bleeding, medications, underlying comorbidities, and acquired coagulopathies, which can have components of both transfusion-related coagulopathy (discussed below) and trauma-related coagulopathy. FAMILY HISTORY Young age at presentation, prolonged duration of symptoms, failure of previous hemostatic challenges, and positive family history can be indicators of an inherited disorder. VWD is generally inherited in an autosomal dominant pattern (except for types 2N and 3, which are recessive), but can have variable penetrance and expressivity (particularly type 1). Factor deficiencies in FVIII and FIX are sex linked, and as such affected males present with more severe disease. PHYSICAL EXAMINATION The physical examination can provide helpful clues as to the etiology of the bleeding disorder, although many patients with mild bleeding disorders will have normal examinations. The skin, mucosa, and sites of skin compromise, such as vascular access sites or surgical or traumatic wounds, should be examined for ecchymoses, petechiae, oozing, and associated soft tissue hematomas. Although there is much interest in developing a global assay of hemostasis, to date there is no single assay that can predict bleeding or thrombosis. The clinician must frequently use the results of several assays combined with the clinical presentation to arrive at a diagnosis or an assessment of bleeding risk. The primary use of coagulation testing should be to confirm the presence and type of bleeding disorder in a patient with a suspicious clinical history. P a g e 7 | 21  MODULE MTP09/HEMA02 – HEMATOLOGY 2 Most coagulation assays are performed in sodium citrate anticoagulated plasma that is recalcified for the assay. Because the anticoagulant is in liquid solution and needs to be added to blood in proportion to the plasma volume, incorrectly filled or inadequately mixed blood collection tubes will give erroneous results. Other patient factors can influence laboratory results. An elevated hematocrit (>55%) can result in an incorrect coagulation measurement because of a decreased plasma-to-anticoagulant ratio. An accurate result is obtained by adjusting the amount of sodium citrate solution added to the blood on the basis of the estimated plasma volume, using the formula: (100 - Hematocrit ) x (total tube volume) ( 595 - Hematocrit) = Volume of Na Citrate(3.2% )required Complete Blood Count and Peripheral Smear The complete blood count (CBC) is a necessary component of the evaluation of the bleeding patient and is often instrumental in identifying an underlying platelet defect or evidence of chronic or ongoing blood loss. Thrombocytopenia is often first identified by automated cell counting. An assessment of reticulated platelets or estimation of average platelet size from the mean platelet volume can also be useful clues to the underlying cause of thrombocytopenia. Red cell indices and morphology may exhibit abnormalities relevant to the bleeding patient. Hypochromia and microcytosis may be evidence of iron deficiency and suggest chronic blood loss, while schistocytes reflect an underlying microangiopathy (such as DIC) Prothrombin Time (PT) P a g e 8 | 21  MODULE MTP09/HEMA02 – HEMATOLOGY 2 The PT, developed by Quick et al. in 1935, assesses the classic extrinsic coagulation system, including FI (fibrinogen), FII (prothrombin), FV, FVII, and FX. The PT measures the time for clot formation of the citrated plasma after recalcification and addition of thromboplastin. Thromboplastin is a combination of tissue factor, the membrane receptor for FVII and FVIla, and phospholipids. The speed of the PT reaction is influenced not only by the plasma concentrations of the extrinsic and common pathway clotting factors but also by the concentration and properties of the tissue factor. In the past, the use of various thromboplastins and instruments resulted in varying levels of anticoagulation when the PT ratio was used to target warfarin therapy. To adjust for this variability, the overall sensitivity of different thromboplastins to reduction of the vitamin K-dependent clotting factors (Fil, FVII, FIX, and FX) in anticoagulated patients is now expressed as the international sensitivity index (ISI). The INR is then determined based on the formula: Although the INR was developed to assess anticoagulation due to reduction of vitamin K-dependent coagulation factors, it can be helpful in the evaluation of patients with liver disease, particularly when comparing values from testing performed at different laboratories. The INR is also generally not a reliable measure of warfarin anticoagulation in patients with an elevated baseline INR, such as in the case of antiphospholipid antibodies directed against prothrombin, which can prolong the PT at baseline. Activated Partial Thromboplastin Time (APTT) P a g e 9 | 21  MODULE MTP09/HEMA02 – HEMATOLOGY 2 Like the PT/INR, the aPTT is performed on recalcified citrated plasma. It assesses the intrinsic and common coagulation pathways and was originally described as a "partial thromboplastin" time because the reagents did not induce hemophilic plasma to clot in a normal period of time as do "complete" thromboplastins. The aPTT test system also includes an activator of the intrinsic coagulation system, such as nonparticulate ellagic acid or the particulate activators kaolin, celite, or micronized silica. These provide a large surface area and increase the precision and reproducibility of the test. The aPTT is commonly prolonged in patients with antiphospholipid antibodies, and reagents vary widely in their sensitivity to these antibodies. Heparin contamination is another common iatrogenic or artifactual cause of a prolonged aPTT, particular in hospitalized patients, where it also may be monitored by its anti-Xa activity. Thrombin Clotting Time (TCT) Thrombin clotting time (TCT), or simply thrombin time, is a simple test that measures the time for clot formation to occur in citrated plasma after the addition of thrombin. It therefore reflects the action of thrombin on fibrinogen with the formation of fibrin. During this process, thrombin cleaves the α and β chains of fibrinogen at their amino-termini, releasing fibrinopeptides A and B. The residual fibrin monomer of one fibrinogen molecule interacts with others to form polymerized fibrin. A prolonged TCT is caused by a deficiency or structural abnormality (dysfibrinogenemia) of fibrinogen or by the presence of an inhibitor of the thrombin-fibrinogen reaction. It is abnormal in patients with familial hypofibrinogenemia or dysfibrinogenemia and in patients with acquired deficiencies of fibrinogen (as in decreased production due to liver dysfunction, increased consumption due to DIC, or acquired dysfibrinogenemia). Monoclonal gammopathies, particularly IgM and IgA isotypes, also interfere with fibrin polymerization both in the TCT and in vivo. Clinically, the most important inhibitor causing prolongation of the TCT is unfractionated heparin. Ecarin Clotting Time Ecarin is a metalloprotease isolated from the viper Echis carinatus that cleaves prothrombin to generate meizothrombin, a proteolytically active intermediate form of thrombin. P a g e 10 | 21  MODULE MTP09/HEMA02 – HEMATOLOGY 2 Reptilase Time The snake venom reptilase (from Bothrops atrox) induces fibrin clot formation in plasma by a direct action on fibrinogen. In contrast to the action of thrombin, which cleaves fibrinopeptides A and B, reptilase cleaves only fibrinopeptide A. Mixing Studies Mixing studies are used to evaluate a prolonged aPTT, or less commonly PT, to distinguish between a factor deficiency and an inhibitor. In this assay, normal plasma and patient plasma are mixed in a 50:50 ratio, and the aPTT or PT is determined immediately after incubation at 37°C at varying time points, typically 30 minutes, 60 minutes, and/or 120 minutes. With isolated factor deficiencies, the aPTT will correct with mixing and stay corrected with incubation. With aPTT prolongation due to an antiphospholipid antibody, the mixing and incubation will show no correction.34 In acquired neutralizing factor antibodies, such as an acquired FVIII inhibitor, the initial assay may or may not correct immediately after mixing but will prolong or remain prolonged with incubation at 37°C. Failure to correct with mixing can also be caused by the presence of other inhibitors or interfering substances, such as heparin, fibrin split products, and paraproteins. P a g e 11 | 21  MODULE MTP09/HEMA02 – HEMATOLOGY 2 REFERENCE: Marder, Victor.,Hemostasis and Thrombosis, 6th edition, Chapter 50 P a g e 12 | 21  MODULE MTP09/HEMA02 – HEMATOLOGY 2 TOPIC 3: Historical Background of Hemostasis Hemostasis is derived from a Greek word, which means stoppage of blood flow. The process is a combination of cellular and biochemical events that function together to keep blood in the liquid state within the veins and arteries and prevent blood loss following injury through the formation of a blood clot.1,2 It consists of a complex regulated system which is dependent on a delicate balance among several systems. The systems involved in the hemostatic process include the vascular system, coagulation system, fibrinolytic system, platelets, kinin system, serine protease inhibitors, and the complement system. The systems work together when the blood vessel endothelial lining is disrupted by mechanical trauma, physical agents, or chemical trauma to produce clots. The clots stop bleeding and are eventually dissolved through the fibrinolytic process. As a result, there is a delicate balance between the production and dissolution of clot during the hemostatic process. A disruption of this balance may precipitate thrombosis or hemorrhage as a result of hypercoagulation or hypocoagulation, respectively P a g e 13 | 21  MODULE MTP09/HEMA02 – HEMATOLOGY 2 TOPIC 4: Historical Development of Clinical Hemostasis Open the link below to view the image: https://www.stago- us.com/hemostasis/history-of- coagulation/ P a g e 14 | 21  MODULE MTP09/HEMA02 – HEMATOLOGY 2 Initial discoveries  Theories on the coagulation of blood have existed since antiquity. Physiologist Johannes Müller (1801–1858) described fibrin, the substance of a thrombus.  Its soluble precursor, fibrinogen, was thus named by Rudolf Virchow (1821–1902),  Isolated chemically by Prosper Sylvain Denis (1799–1863).  Alexander Schmidt suggested that the conversion from fibrinogen to fibrin is the result of an enzymatic process, and labeled the hypothetical enzyme "thrombin" and its precursor "prothrombin".  Arthus discovered in 1890 that calcium was essential in coagulation.  Platelets were identified in 1865, and their function was elucidated by Giulio Bizzozero in 1882.  The theory that thrombin is generated by the presence of tissue factor was consolidated by Paul Morawitz in 1905. At this stage, it was known that thrombokinase/thromboplastin (factor III) is released by damaged tissues, reacting with prothrombin (II), which, together with calcium (IV), forms thrombin, which converts fibrinogen into fibrin (I). P a g e 15 | 21  MODULE MTP09/HEMA02 – HEMATOLOGY 2 TOPIC 5: Megakaryopoiesis Mature platelets (thrombocytes), metabolically active cell fragments, are the second critical component in the maintenance of hemostasis. These anuclear cells circulate in the peripheral blood after being produced from the cytoplasm of bone marrow megakaryocytes, the largest cells found in the bone marrow. Bone marrow megakaryocytes are derived from pluripotential stem cells. The sequence of development from megakaryocytes to platelets is thought to progress from the proliferation of progenitors to polyploidization, that is, nuclear endoreduplication, and fi nally to cytoplasmic maturation and the formation of platelets. A. Stages of Development The Developmental Sequence of Platelets Early Development Two classes of progenitors have been identified: - The burst-forming-unit megakaryocyte (BFU-M) and the colony-forming-unit megakaryocyte (CFU-M). The BFU-M is the most primitive progenitor cell committed to megakaryocyte lineage. - The next stage of megakaryocyte development is a small, mononuclear marrow cell that expresses platelet- specific phenotypic markers but is not morphologically identifiable as a megakaryocyte. These transitional cells represent 5% of marrow megakaryocyte elements. Some transitional immature megakaryocyte cells may be capable of cellular division, but most are nonproliferating while actively undergoing endomitosis. Megakaryocytes The final stage of megakaryocyte development is the morphologically identifiable megakaryocyte. These cells are readily recognizable in the P a g e 16 | 21  MODULE MTP09/HEMA02 – HEMATOLOGY 2 marrow because of their large size and lobulated nuclei. These cells are polyploid (Table 23.4). Megakaryocytes are the largest bone marrow cells, ranging up to 160 mm in size. The nuclear-cytoplasmic (N:C) ratio can be as high as 1:12. Nucleoli are no longer visible. A distinctive feature of the megakaryocyte is that it is multilobular, not multinucleated. The fully mature lobes of the megakaryocyte shed platelets from the cytoplasm on completion of maturation. Platelet formation begins with the initial appearance of a pink color in the basophilic cytoplasm of the megakaryocyte and increased granularity. Mature Platelets Platelets have an average diameter of 2 to 4 mm, with younger platelets being larger than older ones. In contrast to megakaryocytes, platelets have no nucleus. The cytoplasm is light blue, with evenly dispersed, fi ne red- purple granules. Platelets circulate at the center of the flowing bloodstream through endothelium-lined blood vessels without interacting with other platelets or with the vessel wall. Platelets are extremely sensitive cells and may respond to minimal stimulation by forming pseudopods that spontaneously retract. Stronger stimulation causes platelets to become sticky without losing their discoid shape; however, changes in shape to an irregular sphere with spiny pseudopods will occur with additional stimulation. This alteration in cellular shape is triggered by an increase in the level of cytoplasmic calcium. Such changes in shape accompanied by internal cellular contractions can result in the release of many of the internal organelles. A loss of viability is associated with this change to a spiny sphere. P a g e 17 | 21  MODULE MTP09/HEMA02 – HEMATOLOGY 2 B. Platelet Structure The Glycocalyx Ultrastructure examination of the platelet reveals that the cellular membrane is surrounded externally by a fluffy coat or glycocalyx. This glycocalyx is unique among the cellular components of the blood. It is composed of plasma proteins and carbohydrate molecules that are related to the coagulation, complement, and fi brinolytic systems. The glycoprotein receptors of the glycocalyx mediate the membrane contact reactions of platelet adherence, change of cellular shape, internal contraction, and aggregation. Cytoplasmic Membrane Adjacent to the glycocalyx is the cytoplasmic membrane whose chemical composition and physical structure. Extending through the plasma membrane and into the interior of the platelet is an open canalicular or surface- connecting system. It is this system that forms the invaginated, sponge-like portion of the cell that provides an expanded reactive surface to which plasma clotting factors are selectively adsorbed. Microfi laments and Microtubules Directly beneath the cell membrane is a series of submembrane filaments and microtubules that form the cellular cytoskeleton. In addition to providing the structure for maintaining the circulating discoid shape of the cell, the cytoskeleton also maintains the position of the organelles. Granules Three different types of storage granules related to hemostasis are present in the mature platelet. These granules are alpha granules, dense or delta granules, and lysosomes. The alpha granules are the most abundant. Alpha P a g e 18 | 21  MODULE MTP09/HEMA02 – HEMATOLOGY 2 granules contain heparin-neutralizing platelet factor 4 (PF 4), beta- thromboglobulin, platelet-derived growth factor, platelet fibrinogen, fi bronectin, von Willebrand factor (vWF), and thrombospondin. C. Platelet Functions Platelets normally move freely through the lumen of blood vessels as components of the circulatory system. Maintenance of normal vascular integrity involves nourishment of the endothelium by some platelet constituents or the actual incorporation of platelets into the vessel wall. This process requires less than 10% of the platelets normally in the circulating blood. For hemostasis to occur, platelets not only must be present in normal quantities but also must function properly. This section discusses the hemostatic functions of platelets, including platelet adherence and aggregation. Platelet Adhession If vascular injury exposes the endothelial surface and underlying collagen, platelets adhere to the subendothelial collagen fi bers, spread pseudopods along the surface, and clump together (aggregate). Platelet adhesion to subendothelial connective tissues, especially collagen, occurs within 1 to 2 minutes after a break in the endothelium. Epinephrine and serotonin promote vasoconstriction. ADP increases the adhesiveness of platelets. Considerable evidence indicates that the adhesion and aggregation of platelets are mediated by the binding of large soluble macromolecules to distinct glycoprotein receptors anchored in the platelet membrane. This increase in adhesiveness causes circulating platelets to adhere to those already attached to the collagen. The result is a cohesive platelet mass that rapidly increases in size to form a platelet plug. Platelet aggregation Platelet aggregation is the gold standard test to determine platelet function. Platelet aggregation in vivo is a much more complex and dynamic process than previously thought. Over the last decade, it has become clear that platelet aggregation represents a multistep adhesion process involving distinct receptors and adhesive ligands, with the contribution of individual receptor-ligand interactions to the aggregation process dependent on the prevailing blood fl ow conditions. It is now believed that three distinct mechanisms can initiate platelet aggregation. P a g e 19 | 21  MODULE MTP09/HEMA02 – HEMATOLOGY 2 Platelet Plug consolidation and Stabilization The permanently anchored platelet plug requires additional consolidation and stabilization. Fibrinogen, under the influence of small amounts of thrombin, provides the basis for this consolidation and stabilization. This process involves the precipitation of polymerized fibrin around each platelet. The result is a fibrin clot that produces an irreversible platelet plug. REFERENCE: Turgeon, Louisse.,Clinical Hematology: Theory and Principles, Chapter 23 P a g e 20 | 21  MODULE MTP09/HEMA02 – HEMATOLOGY 2 For further reading please refer to the link provided: Biology Help: Arteries - Structure and Function https://www.youtube.com/watch?v=L2TySbJce1k Pupura | Bleeding Disorders https://www.youtube.com/watch?v=M2hrszR32lk Hemophilia - causes, symptoms, diagnosis, treatment, pathology https://www.youtube.com/watch?v=nkC1vZaUpxs Hemostasis Introduction https://www.youtube.com/watch?v=1_-ZPbq-vDI The Megakaryocytes https://www.youtube.com/watch?v=yMwYyhmgdS8 Platelet Structure | Thrombocytes Are The Babies of Megakaryocytes https://www.youtube.com/watch?v=95A-j-eL9B8 Platelet Adhesion and Aggregation https://www.youtube.com/watch?v=0pnpoEy0eYE P a g e 21 | 21