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Platelets adhere to exposed subendothelium through von Willebrand factor (vWF), a large multimeric adhesive protein that bridges platelets to collagen. Upon activation (by thrombin, ADP, collagen), platelets change shape, secrete granule contents, and expose phosphatidylserine for tenase complex assembly. Platelet aggregation is mediated by fibrinogen bridging across GPIIb/IIIa integrin. Von Willebrand disease results from deficiency or dysfunction of vWF, causing defective adhesion and often factor VIII deficiency (vWF carries factor VIII).
Study the three phases of platelet activation: adhesion (vWF-mediated), activation (agonist-induced shape change and secretion), and aggregation (fibrinogen-mediated crosslinking). Understand why vWF multimers matter—larger multimers are more thrombotic.
Platelets are not 'miniature cells'—they lack a nucleus and cannot synthesize proteins de novo. vWF deficiency leads to prolonged bleeding time (not clotting time), reflecting defective primary hemostasis. Type 2 vWD has complex genetics and variable phenotypes.
From your study of hemostasis, you know that stopping bleeding requires two sequential processes: primary hemostasis (the platelet plug) and secondary hemostasis (the coagulation cascade producing fibrin). Platelet function sits entirely within primary hemostasis, but it is not a single step — it is a coordinated three-phase program: adhesion, activation, and aggregation.
Adhesion is the problem-solving phase. Resting platelets do not stick to intact endothelium — the endothelial surface actively repels them through nitric oxide and prostacyclin (PGI2) secretion. When a vessel is damaged, subendothelial collagen and von Willebrand factor (vWF) are exposed. At the high shear stress of arterial flow, free vWF unfolds and changes conformation, binding collagen on one end and platelet GPIb receptors on the other. This bridging function is the reason vWF exists: simple diffusion-based collagen-platelet interactions would be too slow and too weak at arterial flow rates. vWF's effectiveness scales with its size — the largest ultra-large multimers (ULvWF, released from Weibel-Palade bodies during endothelial activation) are the most thrombogenic, and the ADAMTS13 enzyme that cleaves them into smaller forms is an important regulatory brake. When ADAMTS13 fails, ULvWF accumulates and drives pathological microvascular thrombosis — the mechanism of thrombotic thrombocytopenic purpura (TTP).
Activation transforms the adherent platelet from a passive disc into an active signaling cell. Collagen, thrombin, ADP, and TXA2 all converge on intracellular signaling cascades that produce three simultaneous outputs: shape change (the platelet extends pseudopods, dramatically increasing surface area), degranulation (alpha granules releasing fibrinogen, vWF, factor V, and P-selectin; dense granules releasing ADP, serotonin, and calcium to recruit more platelets), and phosphatidylserine (PS) flip (the inner leaflet phospholipid migrates to the outer leaflet, providing the anionic surface required for the tenase and prothrombinase complexes of the coagulation cascade). The last step is the molecular bridge between primary and secondary hemostasis — platelet activation directly enables coagulation by providing the phospholipid scaffold.
Von Willebrand disease (vWD) is the most common inherited bleeding disorder, and its clinical presentation illustrates which hemostatic system is affected. Because vWF mediates the initial adhesion step of primary hemostasis, vWD patients present with mucocutaneous bleeding — nosebleeds, gum bleeding, heavy menstrual periods, and prolonged bleeding from minor cuts. This is the classic presentation of a platelet plug defect, in contrast to coagulation factor deficiencies (hemophilia A/B) which present with deep tissue bleeding — hemarthroses, intramuscular hematomas. Laboratory testing reflects this: vWD prolongs the bleeding time and PFA-100 closure time (tests of primary hemostasis) but initially leaves the PT and PTT normal — unless the vWF deficiency is severe enough to reduce factor VIII levels, since vWF normally carrier-protects factor VIII from premature proteolysis. Type 1 vWD (partial quantitative deficiency) is mild and common; Type 3 (near-absent vWF) is severe and rare; Type 2 involves qualitative defects with variable clinical severity depending on which aspect of vWF function is disrupted.