Von Willebrand Factor (Answer: B)
The correct answer is B. Von Willebrand's Factor (vWF) is the compound secreted by vascular endothelial cells that promotes platelet adhesion to sub-endothelial collagen.
Mechanism of Action
vWF serves as the essential molecular bridge between exposed subendothelial collagen and platelet surface receptors, making it the primary mediator of platelet adhesion following vascular injury. 1
Synthesis and Storage
- vWF is synthesized in endothelial cells and stored in specialized organelles called Weibel-Palade bodies 1
- Upon vascular injury or various stimuli, vWF is released from these storage granules into circulation 1
- The primary function of vWF is the promotion of platelet adhesion by anchoring platelets to the subendothelial matrix 1
Functional Mechanism
- Platelet adhesion is initiated by cross-linking platelet surface glycoprotein Ib (GPIb) to exposed subendothelial collagen through vWF 2
- This GPIb-vWF interaction constitutes the critical first step of the platelet response to vascular injury and precedes both platelet aggregation and the release reaction 2
- Under high shear stress conditions (>1000 s⁻¹), vWF undergoes conformational changes from a globular to an unfolded state, exposing binding sites for platelet GPIb 3
Why the Other Options Are Incorrect
Platelet Derived Growth Factor (Option A)
- PDGF is a mitogenic factor involved in cell proliferation and wound healing, not in the immediate adhesion of platelets to collagen
- It does not function as a bridge between platelets and subendothelial matrix
Calcium (Option C)
- While calcium is essential for platelet activation and coagulation cascade function, it is not secreted by endothelial cells specifically to promote platelet adhesion
- Calcium acts as a cofactor in multiple steps but is not the adhesive molecule itself
Prostacyclin/PGI2 (Option D)
- Prostacyclin is actually an anti-thrombotic agent secreted by endothelial cells
- It inhibits platelet activation and promotes vasodilation, functioning opposite to what the question asks
- This is a common pitfall—prostacyclin prevents rather than promotes platelet adhesion
Clinical Relevance
The vWF-collagen-platelet axis is critical for normal hemostasis 4. Deficiency or dysfunction of vWF leads to von Willebrand disease, the most common inherited bleeding disorder, characterized by impaired platelet adhesion and prolonged bleeding times 1.