FFP2 Platelet and Vessel Wall Interactions 2026
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Flashcards in this deck (48)

  • What is the primary role of platelets in the vasculature?

    • Circulate as quiescent cells and survey vascular integrity
    • Undergo explosive activation upon vessel wall damage

  • How are platelets described metaphorically in the lecture?

    They are described as the 'unmanned drones of the vasculature'

  • What is the cellular origin and nuclear status of platelets?

    • Anucleate discs derived from megakaryocyte

  • What is the typical size of platelets according to the notes?

    Very small - 2-4

  • What is the approximate lifespan of a platelet?

    Long life-span ~ 10 days

  • What is the normal platelet count range provided in the notes?

    Normal platelet count: 150 - 400 x 10 /litre

  • Name the main ultrastructural components shown in the platelet diagram.

    • Glycogen
    • Phospholipid membrane
    • Open canalicular system
    • Mitochondrion
    • Dense granules
    • Alpha granules

  • List the platelet surface receptors provided in the text.

    • Gp VI
    • Gp Ib-IX-V complex
    • Gp IIb-IIIa complex

  • Which substances are contained in platelet dense granules according to the text?

    • Ca
    • ADP / ATP
    • Serotonin

  • Which substances are listed as contents of the (alpha) granules in the text?

    • VWF
    • Fibrinogen
    • Factor V
    • PF4

  • What is Von Willebrand factor (VWF)?

    • Globular multimeric circulating plasma protein

  • Which binding sites does VWF contain?

    • Binding sites for platelets
    • Binding sites for sub-endothelial tissue

  • What is required for VWF binding sites to become exposed?

    • VWF must be 'tethered' for its binding sites to be exposed

    VWF unwinding and tethering illustration

  • What subunits compose the GPIb‑V‑IX complex receptor on platelets?

    • GPIba
    • GPIb
    • GPIX
    • GPV

  • How is the GPIb‑V‑IX complex connected within the platelet?

    • It is coupled to the underlying cytoskeleton.

  • Which molecular interactions are shown for the GPIb‑V‑IX complex in the schematic?

    • Interactions with VWF, P‑selectin, Thrombin, Filamin, and Calmodulin

    GPIb‑V‑IX schematic

  • What is the VWF-GPIba complex?

    • Interaction between a circulating plasma protein (Von Willebrand Factor; VWF) and a platelet surface receptor (glycoprotein Ib; GPIba).

  • What is the primary function of the VWF-GPIba complex?

    • It is crucial for platelet adhesion in high shear conditions.

  • What are the sequential steps by which von Willebrand factor (VWF) mediates platelet adhesion at a vascular injury site?

    • VWF becomes tethered to the site of vascular injury
    • VWF unravels, exposing binding sites for GPIbα
    • Platelets bind to the site of injury via GPIb

    VWF tethering illustration

  • Which change in VWF exposes platelet-binding sites for GPIbα?

    VWF unravels, which exposes binding sites for GPIbα.

  • What is Von Willebrand disease (VWD) and how common is it?

    • Von Willebrand disease (VWD): the most common bleeding disorder
    • Prevalence: ~1 in 1000

  • What general cause underlies Von Willebrand disease according to the text?

    • Cause: Genetic defects in von Willebrand factor (VWF)

  • List the specific functional consequences of VWF genetic defects described in the text.

    • Consequences:
    • Low VWF levels
    • Inability of VWF to bind collagen or platelets
    • Failure to bind sub-endothelial tissue or platelets

  • What is Bernard-Soulier syndrome?

    • Bernard-Soulier syndrome is a rare genetic defect in GPIba that prevents normal von Willebrand factor (VWF) interaction.

  • How does the GPIba defect in Bernard-Soulier syndrome affect platelet function?

    • The GPIba defect prevents normal VWF–GPIb interaction, which prevents normal thrombus formation.

  • What is the primary clinical consequence of Bernard-Soulier syndrome?

    • A bleeding tendency due to impaired thrombus formation.

  • What are the main steps of platelet activation?

    • Change in platelet shape
    • Activation of cell surface integrins (Gpllb/llla)
    • Granule secretion
    • 'Flip/Flop' of plasma membrane

  • Which cell surface integrin is listed as activated during platelet activation?

    • Gpllb/llla

  • What are the purposes of platelet 'activation' in haemostasis?

    • Adopt structural changes to help clot formation
    • Secretion of factors that facilitate clot formation
    • Activate cell surface molecules to make them 'sticky' for other platelets and coagulation factors

  • When do platelets undergo a change in shape during adhesion?

    Within seconds of adhesion

  • What morphological changes do platelets exhibit after adhesion?

    • Spherical
    • Pseudopodia

  • What cytoskeletal change underlies platelet shape change?

    Reorder of actin and tubulin polymers of the cytoskeleton

    Answer image: SEM images of platelets in different states

  • What are the main platelet granule types and their primary contents/functions?

    • Dense granules: rich in chemical messengers that promote further platelet activation and aggregation
    • Alpha granules: rich in clotting proteins and contribute to further clot formation

  • What is the open canalicular system (OCS) in platelets?

    An extensive system of internal membrane tunnels that serves as a conduit system for granule release.

  • What structural feature on the platelet surface forms the entrances into the OCS?

    • Periodic invaginations on a flat platelet surface

    SEM images of platelet surface showing invaginations leading to the OCS

  • What is the primary clinical use of inhibitors of platelet activation?

    • To prevent arterial thrombosis (lesions rich in aggregated platelets)

  • What is the most common example of an inhibitor of platelet activation?

    • Aspirin

    Photograph of an aspirin pill

  • By what mechanism do inhibitors of platelet activation act?

    • They inhibit signaling pathways that lead to platelet activation

  • What does 'flip-flop' of phospholipids refer to in platelet membranes?

    The transverse movement where negatively charged phospholipids that were on the inner leaflet become externalized to the outer leaflet of the platelet membrane.

  • Why is phospholipid externalization on activated platelets important for coagulation?

    • It is necessary for vitamin K-dependent coagulation factor binding

    lipid bilayer diagram

  • What triggers activation of the platelet membrane glycoprotein complex GPIIb/IIIa and what is its primary function?

    • Trigger: Platelet activation causes membrane activation of the glycoprotein complex IIb/IIIa
    • Primary function: Acts to cause platelet aggregation

  • Which major ligands does the platelet receptor GPIIb/IIIa bind?

    • Ligands: von Willebrand factor (VWF) and fibrinogen

    GPIIb-IIIa complex diagram

  • What is the consequence of GPIIb/IIIa deficiency for platelet function at a vessel injury site?

    • Platelet plug fails to accumulate at the site of vessel injury when GPIIb/IIIa is absent.

  • Which clinical disorder is caused by deficiency of the GPIIb/IIIa receptor?

    • Glanzmann's thrombasthenia

  • How does platelet aggregation compare between wild-type and beta3-deficient (3-/-) subjects in the provided experiment?

    • Beta3-deficient (3-/-) subjects show reduced platelet aggregation compared with wild-type (wt).

    Answer image: Panel showing reduced aggregation in beta3-deficient subjects

  • What is abciximab classified as?

    • A monoclonal antibody directed against GPIIb/IIIa

  • What is the primary mechanism of action of abciximab on platelets?

    • Blocks GPIIb/IIIa, thereby inhibiting platelet aggregation

  • What is the main clinical use of abciximab described in the text?

    • Prevents formation of blood clots during procedures to open blocked arteries
Study Notes

Platelets — quick overview

  • Definition: Small, anucleate cell fragments from megakaryocytes that patrol the vasculature and respond to injury.
  • Size: \(2\)–\(4\,\mu m\).
  • Lifespan: \(\sim 10\ \text{days}\).
  • Normal count: \(150\)–\(400\times 10^9\ \text{L}^{-1}\).
  • Function: maintain vascular integrity, form primary haemostatic plug after injury.

Ultrastructure & key components

  • Major compartments: phospholipid membrane, open canalicular system (OCS), mitochondria, dense granules, alpha granules.
  • Surface receptors (important): GPIb-IX-V complex, GPVI, GPIIb/IIIa.
  • Dense granules: contain Ca2+, ADP/ATP, serotonin — promote activation and recruitment.
  • Alpha granules: contain VWF, fibrinogen, factor V, platelet factor 4 — supply clotting and adhesive proteins.

Image — platelet ultrastructure

Platelet ultrastructure schematic

How platelets find and stick to injured vessel (adhesion)

  • First step: exposed subendothelial matrix (e.g., collagen) at injury site captures circulating proteins and platelets.
  • Key mediator: Von Willebrand factor (VWF) — a multimeric plasma protein that must tether/unravel to expose platelet-binding sites.
  • When VWF unravels it exposes binding sites for GPIba (part of the GPIb-IX-V complex) and for collagen.

Image — VWF unwinding

VWF unwinding and tethering to subendothelium

GPIb-IX-V complex (platelet receptor)

  • Composed of subunits GPIba, GPIb, GPIX, GPV and linked to the cytoskeleton.
  • Function: binds unfolded/tethered VWF under high shear, initiating platelet tethering and slowing of platelets near the lesion.

Image — GPIb-IX-V schematic

GPIb-IX-V receptor schematic

VWF–GPIba interaction — why it matters

  • Under high shear (arteries), direct platelet adhesion to collagen is inefficient; VWF acting through GPIba is essential.
  • If VWF tethering or GPIba binding is impaired, platelet adhesion at sites of high shear fails and bleeding occurs.

Important clinical defects in adhesion

  • Von Willebrand disease (VWD): most common bleeding disorder; caused by quantitative (low VWF) or qualitative (impaired binding to collagen/platelets) defects in VWF.
  • Bernard–Soulier syndrome: rare genetic defect in GPIb\/a (GPIb-IX-V) that prevents normal VWF interaction and causes bleeding tendency.

Platelet activation — overview

  • Activation converts a quiescent platelet into a haemostatic cell capable of adhesion, secretion, and aggregation.
  • Four main changes on activation:
  • Shape change (discoid → spherical with pseudopodia).
  • Activation of integrins (notably GPIIb\/IIIa).
  • Granule secretion (dense and alpha granules).
  • Phospholipid "flip-flop" exposing anionic surface.

Image — platelet shape change (SEM)

Platelet states: rest, round, active

1) Shape change

  • Occurs within seconds after adhesion; actin and microtubule reorganisation produces spherical shape and filopodia to increase surface contact and stability.

2) Granule release

  • Dense granules: release small-molecule agonists (ADP, Ca2+, serotonin) that amplify activation and recruit platelets.
  • Alpha granules: release adhesive and clotting proteins (VWF, fibrinogen, factor V, PF4) to support aggregation and coagulation.

Open canalicular system (OCS)

  • Membrane invaginations forming tunnels linking interior granules to the surface; acts as a conduit for rapid secretion and membrane expansion during activation.

Image — OCS (platelet surface invaginations)

Open canalicular system invaginations

3) Phospholipid "flip-flop"

  • Activation externalises negatively charged phospholipids (e.g., phosphatidylserine) previously on the inner leaflet.
  • This anionic surface is required for assembly/binding of several coagulation complexes and vitamin K–dependent factors on the platelet surface.

Image — lipid bilayer and flip-flop

Lipid bilayer and transverse diffusion

4) GPIIb\/IIIa activation and platelet aggregation

  • GPIIb\/IIIa (integrin αIIbβ3) undergoes a conformational change on activation, allowing it to bind fibrinogen and VWF and cross-link adjacent platelets, forming an aggregate.
  • Without functional GPIIb\/IIIa, platelets cannot form a stable plug (see Glanzmann thrombasthenia).

Image — GPIIb\/IIIa interacting with fibrinogen/VWF

GPIIb-IIIa complex binding fibrinogen

Clinical correlates related to aggregation

  • Glanzmann thrombasthenia: genetic deficiency of GPIIb\/IIIa → defective aggregation and mucocutaneous bleeding.
  • GPIIb\/IIIa inhibitors (e.g., Abciximab): monoclonal antibodies that block the receptor to prevent platelet aggregation during coronary interventions.

Image — aspirin (common platelet inhibitor)

Aspirin pill

Pharmacologic inhibition of platelet activation

  • Aspirin: irreversibly inhibits COX-1 in platelets, reducing thromboxane A2 synthesis and impairing activation/aggregation.
  • GPIIb\/IIIa antagonists (Abciximab): block final common pathway of platelet aggregation.

Key takeaways (high-yield)

  • Platelet adhesion under high shear relies on VWF tethering and the GPIb-IX-V receptor.
  • Activation causes shape change, granule release, phospholipid externalisation, and integrin activation (GPIIb\/IIIa) — together produce aggregation and support coagulation.
  • Major disorders: VWD (VWF defects), Bernard–Soulier (GPIb defect), Glanzmann thrombasthenia (GPIIb\/IIIa defect).
  • Antiplatelet drugs target different stages: aspirin (activation signaling) and GPIIb\/IIIa inhibitors (aggregation).

Quick self-test questions

  • What receptor mediates VWF-dependent adhesion under high shear?
  • Which granule type releases ADP and serotonin?
  • Name two clinical syndromes caused by defects in GPIb or GPIIb\/IIIa.