Platelets And Haemostasis
Leaks in the cardiovascular system can lead to loss of blood and must
be rapidly plugged. This is the purpose
of haemostasis, a complex process that includes formation of the blood
clot, a tough mesh of fibrin entrapping platelets and blood cells. Its
complexity is in part due to the precarious balance that must be maintained
between providing a rapid and effective means of stopping leaks, and
inappropriate formation of clots in blood vessels (thrombosis).
Thrombosis is associated with many serious conditions such as coronary artery
disease. Platelets play a critical role in haemostasis. They circulate
in the blood but are not true cells, being small (∼3 μm)
vesicle-like structures formed
from megakaryocytes in the bone marrow. They have a lifespan of ∼4 days.
Platelets have multiple surface receptors and clearly visible dense granules.
These contain mediators such as serotonin and adenosine diphosphate (ADP),
which are released on activation.
Primary haemostasis
The immediate response to damage of
a blood vessel wall is vasocon- striction, which reduces blood flow and
thus loss; it is an intrinsic property of the blood vessel. This is followed by
a sequence of events that eventually leads to sealing of the wound by a clot
(Fig. 9a). Damage to the vessel wall exposes collagen, to which a plasma
protein called von Willebrand factor (vWF) binds. Tissue factor (TF) is
also exposed (see below). Platelets have glycoprotein (GP) recep-
tors which avidly bind to vWF, tethering the platelet. Further receptors
including integrins (Chapter 3) bind directly to collagen. Together
these cause adhesion of the platelet to the damaged area. Binding to
these receptors also initiates platelet activation, partly by increasing
intracellular Ca2+. Platelets change shape, put out pseudopodia and make thromboxane
A2 (TXA2) via cyclooxygenase (COX). TXA2 stimulates release of serotonin,
ADP and other compounds from the platelet granules. TXA2 and serotonin
also enhance the vasoconstric- tion. The process propagates because ADP directly
activates more platelets via purinergic (P2Y) receptors. It also causes
activation of fibrinogen (GPIIb/IIIa) receptors on their surface, which
bind to fibrinogen in the plasma causing the platelets to become sticky
and aggregate, forming a soft platelet plug (Fig. 9a). This is
stabilized during clotting by conversion of the fibrinogen to fibrin.
Note that thrombin (see below) is also a potent platelet activator.
Formation of the blood clot
(coagulation) The process
leading to formation of the blood clot involves sequential conversion of proenzymes
to active enzymes (clotting factors; e.g. factor X → Xa) in an
amplifying cascade. Most clotting factors are produced in the liver, which
requires vitamin K, and many (e.g. thrombin, factor X) require Ca2+ to
act. The ultimate purpose is to produce a massive burst of thrombin (factor
IIa; a protease which cleaves fibrinogen to fibrin), and thus rapid formation
of the clot. The cell-based model of clotting (Fig. 9b) has replaced the
older extrinsic and intrinsic pathways. Most of the action in this model only
occurs on cell or platelet surfaces (hence its name).
The initial phase of
clotting occurs when cells that express a protein called tissue factor (TF;
thromboplastin) become exposed to plasma as a result of vascular damage. Such cells include fibroblasts, monocytes
and damaged endothelial cells. Factor VIIa from the plasma is then able
to bind to TF (TF : VIIa), and this complex consequently activates a key
protein in the clotting process, factor X. It is this, when combined
with cofactor Va to form prothrombinase, that converts prothrombin
to thrombin; importantly, it can only do so when tethered to the
cell by phospholipids. However, insufficient thrombin is produced at this stage
for clot formation, and the signal has to be amplified.
The amplification phase takes
place on the surface of platelets (Fig. 9b). The small amount of
thrombin produced above activates nearby platelets, and also cofactor V on
their surface. Cofactor VIII is normally bound to plasma vWF, which
protects it from degradation. Thrombin cleaves factor VIII from vWF and activates
it, when it also binds to the platelet surface. The end product is a large
number of activated platelets, each with a large surface area (due to
pseudopodia) covered with the active cofactors, all stuck together by
fibrinogen (see above).
The scene is now set for the propagation
phase. Thrombin acti- vates a short cascade that leads to activation of factor
IX (also acti- vated by TF:VIIa). Factor IXa forms a complex with cofactor
VIIIa on the platelet surface to form tenase, a much more powerful
activator of factor X than TF:VIIa. The large amount of factor Xa thus
generated binds to cofactor Va also on the platelet surface to form a similarly
large amount of prothrombinase. There is consequently a massive burst of
thrombin production, 1000-fold greater than in the initial phase. Thrombin
cleaves the fibrinogen bound around the platelets to form fibrin
monomers, which spontaneously polymerize to a fibrous mesh of fibrin,
entrapping the platelets and other blood cells. The fibrin polymer is finally
cross-linked by factor XIIIa (also activated by thrombin) to create a
tough network of fibrin fibres and a stable clot. Retraction of entrapped platelets contracts the
clot by ∼60%, making it tougher and
assisting repair by drawing the edges of the wound together.
Inhibitors of
haemostasis and fibrinolysis Because thrombin both activates and is produced
by the mechanisms described above, there is an element of positive feedback,
and the whole process is intrinsically unstable. Multiple inhibitory mechanisms
counteract this to prevent inappropriate clotting. Undamaged endothelium
produces prostacyclin and nitric oxide (Chapter 24), which impede
platelet adhesion and activation and so limit them to damaged areas. Plasma antithrombin
inhibits thrombin, factor Xa and tenase, and is strongly potentiated by heparin
and heparans on endothelial cells. Thrombomodulin (also on
endothelial cells) binds thrombin and converts it so it no longer cleaves
fibrinogen but instead activates protein C (APC), which with protein
S inactivates factors Va and VIIIa, and hence tenase and prothrombinase.
Finally, the clot is broken down by plasmin, a process called fibrinolysis.
This occurs when plasma plasminogen binds to fibrin, and is converted to
plasmin by tissue plasminogen activator (tPA). Plasmin is itself
inactivated by α2-antiplasmin.
