Lec 08 :

Platelets and Hemostasis II

Assist. Prof. Dr. Mudhir S. Shekha

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CONTROL OF TROMBOPOIESIS.

• Thrombopoietin

• Megakaryocyte – colony stimulating activity (Meg-CSA)

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FORMATION OF PLATELETS.

• Stages in platelets

• production.(10 days)– Megakaryoblast.

– Promegakaryocyte.

– Megakaryocyte.

• Sites – Bone Marrow

• Stem cells – PHSC

• CFU- Meg

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MEGAKARYOBLAST

• Earliest recognizable

cells.

• CFU-Meg differentiate

to form Megakaryoblast.

• Diameter – 20-30 mm

• Cytoplasm – small, blue,

Non-granular

• Nucleus – large,

oval/kidney shaped

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PROMEGAKARYOCYTE

• Megakaryoblast – Endoreduplication of nuclear chromatin.

• Nuclear chromatin replicates in multiple of 2 without division

of cell.

• Large cell with 32 times diploid content of nuclear DNA

formed.

• Cytoplasm – Granular.

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MEGAKARYOCYTE.

• Diameter – large cell with 30-90mm in diameter.

• Nucleus- single multiloaded.

• Cytoplasm – abundant with red purple granules.

• Cell margin – irregular with many Pseudopodia which gets detached into

blood & forms platelets.

• One Megakaryocyte – 4000 platelets

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• The pluripotential stem cell produces a progenitor

committed to megakaryocyte differentiation (colony

forming unit–megakaryocyte [CFU-MK]), which can

undergo mitosis.

• Eventually the CFU-MK stops mitosis and enters

endomitosis.

• During endomitosis, neither cytoplasm nor nucleus

divides, but DNA replication proceeds and gives rise to

immature polyploid progenitors, which then enlarge and

mature into morphologically identifiable, mature

megakaryocytes that shed platelets.

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Von Willebrand factor (vWF)• An adhesive protein formed by the vascular

endothelial cells and secreted in the plasma.

• It is formed by megakaryocytes and stored in the

platelets.

• Blood clotting factor VIII circulated in plasma

bound to vWF

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Thrombus formation• The function of platelets is the maintenance of

hemostasis. This is achieved primarily by the

formation of thrombi, when damage to the

endothelium of blood vessels occurs. On the

converse, thrombus formation must be inhibited

at times when there is no damage to the

endothelium.

• It depends on:

• the vascular wall, platelets and the clotting

factors 10

Hemostasis has three major steps

• 1) vasoconstriction,

• 2) temporary blockage of a break by a platelet plug

• 3) blood coagulation, or formation of a clot that

seals the hole until tissues are repaired.

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• The inner surface of blood vessels is lined with a thin layer of

endothelial cells that, in normal hemostasis, acts to inhibit

platelet activation by producing nitric oxide, endothelial-

ADPase, and PGI2. Endothelial-ADPase clears away the

platelet activator, ADP

• Endothelial cells produce a protein called von Willebrand

factor (vWF), a cell adhesion ligand, which helps endothelial

cells adhere to collagen in the basement membrane.

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• When the endothelial layer is injured,

collagen, vWF and tissue factor from the

sub- endothelium is exposed to the

bloodstream.

• When the platelets contact collagen or vWF,

they are activated. They are also activated by

thrombin.

• They can also be activated by a negatively

charged surface, such as glass.

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• Non-physiological flow conditions caused by

arterial stenosis or artificial devices can

also lead to platelet activation

• Calcium ions are essential for binding of

these coagulation factors.

• Larger platelets are metabolically and

functionally more active than small platelets.

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• Surface projections represent protractions

of platelets granules

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Thromboxane A2 synthesis• Platelet activation initiates the arachidonic acid

pathway to produce TXA2. TXA2 is involved in

activating other platelets and its formation is

inhibited by COX inhibitors, such as aspirin.

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Adhesion and aggregation

• Platelets aggregate, or clump together, using

fibrinogen and von Willebrand factor (vWF) as

a connecting agent.

• The most abundant platelet aggregation

receptor is glycoprotein IIb/IIIa (gpIIb/IIIa); this

is a calcium-dependent receptor for

fibrinogen, fibronectin, vitronectin,

thrombospondin, and vWF. Other receptors

include GPIb-V-IX complex (vWF) and GPVI

(collagen).18

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• Activated platelets will adhere, via glycoprotein

(GP) Ia, is exposed by endothelial damage.

• Aggregation and adhesion act together to form

the platelet plug. Myosin and actin filaments in

platelets are stimulated to contract during

aggregation, further reinforcing the plug.

• Platelet aggregation is stimulated by ADP,

thromboxane, and α2 receptor-activation, but

inhibited by other inflammatory products like

PGI2 and PGD2.

• Platelet aggregation is enhanced by

exogenous administration of anabolic steroids.20

Wound repair• The blood clot is only a temporary solution to

stop bleeding; vessel repair is therefore

needed.

• The aggregated platelets help this process by

secreting chemicals that promote the

invasion of fibroblasts from surrounding

connective tissue into the wounded area to

completely heal the wound or form a scar.

• The obstructing clot is slowly dissolved by

the fibrinolytic enzyme, plasmin, and the

platelets are cleared by phagocytosis.21

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