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Platelets

Terms in this set (30)

Platelets
Normal counts between 150,000 - 350,000 / microL
Life span = 10 days
1/3 pooled in the spleen at any given time
- Arise from megakaryocytes
Endo-reduplication
Nuclear material undergoes replication but the cell does not divide.
How megakaryocytes get so big...
TPO
major hormone that controls thrombopoiesis and stimulates platelet production.
- does this by binding to the C-Mpl receptor on megakaryocytes and platelets.
- Made in liver and kidney
- Degraded by megakaryocytes and platelets, # of these cells controls the levels of this hormone
TPO
Romiplostim and Eltrombopag are agonists of this hormone that bind to C-Mpl and increase platelet production
Glycoprotein Ib
When a blood vessel is damaged, platelets intially adhere to the exposed subendothelial matrix via _______________ receptors on their membranes.
VWF
When a blood vessel is damaged, platelets intially adhere to the exposed subendothelial matrix via glycoprotein Ib receptors on their membranes. Glycoprotein Ib is the receptor for ______________ which is secreted by endothelial cells and deposted from plasma on the damaged area.
Platelets
Express receptors for:
- ADP
- Epinephrine
- Thromboxane A2
- Thrombin
- Serotonin
- Etc....
* Exposure to these can lead to activation
Platelet activation
Steps include:
- Shape change from disc to sphere
- Release reaction: alpha granules and dense bodies release contents (includes VWF, serotonin, ADP)
- Conformational change in glycoprotein Alpha2bBeta3 occurs, enabling binding to fibrinogen and VWF
Thromboxane A2
Potent platelet aggregating agent and vasoconstrictor. Produced by platelets from arachidonic acid by Cox-1.
Aspirin irreversibly inhibits Cox-1 and prevents its formation.
Platelet membrane
In vivo, the phospholipid required for the fibrin forming cascade is provided by?
abciximab
This medicine blocks the alpha2bbeta3 receptor, which is more important to give than aspirin in preventing occlusive thrombus formation in the setting of (MI, angina, stroke)
Yes
Is bleeding time prolonged in platelet disorders?
Mucocutaneous
What is more likely to occur in platelet disorders?
Joint/muscle bleeding or mucocutaneous bleeding?
Platelet disorder
Symptoms may include:
- mucocutaneous bleeding
-epistaxis
- gingival bleeding
- Bruising
- menorrhagia
- GI bleeding
- petechiae
Von Willebrand Disease
Autosomal dominant disorders characterized by low levels of ________ or qualitative abnlts.
Most common inherited bleeding disorder, prevalence = 1%
- Platelet adhesion to damaged epithelium is impaired
- Mucocutaneous bleeding
Diagnosis:
- Prolonged bleeding time
- Prolonged PTT (due to low factor VIII)
- Decreased ristocetin cofactor
- Measure ____ antigen / multimeric pattern
*Treatment:
- DDAVP
- Infusion of purified factor VIII products that contain _____
Afribrinogenemia
Rare disorder in which both platelet aggregation and fibrin clot formation are impaired.
Glanzmann Thrombasthenia
Rare AR disorder characterized by quantitative or qualitative abnlts of Alpha2bBeta3 receptor
Glanzmann Thrombasthenia
Rare AR disorder characterized by quantitative or qualitative abnlts of Alpha2bBeta3 receptor
-Platelet aggregation, mediated by the Alpha2bBeta3 receptor, is severely impaired or absent.
- Alpha2bBeta3 receptor binds fibrinogen enabling bridging of platelets
- adhesion intact (bc VWF mediates)
Clinical:
- Severe mucocutaneous hemorrhage
Diagnosis:
- Normal platelets
- Markedly prolonged bleeding time
- Impaired platelet aggregation
Treatment:
- Avoid trauma, suppress menses, platelet transfusion
Bernard Soulier Syndrome
Rare, AR disorder due to deficiency or abnlty of glycoprotein Ib.
- Results in impaired adhesion of platelets and mucocutaneous bleeding
- Same clinical picture as VW disease, absent receptor here
Diagnosis:
- Prolonged bleeding time
- *Thrombocytopenia
Treatment
- Platelet transfusion, VIIa for severe bleeding
Bleeding
Rarely occurs if the platelet count is above 75,000.
Does not usually occur until the platelet count falls below 10-20,000/ microL.
thrombocytopenia
Results from:
- Impaired BM production of platelets
- Destruction
- Consumption
- sequestration
alcohol
Substance that suppresses thrombopoiesis
Immune thrombocytopenia
Premature removal of antibody-coated platelets by macrophages, primarily in the spleen, with variable abnormalities in platelet production due to immune damage of megakaryocytes.
Chronic ITP
Autoreactive IgG against Alpha2bBeta3 or GPIb on platelets
- Young women
- Mucocutaneous bleeding, menorrhagia, petchiae
- NO SPLENOMEGALY
Dx: of exclusion
Bone marrow: Normal-increased megakaryocyte #s
Treatments:
- steroids, IVIG, Splenectomy, Anti-D Ab, Rituximab, TPO agonists
Acute ITP
Antibodies directed against viral antigens (that adsorb onto platelets)
- Young Children
- 1-3 weeks post viral illness
- Usually self limited
HIT
Most common cause of drug induced thrombocytopenia
3-5% of pts who receive heparin
- Results from antibodies against heparin complexed to platelet factor 4
Thrombotic thrombocytopenic purpura
Associated with deficiency of VWF metalloproteinase, results in failure to break down high MW multimers of VWF.
- thrombocytopenia
- microangiopathic hemolysis
- Transient neurologic sx
Hypersplenism
Sequestration of up to 90% of platelets leads to reduction in circulating platelets.
Thrombocytosis
May result from:
- primary bone marrow disorder resulting in autonomous prolfieration of platelets.
- *Jak2 mutation often seen
- Increased incidence of bleeding and thombosis
Occurs in myeloproliferative disorders:
1- Polycythemia Vera*
2- CML
3- Essential Thrombocythemia*
4- Agnogenic Myeloid Metaplasia
Thrombocytosis
May result from:
- Infection, inflammation, malignancy, bleeding, iron deficiency
- Platelet function is normal
- Is a reactive process
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