Platelets

What are platelets?

Anucleate fragments of megakaryocyte cytoplasm

What cells produce platelets?

Megakaryocytes

What is the largest cell in bone marrow?

Megakaryocyte

What hormone regulates platelet production?

Thrombopoietin

What are the two main functions of platelets?

Primary hemostasis and fibrin formation support

What prevents platelet adhesion in normal vessels?

Intact endothelium

What happens to platelets after vessel injury?

They activate and adhere

What is the result of platelet activation?

Primary platelet plug formation

What do endothelial cells release to inhibit platelets?

Nitric oxide and prostacyclin

What are the steps in platelet plug formation?

Adhesion, activation, aggregation, secretion

What receptors bind collagen?

GPIa

What receptor binds vWF?

GPIb

What receptor binds fibrinogen?

GPIIb

What does vWF do in platelet function?

Mediates adhesion and carries factor VIII

What is found in the platelet organelle zone?

Granules and mitochondria

What do dense granules contain?

Ca2+, ADP, ATP, serotonin

What do alpha granules contain?

Hemostatic proteins

What are lysosomes in platelets?

Organelles with enzymes

What are peroxisomes?

Organelles involved in metabolism

What is platelet adhesion?

Attachment to subendothelial collagen

What receptors mediate direct adhesion?

GPIa

What mediates adhesion at high shear rates?

GPIb

Is platelet adhesion reversible?

Yes in early stages

What happens during platelet activation?

Release of ADP and TxA2

What does thromboxane A2 do?

Promotes aggregation and vasoconstriction

What drug inhibits TxA2 production?

Aspirin

What is platelet aggregation?

Platelets sticking together

What receptor is required for aggregation?

GPIIb

Is aggregation energy dependent?

Yes

What is required for aggregation?

Fibrinogen and Ca2+

What happens to arachidonic acid in platelets?

Converted to TxA2

What enzyme converts arachidonic acid?

Cyclooxygenase

What can cause pseudothrombocytopenia?

Platelet clumping

What is platelet satellitism?

Platelets surrounding neutrophils

What is bleeding time?

Test of platelet function in vivo

What causes prolonged bleeding time?

Platelet dysfunction or low count

What disorders prolong bleeding time?

vWD, Glanzmann, Bernard-Soulier

What symptoms occur with platelet disorders?

Mucosal bleeding and petechiae

What are causes of thrombocytopenia?

Decreased production, destruction, sequestration

What is the most common cause of thrombocytopenia?

Increased destruction

What is ITP?

Immune thrombocytopenic purpura

What causes ITP?

Autoantibodies against platelets

Where are platelets destroyed in ITP?

Spleen

What antibodies are involved in ITP?

IgG

What receptors are targeted in ITP?

GPIIb

What is acute ITP?

Short-term thrombocytopenia <6 months

Who commonly gets acute ITP?

Children after viral infection

What is chronic ITP?

Thrombocytopenia >6 months

Who commonly gets chronic ITP?

Adults, especially females

What is a key blood film finding in ITP?

Large platelets

What are treatments for ITP?

Steroids, IVIG, splenectomy

What are non-immune causes of thrombocytopenia?

Consumption and mechanical destruction

What are MAHAs?

Microangiopathic hemolytic anemias

What conditions are MAHAs?

DIC, HUS, TTP, HELLP

What blood film finding is seen in MAHA?

Schistocytes

What is HELLP syndrome?

Hemolysis, elevated liver enzymes, low platelets

When does HELLP occur?

Third trimester of pregnancy

What causes primary thrombocytosis?

Myeloproliferative disorders

What causes secondary thrombocytosis?

Inflammation or illness

What is thrombocytopathy?

Platelet function disorder

What symptoms occur in thrombocytopathy?

Bleeding and bruising

What is Bernard-Soulier syndrome?

Defect in GPIb receptor

What happens in Bernard-Soulier syndrome?

Platelets cannot bind vWF

What is seen on smear in Bernard-Soulier?

Giant platelets

What is Glanzmann thrombasthenia?

Defect in GPIIb

What happens in Glanzmann?

No platelet aggregation

What is Gray platelet syndrome?

Lack of alpha granules

How do platelets appear in Gray platelet syndrome?

Large and gray

What is the most common cause of acquired platelet dysfunction?

Drugs

How does aspirin affect platelets?

Inhibits COX-1 and TxA2

What do NSAIDs do to platelets?

Inhibit COX enzymes

What other substances affect platelet function?

Alcohol and antibiotics