L8: Platelets in health and disease

Question 1

Signs of platelet disorders

Answer 1

• Mucocutaneous bleeding
• Purpura
• Petechiae (non-blanching rash)
• Haematoma

Question 2

Platelets

Answer 2

• More red cells than platelets and less white cells
• Normal range 150-400x10^9/L
• Produced in bone marrow

Question 3

Megakaryoblast

Answer 3

• First step in platelet development

- Large, little cytoplasm, immature nucleole

Question 4

Megakaryocyte

Answer 4

• More cytoplasm, granulated, multinucleated
• Located next to BM sinusoidal endothelial cells, filopodia extend into capillaries
• Each megakaryocyte –> 4000 platelets
• Mature megakaryocyte contain 4-64x amount of haploid DNA (mostly 18-32N)

Question 5

Steps in platelet development

Answer 5

1. Megakaryocyte development in adult BM
2. Endomitosis to create polyploid nucleus
3. Cytoplasmic maturation
4. Pre-platelet formation and release
5. Pre-platelet to pro-platelet interconversion
6. Platelet release

Question 6

Thrombocytopoiesis

Answer 6

Stem cell -> megakaryoblast -> megakaryocyte -> platelets

Hormones:

• Steel (stem cell) factor
• LIF (leukaemia inhibitory factor)
• IL6 and IL11
• Thrombopoietin (most impt)

Question 7

Platelet homeostasis

Answer 7

• Maintained at constant level
• 1/3 do not circulate but remain in spleen
• Require 400,000 new platelets/uL blood/day
• Lifespan 7-10 days
• Consumption: senescence (old) and utilisation in haemostasis

Question 8

Platelet ultrastructure

Answer 8

Complex cytoskeleton
Electron-dense granules: Ca2+, Mg2+, ATP, ADP and serotonin
Alpha-granules: coagulation factors, platelet-derived growth factor, TGF-b, heparin neutralising factor (PF4), thrombospondin

Question 9

Primary haemostasis

Answer 9

= Process of forming platelet plug at site of vessel injury

1. Injury: endothelial injury –> vascular constriction
2. Initiation: exposed collagen bound by VWF in blood, VWF binds platelet via platelet receptor GP 1B-IX-V
3. Extension:
   - Platelet activated -> exposes integrin αIIbβ3 -> proteins link between platelets (e.g. fibrinogen)
   - Granules released recruiting more platelets
4. Stabilisation: using clotting cascade, forming fibrin

Question 10

Clinical: problems with primary haemostasis

Answer 10

• Congenital lack of VWF = Von Willebrand disease
• Lack of GP-1b on platelets = Bernard-Soulier syndrome

Leads to haemorrhagic diathesis characterised by abnormal platelet adherence

Question 11

Platelet quiescence

Answer 11

NO, PGI2 released from endothelium

Prevent platelet aggregation

Question 12

Platelet aggregation

Answer 12

Most platelets do not adhere directly to sub-endothelial structure, but to each other = aggregation

• Adherence to collagen -> prostaglandin synthesis activated -> thromboxane A2 released
• Thromboxane A2 triggers release of platelet granules (coagulation factors , ADP, fibrinogen)
• Leads to aggregation and clotting cascade activation

Question 13

Thromboxane A2 synthesis and action

Answer 13

Synthesis:

• Within platelets phospholipids generate arachidonic acid (via phospholipase)
• Arachidonic acid -> prostaglandins via cyclo-oxygenase
• Prostaglandins -> thromboxane A2 via thromboxane synthase

Action:
Thromboxane A2 slows conversion of ATP to cAMP –> lower platelet cAMP –> lower Ca2+ –> granule release

Question 14

Prostacyclin

Answer 14

• Increases platelet cAMP –> increase Ca2+
• Inhibits granule release
• Prevents platelet aggregation on normal endothelium

Question 15

Aspirin action

Answer 15

Aspirin binds to cyclo-oxygenase irreversibly leading to covalent acetylation
–> reduces thromboxane A2
Enzyme modification permanent

If stopping aspirin, 10% of platelets free of aspirin each day (at 5 days 50% free)

Question 16

Causes of high platelets

Answer 16

Increased production:

Myeloproliferative neoplasms: polycythemia vera, essential thrombocythemia, primary myelofibrosis

Question 17

Presentation of thrombocytopenia

Answer 17

• Epistaxis, easy bruising, petechiae, oral blood blisters
• Low platelets in FBC
• If no splenomegaly or hepatomegaly then no underlying malignancy
• Most likely scenario is ITP, sometimes viral trigger and presents acutely

Lab phenomenon: Sometimes EDTA (used in FBC) causes clumping and appears as thrombocytopenia (low platelet count)

Question 18

Causes of thrombocytopenia

Answer 18

Decreased production:

• Selective decrease in megakaryocytes (viral, drugs)
• General BM failure (aplasia, leukaemia)

Increased destruction:

• ITP
• Other autoimmune e.g. SLE
• Drugs
• DIC
• Viral infection

Other: hypersplenism, massive transfusion

Question 19

Immune thrombocytopenia - mechanism

Answer 19

• Sensitised platelet has anti-platelet antibodies bound to it
• Macrophages respond to Fc portion of antibody on platelet –> digest platelets
  Less platelets, often larger platelets, bone marrow aspirate shows elevated megakaryocytes

Question 20

Treatment of ITP

Answer 20

• Prednisolone (immune suppression steroid)
• If relapse on prednisolone, splenectomy
• New agents for ITP: thrombopoietin (TPO)-mimetic –> binds to TPO receptor and stimulates platelet production

Question 21

Hereditary platelet defects

Answer 21

Glanzmann’s thrombasthesthenia: no aIIbB3 (rare)
Secretory defects
Minor receptor defects (more common)

(acquired defects vastly more common)

Question 22

Testing platelet function

Answer 22

• Platelet function assay (screen)

- Platelet aggregation testing