Lecture on platelets for medical and paramedical students

1. Lecture:34
Platelets: structure and Functions
Dr Shamshad

2. Objectives:
1. Describe the structure of platelets
2. Describe the process of thrombopoiesis
3. Discuss the functions of platelets
4. List common disorders of platelets.

3. Platelets/Thrombocytes:Mediators of hemostasis
Disc like of 1-4 μm diameter. Nucleus-free granulated cells.
● Life span: 10 days,Half life 8-12 days
● Synthesis: Site: Bone marrow from megakaryocytes,
Megakaryocytes fragment/bud off into minute platelets
● either in the bone marrow or soon after entering the blood, as
they pass through capillaries.
➔ Normal count:150,000 and 300,000 /mm3

4. Control of synthesis:
Interleukins (IL3) and TPO(Thrombopoietin)/GM-
CSF(granulocyte-macrophage-colony stimulating factor)
Mechanism of action:
➢ IL:stimulate the development of PHSC (progenitor
hematopoietic stem cells) to committed stem cells
➢ TPO: stimulate differentiation of committed stem cells into
megakaryocytes

5. ● Elimination:from the circulation mainly by the tissue
macrophage/RE system.

6. Thrombopoiesis

7. Structure of platelet:
1
2
3
4

8. 1:Peripheral zone
1:Glycocalyx :Outermost coat
Contain mucopolysaccharides & Mg2+ dependent ATPase, plasma
proteins(IgG,IgM), fibrinogen,Factor V & VIII.
Function:Adsorbs plasma proteins,glycoproteins & glycolipids
2:Platelet membrane : Lipid bilayer of glycoproteins & proteoglycans
Function:Maintain cytoplasmic integrity.
Glycoprotein (Ia,Ib,IIa,III..) : Essential for platelets adhesion &
aggregation.

9. 2:Membrane system
Function:
➔ Serves as a pathway for release of intracellular materials.
➔ Platelets factor III plays important role in activating clotting
mechanism
➔ Provide increased surface areas for absorption and storage of
coagulation factors from the plasma.
➔ Specific receptors: for agonists such as ADP,TxA2,Adrenaline
Thrombin.

10. I:Complex open canalicular system: Made of Phospholipids
Surface connected canalicular system.
Invagination of plasma membrane
Functions:
➔ secretion of platelet granules
➔ Uptake of plasma proteins
➔ Increased surface area during
➔ platelet activation

11. II:Dense tubular system
Site for sequestration of Calcium ions & localization of enzymes
required for prostaglandin synthesis
Functions:
➔ Triggers platelets contraction thus causing activation of platelets
➔ Calcium also play important role as Coagulation factor(factor IV)
➔ Site of Thromboxane (TXA2) synthesis

12. 3:Structural zone
● Network of protein filaments
● Submembranous cytoskeleton
● Contractile proteins (Actin G&F, Myosin)
3 types of microfilaments
I:Microtubules:Beneath the cell membrane
Maintains discoid shape of the resting platelet
II:Microfilaments associated with thrombosthenin which contain
Actin G & F and Myosin
Meditate contractile events
III:Intermediate filaments

13. 4:Organelle zone: Metabolic activity zone
Internal platelet components
Mitochondria
Golgi apparatus and ER
Granules: 3 types
1) Alpha granules
2) Dense Bodies
3) Lysosomal granules

14. Function:Mitochondria and enzyme systems forms ATP & ADP
ER and Golgi apparatus synthesize various enzymes , store large
quantities of calcium ions.
Enzyme systems synthesizes:
● Prostaglandins:Role in vascular & other local tissue reactions
● Fibrin-stabilizing factor:Important role in blood coagulation
● Growth factor:Multiplication & growth of vascular endothelial
cells, vascular smooth muscle cells & fibroblasts that helps in
repair damaged vascular walls.

15. Alpha granules
Most abundant 20-200/platelet .Membrane bound oval granules
➢ Platelet Factor 4(PF4)/anti heparin : Neutralizes heparin
➢ Coagulation Factor V & Factor VIII :Related to antigen growth
factor
➢ Beta -Thromboglobulin (Beta-TG):Promotes smooth muscle
growth thus help vessel repair
➢ Platelet derived growth factor(PDGF): Promotes smooth muscle
growth & involved in atherosclerosis & lipid metabolism
➢ Thrombospondin: Promotes platelets to platelet interaction
In von Willebrand disease platelets lack factor VIII related antigen.

16. Dense bodies/Delta granules:
2-10/platelet:species specific
➢ ADP :Promotes aggregation of platelets by attracting more platelets
➢ ATP : Provides energy
➢ GTP and GDP : In Signal transduction
➢ Serotonin and Histamine : Vasoconstriction.
➢ Calcium ions: Primary and secondary messenger and Regulation of
platelet activation and aggregation.

17. Lysosomes
Contain enzymes called hydrolases
Neutral proteases ,Acid hydrolases and Antibacterial enzymes
Function:
Act to digest materials brought into the platelets by endocytosis

18. Von Willebrand factor(vWF)
Large multimeric protein
Synthesis: In the endothelial cells and megakaryocytes
Found in :Subendotheilium plasam, alpha granules of platelets,
Function
-Mediate platelet adhesion to the collagen in the subendothelium
-Bind factor VIII to protect it from proteolysis in the plasma.

20. Structure and Function
of platelet:

21. Functions of platelets
On the non activated platelet cell membrane surface coat of
glycoproteins repulses adherence to normal endothelium of the
vessels.
Injury or disruption of the continuity of the vessels causes
adherence to injured areas of the vessel wall, especially to injured
endothelial cells and even more so to any exposed collagen from
deep within the vessel wall.
It contains large amounts of phospholipids that activate multiple
stages in the blood clotting.

22. Functions of platelets
1. Adhesion
2. Activation by changing its shape
3. Secretion or release of granules
4. Aggregation.
In circulation:Checks the blood vessel continuity by endothelial
lining for gaps.
Formation of primary hemostatic plug.
Provides surface for coagulation factors to make secondary
hemostatic plug.
Helps in healing injured tissue.

27. Platelets disorders
Quantitative Qualitative

28. Thrombocytopenia
Platelet count Symptoms
50-100,000 Prolonged bleeding following trauma
<50,000 Easy bruising ,purpura following minor
trauma
<20,000 Spontaneous bleeding, petechiae, internal
or intracranial bleeding

29. Thrombocytopenia: Causes
Decreased
production
1
Bone Marrow damage, aplasia, drugs,
malignancy,Toxins Hepatitis
Congenital defects2
Rubella , Autosomal dominant,
Fanconi anemia
In effective production3
Vit B12 and Folic acid
deficiency

30. Immune6 Drugs induced, SLE, AIDS, ITP
Abnormal distribution4
Splenomegaly,Liver diseases,
Myelofibrosis
5 Non immune :DIC,Hemolytic -uremic
syndrome,TTP,HELP syndrome
Increased
destruction

31. Inhereted disorder of platelet functioin
➔ Defect in platelet -vessel wall interaction:
vonWillebrand disease
Bernard-Soulier synderome
➔ Defect in platelet- platelet interaction:
Congenital afirbinogenimia
Glanxmann throbasthenia
➔ Defects in platelets granules ,secretion & signal transduction:
Abnormalities of granules, signal transduction defects,
➔ Defects of platelet procoagulant functions:
➔ Defect in structural/ cytoskeletal components
➔ Others

32. Thrombocytosis
Causes:
➢ Myeloproliferation
➢ Essential thrombocytopenia
➢ Polycythemia vera
➢ CML
➢ Myeloid metaplasia
➢ Secondary /reactive thrombocytosis
➢ Systemic inflammation
➢ Malignancy
➢ Hemorrhage
➢ Postsplenectomy