platelets

DR NILESH KATE

MBBS,MD ASSOCIATE PROF

DEPT. OF PHYSIOLOGY

PLATELETS

OBJECTIVES Structure &

composition. Properties & functions Normal count &

variations. Formation of platelets. Life span & fate of

platelets.

Wednesday, May 3, 2023

STRUCTURE & COMPOSITION. Platelets – small plate like. Thrombocytes. – thrombo –

clot; cytes—cells. Size – smallest blood cells ;

2-4 μm. Shape – spherical or oval

discoid Volume 5.8 μm3. Colour – colourless

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STRUCTURE & COMPOSITION Leishmann’s staining

– faint blue cytoplasm with reddish purple granules.

Nucleus - Absent.

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ELECTRON MICROSCOPIC PICTURE.

Cell membrane. Microtubules. Cytoplasm.

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CELL MEMBRANE. 6 nm thick trilaminar

membrane. Consists of

Lipids – phospholipids, cholesterol & glycolipids.

Carbohydrates. Proteins –

glycoproteins.

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CELL MEMBRANE. Phospholipids of membrane

contains platelet factor -3 , imp for blood clotting process.

Glycoproteins – forming surface coat of platelets Prevent sticking of platelets to

endothelium. Accelerate adherence to

collagen & damaged endothelium.

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CELL MEMBRANE. Canalicular system – surface

of platelets invaginate to form Canalicular system or surface connecting system.

Receptors -- For combining with collagen & fibrinogen.

Precursors of substances like – Thromboxane A2, PG, LT & platelet factor 3,4 are also present.

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MICROTUBULES. Made up of

Polymerised proteins called Tubulins.

Forms compact bundle below membrane & encircle whole cytoplasm.

Responsible for discoid shape.

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CYTOPLASM. Contains – Golgi

apparatus, Endoplasmic reticulum, Mitochondria, microtubules, microvesicles, filament, granules, glycogen, Lysosomes, protein enzymes & hormonal substances.

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GOLGI APPARATUS, ENDOPLASMIC RETICULUM

Synthesizes various Enzymes.

Endoplasmic Reticulum – stores large amount of calcium.

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MITOCHONDRIA Power house of cell. Produce ATP & ADP.

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CONTRACTILE PROTEINS.

Actin & Myosin – same as those of contractile

proteins of muscle.

Responsible for platelet contraction & clot retraction.

Thrombosthenin

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OTHER PROTEINS

Fibrin stabilizing Factor – Imp in blood coagulation.

Platelet derived growth factor – for growth of vascular

endothelial cells, vascular smooth cells & fibroblast

Involved in repair of damaged blood vessel.

Von-Willebrand factor – for platelet adherence

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GRANULES Dense granules –

contains phospholipids, triglycerides, cholesterol, ATP,ADP, Serotonin (5HT- which hey obtain while passing through GIT)

Alpha granules – contains secreted proteins as clotting factors, & PDGF

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ENZYMES

Adenosine triphosphatase & enzyme for synthesis

of prostaglandins.

Prostaglandins – acts as a local hormone & have

local vascular & tissue reaction.

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PROPERTIES Adhesiveness –

whenever comes in contact with any wet or rough surface gets activated & stick to surface. Factors responsible are –

collagen, thrombin, ADP, thromboxane A2, Ca ion & Von-Willebrand Factor

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PROPERTIES Aggregation – property

to stick to each other Factors responsible are

ADP & Thromboxane A2 Agglutination –

property of clumping together of platelet Its due to platelet

Agglutinins.

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FUNCTIONS Role in haemostasis. Role in clot formation. Role in clot retraction. Role in repair of injured blood vessels. Role in Defense mechanism. Transport & storage function.

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ROLE IN HAEMOSTASIS. Haemostasis – spontaneous arrest of bleeding from injured

blood vessel.

Vasoconstriction – by 5HT & other vasoconstrictors

Temporary haemostatic plug – by platelets due to its

property of adhesiveness & aggregation.

Definite haemostatic plug – also initiated by platelets.

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ROLE IN CLOT FORMATION.

Play imp role in formation of intrinsic prothrombin

activator

It is responsible for onset of blood clotting.

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ROLE IN CLOT RETRACTION. Contraction of

contractile proteins i. e Actin, Myosin & Thrombosthenin.

Responsible for clot Retraction & wound healing.

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ROLE IN REPAIR OF INJURED BLOOD VESSELS.

Platelet derived growth

factor (PDGF) in

cytoplasm of platelet

imp for Repair of

Endothelium.

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ROLE IN DEFENCE MECHANISM. Due to the property of

agglutination , platelets are capable of Phagocytosis.

Mainly in Phagocytosis of carbon particles, viruses & immune complexes.

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TRANSPORT & STORAGE FUNCTION.

Platelet when passes

through GIT takes 5-

HT against

concentration gradient,

stored & transported to

the site of injury.

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NORMAL COUNT & VARIATIONS.

Normal count.

150,000 – 400,000

per mm3

Avg 2.5 L/mm3

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PHYSIOLOGICAL VARIATIONS.

Age – Less in infant, reach adult level by 3 months of age.

Sex – No difference but during mensturation reduced in

females.

Meal – Increases after meal

Muscular exercise – Increases.

High Altitude – Increases.

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PATHOLOGICAL VARIATIONS.

Thrombocytosis.

Increase in count more than 450,000/mm3

After Splenectomy

After hemorrhage, severe injury, major operation &

parturition.

Myloproliferative Disorders – CML, Polycythemia vera,

Myelofibrosis.

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PATHOLOGICAL VARIATIONS. Thrombocytopenia

Decrease in count less than 150,000/mm3

ITP Bone marrow depression Acute leukemia Infections Toxaemia, septicemia &

uremia.

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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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CONTROL OF TROMBOPOIESIS.

Thrombopoietin –

Megakaryocyte –

colony stimulating

activity (Meg-CSA)

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LIFE SPAN & FATE OF PLATELETS.

Life span – 8-12 days

Avg – 10 days.

Fate – Destroyed by tissue macrophage system in

spleen.

Splenomegaly – reduces platelet count.

Splenectomy – increases platelet count.

Wednesday, May 3, 2023

Thank You