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


STRUCTURE & COMPOSITION Leishmann’s staining

– faint blue cytoplasm with reddish purple granules.

Nucleus - Absent.


ELECTRON MICROSCOPIC PICTURE.

Cell membrane. Microtubules. Cytoplasm.


CELL MEMBRANE. 6 nm thick trilaminar

membrane. Consists of

Lipids – phospholipids, cholesterol & glycolipids.

Carbohydrates. Proteins –

glycoproteins.


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.


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.


MICROTUBULES. Made up of

Polymerised proteins called Tubulins.

Forms compact bundle below membrane & encircle whole cytoplasm.

Responsible for discoid shape.


CYTOPLASM. Contains – Golgi

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


GOLGI APPARATUS, ENDOPLASMIC RETICULUM

Synthesizes various Enzymes.

Endoplasmic Reticulum – stores large amount of calcium.


MITOCHONDRIA Power house of cell. Produce ATP & ADP.


CONTRACTILE PROTEINS.

Actin & Myosin – same as those of contractile

proteins of muscle.

Responsible for platelet contraction & clot retraction.

Thrombosthenin


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


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


ENZYMES

Adenosine triphosphatase & enzyme for synthesis

of prostaglandins.

Prostaglandins – acts as a local hormone & have

local vascular & tissue reaction.


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


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.


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.


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.


ROLE IN CLOT FORMATION.

Play imp role in formation of intrinsic prothrombin

activator

It is responsible for onset of blood clotting.


ROLE IN CLOT RETRACTION. Contraction of

contractile proteins i. e Actin, Myosin & Thrombosthenin.

Responsible for clot Retraction & wound healing.


ROLE IN REPAIR OF INJURED BLOOD VESSELS.

Platelet derived growth

factor (PDGF) in

cytoplasm of platelet

imp for Repair of

Endothelium.


ROLE IN DEFENCE MECHANISM. Due to the property of

agglutination , platelets are capable of Phagocytosis.

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


TRANSPORT & STORAGE FUNCTION.

Platelet when passes

through GIT takes 5-

HT against

concentration gradient,

stored & transported to

the site of injury.


NORMAL COUNT & VARIATIONS.

Normal count.

150,000 – 400,000

per mm3

Avg 2.5 L/mm3


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.


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.


PATHOLOGICAL VARIATIONS. Thrombocytopenia

Decrease in count less than 150,000/mm3

ITP Bone marrow depression Acute leukemia Infections Toxaemia, septicemia &

uremia.


FORMATION OF PLATELETS. Stages in platelets

production.(10 days) Megakaryoblast. Promegakaryocyte. Megakaryocyte.

Sites – Bone Marrow Stem cells – PHSC

CFU- Meg


MEGAKARYOBLAST. Earliest recognizable cells. CFU-Meg differentiate to

form Megakaryoblast. Diameter – 20-30 mm Cytoplasm – small, blue,

Non-granular Nucleus – large,

oval/kidney shaped


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.


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


CONTROL OF TROMBOPOIESIS.

Thrombopoietin –

Megakaryocyte –

colony stimulating

activity (Meg-CSA)


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.


Thank You

platelets

Health & Medicine