Lesson 2: Platelets

sequence for progenitor stages

HSC→CMP→CFU-GEMM→CFU-EMk→CFU-Mk

maturation sequence of platelets

megakaryoblast (stage one)

megakaryocyte II

megakaryocyte III

mature megakaryocyte IV

platelets

megakaryoblast

• stage one

• very little basophilic cytoplasm

• no visible granules 

• round nucleus

• visible nucleoli

megakaryocyte II

• stage two 

• primarily basophilic cytoplasm

• more cytoplasm 

• few azurophlic granules 

• indented nucleus

megakaryocyte III

• stage 3

• numerous granules 

• acidophlic cytoplasm 

• large, multilobed nucleus 

megakaryocyte IV

• stage four 

• very granular cytoplasm

• demarcation zones (invagination)

• mutlilobulated nucleus 

overall maturation characteristics of the nucleus of a megakaryocyte

• endomitosis (unique to platelets)

• increasing ploidy (16N is most common and 8N is morphologically identifiable)

• gets bigger 

• becomes multi-lobed 

endomitosis

• replication of DNA

• blocked in anaphase 

• no telophase, karyokinesis, or cytokinesis

overall maturation characteristics of the cytoplasm in megakaryocytes

• increases in size

• more cytoskeletal elements, organelles, and granules 

• starts at 8N

what is the last thing that is seen as megakaryocytes mature?

the demarcation membrane system (DMS)

• invagination of outer membrane

• maintains communicatino with extracellular space

• compartmentalizes the cell

what are all the cytokine regulators for thrombopoiesis

thrombopoietin (TPO)

IL-3, GM-CSF (progenitor stages)

IL-6, IL-11 (maturation stages)

thrombopoietin (TPO)

• made mainly by liver

• also made in kidneys and BM stromal cells

• inversely proportional to PLT in PB

• influences all maturation stages

what CD markers are seen on platelets?

CD 41, 42, 61

what is special about CD 41 and 61?

41= GP IIb

61= GP IIIa

they are receptors for F1

how are individual platelets formed?

created through strands called proplatelets or budding that wiggle through sinusoids 

proplatelets

long, slender protusions of cytoplasm of megakaryocyte

where are megakaryocytes normally found?

in the bone marrow

where are platelets normally found?

in the peripheral blood

what are the four zones of a platelet?

peripheral

membrane 

organelle

structural 

function of peripheral zone 

adhesion and aggregation 

parts of the peripheral zone

• glycocalyx 

• cytoplasmic membrane 

• phospholipid bilayer 

• arachidonic acid 

• intergrins

glycocalyx 

the fluffy outercoat of the platelet 

• made of glycoproteins and adsorbed coag factors

• negatively charged to repel other platelets until needed

• part of the peripheral zone

phospholipid bilayer

• asymmetrical 

• negative on inner and neutral on outer 

• ATP-dependent translocase 

• part of peripheral zone

arachidonic acid

• precursor for stimulators of aggregation and vessel constriction 

• part of peripheral zone

integrins

• receptors for clotting factors 

• have to be activated to bind 

• GP IIb and GP IIIa = factor one 

• GP Ib = receptor for VWF

GP IIb and GP IIIa

needed for both aggregation and adhesion

GP Ib

needed for adhesion

function of structural zone

structure and support

• support plasma membrane

• maintains discoid shape 

• permits shape change during activation 

parts of structural zone

• microtubules 

• cytoskeleton 

microtubules

• tubulin

• just under the cell membrane

• part of the structural zone

cytoskeleton

• made of microfilaments and intermediate filaments 

• actin and myosin 

• contractile proteins 

• forms the pseudopods in activated platelets

• part of structural zone

function of organelle zone

metabolic function, storage, and secretion

mitochondria

• glycolysis and citric acid cycle

• ATP for energy 

• part of organelle zone

glycogen stores

• energy source (ATP)

• part of organelle zone

platelet granules

• storage sites of ressential substances for platelet functions 

• types: alpha granules, dense granules, lysosomes
  

alpha granules 

• most numerous 

• filled with proteins (hemostatic, platelet specific, and thrombospondidin)

dense granules

contains ADP, ATP, serotonin and Ca2+

lysosomes

• contains hydrolytic enzymes

• digests cell debris

function of membrane systems

storage and secretion

open canicular systems (OCS)

• connects the inner contents to the surface 

• invagination of cell membrane 

• part of the membrane zone

dense tubular system (DBS)

• releases Ca2+ 

• connected to the OCS, not the surface 

what are the three steps of the formation of the primary hemostatic plug

1. adhesion 

2. aggregation

3. secretion (overlaps with secondary)

adhesion

• injury exposes collagen and platelets adhere to it 

• GP VI has to contact collagen in order to activate platelet 

• VWF binded to GP Ib is needed for optimal adhesion

• platelet has to be activated

platelet activation

• shape changes 

• biochemical changes 

• agonists help

• pseudopods form 

• granules center

• surface gets sticky 

• contaction of cytoskeleton

aggregation

• platelets stick together forming a large mass

• fibrinogen is the bridge needed to bind the GP IIb and IIIa of the platelets together

• calcium is also required

• happens in three phases 

• happens in 15-30 seconds 

• unstable

phases of aggregation

1. loosely bound

2. alpha granules releases contents

3. irreversible (platelets are no longer viable)

secretion

• gradual 

• happens at the same time as aggregation

• graunle contents released through OCS

• need a strong stimulus and Ca2+

• membrane releases arachidonic acid

• dense bodies release ADP

• ADP mobilizes binding sites for IIb and IIIa on surface 

• brings more platelets to the area

what is a platelet agonist?

induces platelet activation

examples: ADP, serotonin, thromboxan A2, PAF (platelet produced)

collagen, thrombin, epinephrine (injury produced)

how do platelets help in the plasma coagulation process?

act as procoagulants for the fibrin gel clot 

-PF3 is a cofactor 

-surface is reaction template for other coag proteins

how do platelets help in maintanence of vascular endothelium?

platelet derived growth factor (PDGF) promotes growth of endothelial cells 

serotonin is a vasoconstictor 

how does aspirin prevents clots?

it block COX1 (cyclooxygenase) and therefore diminshes thromboxane

how do other meds and alcohol block clotting?

they inhibit other pathways