PLATELETS (THROMBOCYTES)

Lecture by GK Mbassa

Purpose of knowing structure, biochemistry and functions of

platelets

• Understand qualitative platelet abnormalities

• Gain knowledge on hemostasis for treatment of diseases

• Know platelets role in tumor metastases, atherosclerosis and inflammation resulting from cytoplasmic fragments of megakaryocytes, e.g. arachidonic acid.

Morphology of platelets

• Heterogenous in blood smears; discoid, spheroid, elongated, flat

• Granular organelles distributed in cytoplasm.

• Some organelles in centre (granulomere)

• Platelet cytoplasm is hyalomere, which is clear

• Platelet is bounded by thin membrane, smooth or having fine projections

• EDTA anticoagulant minimizes platelets clumping

• Platelets may clump to other cells (erythrocytes and neutrophils), called satellitism.

• Platelet volume in dog, pig, man is 7.6 – 8.3 fl, in cattle, equine, sheep, rat, guinea pig, mouse it is 3.2. – 5.4 fl, while in the cat it is 15.1 fl,

• Platelet counts vary (1- 10 x 1011/l)

• Larger platelets are metabolically and functionally more active than small platelets.

• Scanning electron microscope show platelets to have discoid or lentiform shape, with smooth surfaces, slightly biconcave surface, has shallow indentations at external openings of the open canalicular system

• Surface projections represent protractions of platelets granules

• Surface features of platelets are similar in most species.

• Platelets diameter length is 1.3 – 4.7 µm in dog, cat, equines, cow, sheep and goat.

• Platelet thickness is 0.5 µm

• Transformed platelets acquire pseudopods or projections, found also in normal blood Surface projections occur very fast when blood is taken out of vessel, vary in number and sizes between species

Ultrastructural features of the platelet

• Unit membrane covered with amorphous material (external or exterior coat)

• Bundles of microtubules in matrix beneath membrane

• Internal structure comprises of heterogenous granules (alpha-granules)

• Clycogen particles

• Dense granules

• Mitochondria

• Lysosomes

• Peroxisomes

• Poorly developed Golgi complex

• Endoplasmic reticulum (rarely)

• Spongy like channels, called open canalicular system)

• Open canalicular system communicate with substance of platelet, open to surface at invaginations.

• Open canalicular system is lined by unit membrane, covered by external coat

• Another system of platelet channels is the dense tubular system.

• Dense tubular system occurs under marginal band of microtubules and appears to open to surface, but does not open on the platelet surface.

• Platelets of many animals have similar morphology.

• Platelets have two types of granules, (1) alpha-granules, and (2) dense granules.

• Dense and alpha granules are homogeneously distributed, but vary in electron density, number and size.

Functional organization of the platelet

The platelet is divided into four structural regions

• (1) Peripheral zone

• (2) Sol-gel zone

• (3) Organelle zone

• (4) Membrane system

Peripheral zone

• Composed of external (exterior) coat, unit membrane, sub-membraneous area

• Functions, maintain platelet integrity, receive and transmit stimuli triggering platelet responses (adhesions, aggregations)

• Exterior coat has glycoproteins (glycocalyx) contains mucopolysaccharides and Mg2+ dependent AT Pase, plasma proteins (fibrinogen, IgG, IgM), coagulation factors (vitamin K-dependent factors, factors V and VIII)

• Glycoproteins have receptors for platelet activation and aggregation.

• Seven glycoproteins recognised, including glycoprotein 1b (reaction site for von Willebrand factor, a component of coagulation factor VIII) necessary platelet adhesion to endothelium on injured blood vessel

• Platelet membrane; maintains platelet integrity, rich in phospholipids.

• Platelet phospholipids function in blood coagulation (eg

Sol-gel zone

• Represented by matrix of platelet cytoplasm, contains microfilaments and microtubules, which function as cytoskeletal elements.

• Microfilaments and microtubules maintain discoid platelet shape, form contractile system for shape change, pseudopod formation, internal contractions and granule secretion.

• Microfilaments also function in clot retraction.

• Microfilaments are also associated with thrombosthenin, a contractile protein (has actin-myosin)

• Microtubule tubulin dissolves at 4oC, when exposed to colchicine or vinca alkaloids, leading to platelet shape irregularities.

Organelle zone

• Composed of all internal platelet components, except microtubules, microfilaments (sol-gel zone) components and membrane system.

• Main component of organelle zone are platelet granules, that are morphologically and biochemically heterogeneous, azurophilic granules (alpha-granules under electron microscope)

• Alpha-granules are membrane bound, oval, round, electron dense, contain platelet factor 4 (antiheparin), congulation factor V, fibrinogen, beta-thromboglobulin (a thrombin-sensitive protein), fibronection, factor VIII- related antigen, and a mitogenic or growth factor.

• Platelets in von Willebrand disease lack factor VIII related antigen

• Electron dense granules, called delta granules, or dense bodies contain non metabolic pool ATP and ADP, Ca2+, mono-amines (serotonin, histamine).

• Dense granules vary with species.

External coat

Microfilaments

Alpha granule

Open canalicular system

Dense tubular system

Microtubules

Golgi complex

Lysosome

• Lysosomal granules contain acid hydrolases; acid phosphatase, β-glucuromidase, Contraction of microtubules forces all internal organelles towards the centre squeezing or without squeezing their contents to the exterior via open canalicular system.

• Platelet activation triggers secretion of various platelet constituents.

Membrane system

Memberane system comprises the

• Open canalicular system

• Dense tubular system

Open canalicular system provides a passage for externalization of platelet secretory products and internalization of substances from plasma into the platelet.

Dense tubular system provides a site for sequestration of Ca2+ and localization of enzymes needed for prostaglandin synthesis

Release of Ca2+ from the dense tribular system triggers platelet aggregation

Platelet constituents

Platelet location

Constituent Function

Exterior coat Fibrinogen Platelet aggregation

Membrane Arachidonic acid

Prostaglandin synthesis

Plaletet factor 3

(phospholipid)

Enhances

Coagulation

cAMP Inhibits release reaction

Platelet constituents

Microtubules Tubulin Cytoskeleton

Contractility

Microfilaments Thrombosthenin Shape change, clot reaction, release reaction

Alpha-granules Beta-thromboglobulin

Impedes prostacylin production from endothelial cells

Catalase Enzymic process

Factor VIII-relatedantigen

Platelet adhesion to subendothelium

Fibrinogen Shape change, clot reaction, release reaction

Beta-thromboglobulin

Impedes prostacylin production from endothelial cells

Fibronectin Adherence to extracellular matrix

Promote wound healing