Platelet’s Structure:
- Discoid in shape, 1-4 um (2-3um)
- Has four main structures
1. PERIPHERAL zone
a. Glycocalyx – outer most layer of platelets, also known as the surface coat
- contains glycoproteins (adhesion and aggregation), surface receptors for coagulation factors (prothrombinase)
- functions to act as an open( pore-like indentation are found) communication channel between the external and
internal environment.
b. Plasma membrane
- contains phospholipids and fatty acids e.g. arachidonic acid (important for platelet function and coagulation)
and ATPase sodium-potassium pump.
- functions as a chemical and physical barrier
c. Sub-membrane area
- found underlying the plasma membrane
- main function is to prevent the cytoplasmic constituents (organelles) from coming in contact with the inner part
of the plasma membrane
- contains also filaments which secondarily functions to support the shape of the platelet, form pseudopods and
contraction during adhesion and retraction of platelets.
2. SOL-GEL ZONE:
- contains a circumferential microtubule system and randomly arranged microfilaments.
- the gel-like environment functions to hold the organelles and microtubules/microfilament in place
- also functions to maintain the discoid shape of the platelet and regulate a change in its shape if stimulated for
adhesion and aggregation (platelet activation):
a. Microtubule – primarily serves as the skeletal system
- mainly provides and maintains the discoid-shape of the platelet and controls or governs
platelet contractile response, during which microtubules moves inward and may
reassemble in long parallel bundles for pseudopod formation during platelet shape change.
b. Microfilaments – primarily serves as the muscular system
- mainly responsible for platelet contraction since it contains thrombosthenin.
Microfilaments are found in the area between the microtubules and the plasma membrane.
During platelet activation, the increase in cytoplasmic calcium activates actin transforming
it from being amorphous and globular to filamentous and contractile.
-they are also the ones which anchors the receptors.
***intermediate filaments are found in the cytoplasm, this include desmin and vimentin
which interacts with the microfilaments and tubules in maintaining platelet shape.
*a-actinin and actin-binding protein are transmembrane protein linking microfilaments to the outer
membrane of platelets. Explains how microfilaments may influence contraction by also modulating
the outer platelet cytoskeleton.
3. ORGANELLE ZONE
Organelle Constituents: Function:
Dense granules
Others: guanosine, ADP Stimulates platelet aggregation
di and tri – adenosine Stimulate vasodilation and inhibits platelet activation by increasing cAMP
phosphates levels
Calcium, magnesium Activates phospholipids to release free forms of arachidonic acid that takes
part in platelet aggregation
Serotonin Stimulates vasoconstriction
5-hydroxytryptamine
Alpha granules
a. coagulation Fibrinogen Binds ADP stimulated platelets
Factor V Binds to platelet surface receptors to facilitate prothrombinase complex
formation
VIII:vWF
PF3 An activity: coagulation factors are sequestered inside platelets to
enhance this activity which results to an increase in the reaction efficiency
of the coagulation mechanism by the activated phospholipid membrane.
b. aggregation PF4 Stimulates aggregation and neutralizes heparin action
Thrombospondin Stimulates aggregation
- Discoid in shape, 1-4 um (2-3um)
- Has four main structures
1. PERIPHERAL zone
a. Glycocalyx – outer most layer of platelets, also known as the surface coat
- contains glycoproteins (adhesion and aggregation), surface receptors for coagulation factors (prothrombinase)
- functions to act as an open( pore-like indentation are found) communication channel between the external and
internal environment.
b. Plasma membrane
- contains phospholipids and fatty acids e.g. arachidonic acid (important for platelet function and coagulation)
and ATPase sodium-potassium pump.
- functions as a chemical and physical barrier
c. Sub-membrane area
- found underlying the plasma membrane
- main function is to prevent the cytoplasmic constituents (organelles) from coming in contact with the inner part
of the plasma membrane
- contains also filaments which secondarily functions to support the shape of the platelet, form pseudopods and
contraction during adhesion and retraction of platelets.
2. SOL-GEL ZONE:
- contains a circumferential microtubule system and randomly arranged microfilaments.
- the gel-like environment functions to hold the organelles and microtubules/microfilament in place
- also functions to maintain the discoid shape of the platelet and regulate a change in its shape if stimulated for
adhesion and aggregation (platelet activation):
a. Microtubule – primarily serves as the skeletal system
- mainly provides and maintains the discoid-shape of the platelet and controls or governs
platelet contractile response, during which microtubules moves inward and may
reassemble in long parallel bundles for pseudopod formation during platelet shape change.
b. Microfilaments – primarily serves as the muscular system
- mainly responsible for platelet contraction since it contains thrombosthenin.
Microfilaments are found in the area between the microtubules and the plasma membrane.
During platelet activation, the increase in cytoplasmic calcium activates actin transforming
it from being amorphous and globular to filamentous and contractile.
-they are also the ones which anchors the receptors.
***intermediate filaments are found in the cytoplasm, this include desmin and vimentin
which interacts with the microfilaments and tubules in maintaining platelet shape.
*a-actinin and actin-binding protein are transmembrane protein linking microfilaments to the outer
membrane of platelets. Explains how microfilaments may influence contraction by also modulating
the outer platelet cytoskeleton.
3. ORGANELLE ZONE
Organelle Constituents: Function:
Dense granules
Others: guanosine, ADP Stimulates platelet aggregation
di and tri – adenosine Stimulate vasodilation and inhibits platelet activation by increasing cAMP
phosphates levels
Calcium, magnesium Activates phospholipids to release free forms of arachidonic acid that takes
part in platelet aggregation
Serotonin Stimulates vasoconstriction
5-hydroxytryptamine
Alpha granules
a. coagulation Fibrinogen Binds ADP stimulated platelets
Factor V Binds to platelet surface receptors to facilitate prothrombinase complex
formation
VIII:vWF
PF3 An activity: coagulation factors are sequestered inside platelets to
enhance this activity which results to an increase in the reaction efficiency
of the coagulation mechanism by the activated phospholipid membrane.
b. aggregation PF4 Stimulates aggregation and neutralizes heparin action
Thrombospondin Stimulates aggregation
