Histology - Platelets

PLATELETS

Thromboplastids• Minute, colorless, anucleate corpuscles

• Flat biconcave discs 2-3 micron in diameter, round or oval on the flat

• Fusiform on the edge

• 150,000 – 350,000 per cubic mm

• Life span of 8-11 days

• In stained smears – – Hyalomere – thin pale blue peripheral zone - contain finely filamentous microtubules

– Chromomere –granulomere – thick central zone - alpha granule – contain serotonin

• Platelets contain contractile material with properties similar to those of actinomyosin in muscle

• Activation during blood clotting initiates polymerization of actin and myosin monomers into filamentous form necessary for contraction

Function• Principal– Hemostasis

• To patch small defects in the endothelial lining of blood vessels

• Limit hemorrhage by promoting local coagulation

Hemostasis

• Platelet adhesion – sticking of platelets to solid surfaces

• Adherence – encounter surfaces they are not normally exposed to – glass, plastic or other solids

• Platelet aggregation – sticking of platelets to each other

• In the circulation – no tendency to adhere to each other, to other cells or to the lining of blood vessels

• At sites of vascular injury they adhere to damaged endothelium and to exposed collagen forming a layer of platelets over the denuded area

• The adhering platelets are activated by this contact to break down their ATP and release ADP onto their surface and into surrounding medium

• ADP is a potent inducer of platelet aggregation and other platelets stick to those initially deposited

• These in turn are activated and induce further aggregation

• The mass of platelets on the vessel wall thus continues to enlarge producing a platelet thrombus and finally hemostatic plug

Other complex reaction

Coagulation

• Tissue thromboplastin– Released from the injured tissue of the vessel wall

initiates a series of reactions in the blood plasma that convert prothrombin to thrombin

– Thrombin catalyzes the conversion of plasma fibrinogen to fibrin –

– Fibrin - polymerizes as a feltwork of cross striated fibrils that enmesh erythrocytes and platelets to form a gelatinous clot

- In the process of aggregation and activation, the platelets undergo dramatic morphological changes

- They extend their numerous slender processes, release the content of their granules and ultimately coalesce into coherent viscous mass

- Degranulation releases phospholipids which reacts with other plasma components to produce platelet thromboplastin

- Platelet thromboplastin– acts to promote progression of the clotting process initiated by tissue thromboplastin

• Within an hour or so after its formation, the blood clot shrinks to about half its normal volume

• Shrinkage - Attributed to polymerization of actin and myosin filaments during the viscous metamorphosis of the platelets triggered by thrombin and their interaction to produce contraction of the clot.

• The hemostasis achieved by occluding the lumen is supplemented by active constriction of the injured vessel

Hemostasis: other factors• This is in part a direct consequence of mechanical

stimulation of the vessel wall at the time of injury,

• evidence that diffusible substances released from the platelet mass also play a role.

• The serotonin of the platelets may be involved and proteolytic enzymes activated in the clotting process may result in production of bradykinin and other vasoactive peptides

Other blood components

Plasma proteins

• Albumin

• Globulin

• Clotting factors: fibrinogen

Albumin

• Most abundant and smallest of the plasma proteins

• Synthesized by the liver• Principal function– Maintain the colloid osmotic pressure within the

blood capillaries which prevents excessive loss of fluid to the tissues

– Transport of metabolic products, drugs

globulins

• Divided into several fractions

• Gamma globulins– Immune gammaglobulins(antibodies)• Basis of immulogical defenses of the body against

bacteria, toxins and foreign proteins• Synthesized in the lymphoid organs

• Beta globulins– Function in the transport of hormones, metal ions

and lipid

- Ceruloplasmin – transports copper

– Transferrin – combines with iron, copper and zinc• Main function is to transport iron

• Serum lipoproteins– Involved in lipid transport• Chylomicrons

– Largest– Carry lipids from the intestine to the liver

• Very low density lipoproteins– Carrier of lipid from the liver to adipose tissue

• Low density lipoproteins

Plasma clotting factors

• major defense against serious blood loss• Clotting Factors: Factor VIII, Factor VII, etc • Fibrinogen – Synthesized in the liver