Platelets or thrombocytes

Platelets or thrombocytes are the formed elements of blood. Platelets are small colorless, non-nucleated and moderately refractive bodies. These formed elements of blood are considered to be the fragments of cytoplasm.

Size of Platelets

Diameter : 2.5 μ (2 to 4 μ)

Volume : 7.5 cu μ (7 to 8 cu μ).

Shape of Platelets

Normally, platelets are of several shapes, viz. spherical or rod-shaped and become oval or disk-shaped when inactivated. Sometimes, the platelets have dumbbell shape, comma shape, cigar shape or any other unusual shape. Inactivated platelets are without processes or filopodia and the activated platelets develop processes or filopodia .

STRUCTURE AND COMPOSITION

Platelet is constituted by:

1. Cell membrane or surface membrane

2. Microtubules

3. Cytoplasm.

CELL MEMBRANE

Cell membrane of platelet is 6 nm thick. Extensive invagination of cell membrane forms an open canalicular system . This canalicular system is a delicate tunnel system through which the platelet granules extrude their contents. Cell membrane of platelet contains lipids in the form of phospholipids, cholesterol and glycolipids, carbohydrates as glycocalyx and glycoproteins and proteins. Of these substances, glycoproteins and phospholipids are functionally important.

Glycoproteins

Glycoproteins prevent the adherence of platelets to normal endothelium, but accelerate the adherence of platelets to collagen and damaged endothelium in ruptured blood vessels. Glycoproteins also form the receptors for adenosine diphosphate (ADP) and thrombin.

Phospholipids

Phospholipids accelerate the clotting reactions. The phospholipids form the precursors of thromboxane A2

and other prostaglandin-related substances.

MICROTUBULES

Microtubules form a ring around cytoplasm below the cell membrane. Microtubules are made up of polymerized proteins called tubulin. These tubules provide structural support for the inactivated platelets to maintain the disklike shape.

CYTOPLASM

Cytoplasm of platelets contains the cellular organelles, Golgi apparatus, endoplasmic reticulum, mitochondria, microtubule, microvessels, filaments and granules. Cytoplasm also contains some chemical substances such as proteins, enzymes, hormonal substances, etc.

Proteins

1. Contractile proteins

i. Actin and myosin: Contractile proteins, which are responsible for contraction of platelets.

ii. Thrombosthenin: Third contractile protein, which is responsible for clot retraction.

2. von Willebrand factor: Responsible for adherence of platelets and regulation of plasma level of factor

VIII.

3. Fibrin-stabilizing factor: A clotting factor.

4. Platelet-derived growth factor (PDGF): Responsible for repair of damaged blood vessels and wound

healing. It is a potent mytogen (chemical agent that promotes mitosis) for smooth muscle fibers of blood

vessels.

5. Platelet-activating factor (PAF): Causes aggregation of platelets during the injury of blood vessels,

resulting in prevention of excess loss of blood.

6. Vitronectin (serum spreading factor): Promotes adhe s ion of platelets and spreading of tissue cells in

culture.

7. Thrombospondin: Inhibits angiogenesis (formation of new blood vessels from pre-existing vessels).

Enzymes

1. Adensosine triphosphatase (ATPase)

2. Enzymes necessary for synthesis of prostaglandins.

Hormonal Substances

1. Adrenaline

2. 5-hydroxytryptamine (5-HT; serotonin)

3. Histamine.

Other Chemical Substances

1. Glycogen

2. Substances like blood group antigens

3. Inorganic substances such as calcium, copper, magnesium and iron.

Platelet Granules

Granules present in cytoplasm of platelets are of two types:

1. Alpha granules

2. Dense granules.

Alpha granules

Alpha granules contain:

1. Clotting factors – fibrinogen, V and XIII

2. Platelet-derived growth factor

3. Vascular endothelial growth factor (VEGF)

4. Basic fibroblast growth factor (FGF)

5. Endostatin

6. Thrombospondin.

Dense granules

Dense granules contain:

1. Nucleotides

2. Serotonin

3. Phospholipid

4. Calcium

5. Lysosomes.

NORMAL COUNT AND VARIATIONS

Normal platelet count is 2,50,000/cu mm of blood. It ranges between 2,00,000 and 4,00,000/cu mm of blood.

PHYSIOLOGICAL VARIATIONS

1. Age: Platelets are less in infants (1,50,000 to 2,00,000/cu mm) and reaches normal level at 3rd

month after birth.

2. Sex: There is no difference in the platelet count between males and females. In females, it is reduced

during menstruation.

3. High altitude: Platelet count increases.

4. After meals: After taking food, the platelet count increases.

 

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