Platelets

Platelets

Overview Structure Substances Platelet Count Properties Functions Development Lifespan & Fate Disorders

1 Overview of Platelets

Platelets or thrombocytes are the formed elements of blood.

Platelets are:

  • Small
  • Colorless
  • Nonnucleated
  • Moderately refractive bodies
  • Considered as 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 have several shapes such as:

  • Spherical
  • Oval
  • Disk
  • Rod

Sometimes, platelets may have:

  • Dumbbell shape
  • Comma shape
  • Cigar shape
  • Any other unusual shape.

2 Structure of Platelets

1. Cell membrane / surface membrane
2. Microtubules
3. Cytoplasm

1. Cell Membrane

  • Cell membrane is 6 nm thick.
  • It contains:
Lipids
  • Phospholipids
  • Cholesterol
  • Glycolipids
Other Components
  • Carbohydrates as glycocalyx
  • Glycoproteins
  • Proteins

2. Microtubules

  • Microtubules form a ring around the cytoplasm below the cell membrane.
  • Microtubules are made up of proteins called tubulin.
  • Microtubules provide structural support for inactivated platelets to maintain the disk-like shape.

3. Cytoplasm

Cytoplasm of platelets contains cellular organelles such as:

  • Golgi apparatus
  • Endoplasmic reticulum
  • Mitochondria
  • Microtubule
  • Microvessels
  • Filaments
  • Granules

Platelet Granules

Platelet granules are of 2 types:

  1. Alpha granules
  2. Dense granules

3 Substances Present in Cytoplasm of Platelets

I. Proteins

Protein Action
1. Contractile proteins
Actin, Myosin, Thrombosthenin 		Actin and myosin → contraction of platelets;
Thrombosthenin → clot retraction
2. von Willebrand factor Adherence of platelets; regulation of plasma level of factor VIII
3. Fibrin-stabilizing factor Clotting factor XIII
4. Platelet-derived growth factor (PDGF) Repair of damaged blood vessels; wound healing; potent mitogen (chemical agent that promotes mitosis) for smooth muscle fibers of blood vessels; proliferation of connective tissue
5. Platelet-activating factor (PAF) Aggregation of platelets during injury of blood vessels, resulting in prevention of excess loss of blood
6. Vitronectin (serum-spreading factor) Adhesion of platelets; spreading of tissue cells in culture
7. Thrombospondin Inhibition of angiogenesis (formation of new blood vessels from pre-existing vessels)

II. Enzymes

  1. ATPase
  2. Enzymes necessary for synthesis of prostaglandins

III. Hormonal Substances

  1. Adrenaline
  2. 5-HT (serotonin)
  3. Histamine

IV. Other Chemical Substances

  1. Glycogen
  2. Substances like blood group antigens
  3. Inorganic substances such as:
    • Calcium
    • Copper
    • Magnesium
    • Iron

4 Substances in Platelet Granules

Alpha Granules Dense Granules
1. Clotting factors: Fibrinogen, V and XIII 1. Nucleotides
2. Platelet-derived growth factor 2. Serotonin
3. Vascular endothelial growth factor 3. Phospholipid
4. Basic fibroblast growth factor 4. Calcium
5. Endostatin 5. Lysosomes
6. Thrombospondin

5 Normal Platelet Count and Its Variations

Normal Platelet Count

  • 2,50,000/cu mm of blood
  • It ranges between 2,00,000/cu mm and 4,00,000/cu mm of blood.

Physiological Variations

1. Age
  • Platelets are less in infants:
    • 1,50,000/cu mm to 2,00,000/cu mm
  • Reaches normal level at the 3rd month after birth.
2. Sex
  • There is no difference in platelet count between males and females.
  • In females, platelet count is reduced during menstruation.
3. High Altitude
  • Platelet count increases.
4. After Meals
  • After taking food, platelet count increases.

6 Properties of Platelets

1. Adhesiveness

Adhesiveness is the property of sticking to a rough surface.
  • While having contact with any rough surface:
    • Platelets are activated.
    • Platelets adhere to the surface.

2. Aggregation

Aggregation is the grouping of platelets.
  • Activated platelets group together and become sticky.

Activation of Platelets

  • During activation, platelets change their shape with elongation of long filamentous pseudopodia.
  • These are called:
    • Processes
    • Filopodia
  • Filopodia help the platelets aggregate together.
  • Activation and aggregation of platelets are accelerated by:
    • ADP
    • Thromboxane A₂
    • Platelet-activating factor (PAF)
  • PAF is a cytokine secreted by:
    • Neutrophils
    • Monocytes

3. Agglutination

Agglutination is the clumping together of platelets.

7 Functions of Platelets

1. Role in Blood Clotting
  • Platelets are responsible for formation of intrinsic prothrombin activator.
  • This substance is responsible for onset of blood clotting.
2. Role in Clot Retraction
  • In blood clot, blood cells including platelets are entrapped in between the fibrin threads.
  • Contractile proteins present in cytoplasm of platelets are:
    • Actin
    • Myosin
    • Thrombosthenin
  • These proteins are responsible for clot retraction.
3. Role in Prevention of Blood Loss: Hemostasis

Platelets accelerate hemostasis (arrest of bleeding) by 3 ways:

  1. Platelets secrete 5-HT, which causes constriction of blood vessels.
  2. Due to their adhesive property, platelets seal the damage in blood vessels like capillaries.
  3. By formation of temporary plug, platelets also seal the damage in blood vessels.
4. Role in Repair of Ruptured Blood Vessel
  • Platelet-derived growth factor (PDGF) formed in cytoplasm of platelets is useful for repair of:
    • Endothelium
    • Other structures of ruptured blood vessels.
5. Role in Defense Mechanism
  • By the property of agglutination, platelets encircle small foreign bodies and destroy them.

8 Development of Platelets

Platelets are developed from bone marrow.
  • Pluripotent stem cell gives rise to CFU-M which develops into megakaryocyte.
  • Cytoplasm of megakaryocyte forms pseudopodium.
  • A portion of pseudopodium is detached to form a platelet, which enters the circulation.
  • Production of platelets is influenced by thrombopoietin.
  • Thrombopoietin is a glycoprotein like erythropoietin, secreted by:
    • Liver
    • Kidneys.

9 Lifespan and Fate of Platelets

Average Lifespan

About 10 days.

Fate

Older platelets are destroyed by the tissue macrophage system in spleen.

10 Applied Physiology: Platelet Disorders

Platelet disorders occur because of:
  • Pathological variation in platelet count
  • Dysfunction of platelets

1. Thrombocytopenia

Definition

Decrease in platelet count.

Occurs in

  1. Acute infections
  2. Acute leukemia
  3. Aplastic and pernicious anemia
  4. Chickenpox
  5. Smallpox
  6. Splenomegaly
  7. Scarlet fever
  8. Typhoid
  9. Tuberculosis
  10. Purpura
  11. Gaucher’s disease
Gaucher’s disease:

Inherited metabolic disorder characterized by accumulation of fats in many organs of the body.

Effect

Thrombocytopenia leads to thrombocytopenic purpura.

2. Thrombocytosis

Definition

Increase in platelet count.

Occurs in

  1. Allergic conditions
  2. Asphyxia
  3. Hemorrhage
  4. Bone fractures
  5. Surgical operations
  6. Splenectomy
  7. Rheumatic fever
  8. Trauma
Trauma:

Wound or injury or damage caused by external force.

3. Thrombocythemia

Definition

Persistent and abnormal increase in platelet count.

Occurs in

  1. Carcinoma
  2. Chronic leukemia
  3. Hodgkin’s disease

4. Glanzmann’s Thrombasthenia

  • Inherited hemorrhagic disorder, caused by structural or functional abnormality of platelets.
  • Platelet count is normal.

Characterized by

  • Normal clotting time
  • Normal or prolonged bleeding time
  • Defective clot retraction
This condition leads to thrombasthenic purpura.
Platelets