ACTIVATORS AND INHIBITORS OF PLATELETS

ACTIVATORS AND INHIBITORS OF PLATELETS

ACTIVATORS OF PLATELETS

1. Collagen, which is exposed during damage of blood vessels

2. von Willebrand factor

3. Thromboxane A2

4. Platelet-activating factor

5. Thrombin

6. ADP

7. Calcium ions

8. P-selectin: Cell adhesion molecule secreted from endothelial cells

9. Convulxin: Purified protein from snake venom.

INHIBITORS OF PLATELETS

1. Nitric oxide

2. Clotting factors: II, IX, X, XI and XII

3. Prostacyclin

4. Nucleotidases which breakdown the ADP.

DEVELOPMENT OF PLATELETS

Platelets are formed from bone marrow. Pluripotent stem cell gives rise to the colony forming unit-megakaryocyte (CFU-M). This develops into megakaryocyte. Cytoplasm of megakaryocyte form pseu do podium. A portion of pseudopodium is detached to form platelet, which enters the circulation. Production of platelets is influenced by colony-stimulating factors and thrombopoietin. Colony-stimulating factors are secreted by monocytes and T lymphocytes. Thrombopoietin is a glycoprotein like erythropoietin. It is

secreted by liver and kidneys

 LIFESPAN AND FATE OF PLATELETS

Average lifespan of platelets is 10 days. It varies between 8 and 11 days. Platelets are destroyed by

tissue macrophage system in spleen. So, splenomegaly (enlargement of spleen) decreases platelet count and splenectomy (removal of spleen) increases platelet count.

PLATELET DISORDERS

Platelet disorders occur because of pathological variation in platelet count and dysfunction of platelets.

Platelet disorders are:

1. Thrombocytopenia

2. Thrombocytosis

3. Thrombocythemia

4. Glanzmann’s thrombasthenia.

1. Thrombocytopenia

Decrease in platelet count is called thrombocytopenia. It leads to thrombocytopenic purpura .

Thrombocytopenia occurs in the following conditions:

i. Acute infections

ii. Acute leukemia

iii. Aplastic and pernicious anemia

iv. Chickenpox

v. Smallpox

vi. Splenomegaly

vii. Scarlet fever

viii. Typhoid

ix. Tuberculosis

x. Purpura

xi. Gaucher’s disease.

2. Thrombocytosis

Increase in platelet count is called thrombocytosis.

Thrombocytosis occurs in the following conditions:

i. Allergic conditions

ii. Asphyxia

iii. Hemorrhage

iv. Bone fractures

v. Surgical operations

vi. Splenectomy

vii. Rheumatic fever

viii. Trauma (wound or injury or damage caused by external force) Thrombocythemia

Thrombocythemia is the condition with persistent and abnormal increase in platelet count. Thrombocythemia occurs in the following conditions:

i. Carcinoma

ii. Chronic leukemia

iii. Hodgkin’s disease.

4. Glanzmann’s Thrombasthenia

Glanzmann’s thrombasthenia is an inherited hemorrhagic disorder, caused by structural or functional abnormality of platelets. It leads to thrombasthenic purpura. However, the platelet count is normal.

It is characterized by normal clotting time, normal or prolonged bleeding time but defective clot retraction

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