Platelets

Basics of blood platelets .

1. Platelets

2. George Gulliver in 1841 drew pictures of platelets using the twin lens (compound) microscope invented in 1830 by Joseph Jackson Lister.   William Addison in 1842 drew pictures of a platelet-fibrin clot. Max Schultze in 1865 described what he called "spherules“. James Wright examined blood smears using the stain named for him, and used the term  plates  in his 1906 publication but changed to  platelets  in his 1910 publication which has become the universally accepted term. The term  thrombocyte  (clot cell) came into use in the early 1900s.

3. Literally means, a small plate. Also k/as Thrombocytes ( Thrombus= lump or clot: kytos = cell). Size 2-4µ. Small irregularly shaped blood cells. No nucleus but mitochondria present. Extensive canalicular system present.

4. Microtubular system composed of contractile proteins. Besides contractile proteins, the metabolic pathways are also remarkably similar to skeletal muscle. Normal count= 150,000 to 400,000/mm 3 . Circulating platelets are 2/3 rd of platelet pool of body. Remaining 1/3 rd reside in the spleen.

5. Bleeding Acutely Stressful condition to the body Activation of sympathoadrenal system Release of epinephrine Action on spleen Release of platelets as well as erythrocytes Help in haemostasis

6. Structurally the platelet can be divided into four zones, from peripheral to innermost: Peripheral zone – is rich in glycoproteins required for platelet adhesion, activation, and aggregation. Sol-gel zone – is rich in microtubules and microfilaments, allowing the platelets to maintain their discoid shape. Organelle zone – is rich in platelet granules. Membranous zone – contains membranes derived from megakaryocytic smooth endoplasmic reticulum .

7. Organelle zone – is rich in platelet granules.  Alpha granules contain clotting mediators such as factor V, factor VIII, fibrinogen, fibronectin , platelet-derived growth factor, and chemotactic agents. Delta granules, or dense bodies, contain ADP, calcium, serotonin, which are platelet-activating mediators. γ granules (gamma granules) – similar to lysosomes and contain several hydrolytic enzymes. λ granules (lambda granules) – contents involved in clot resorption during later stages of vessel repair. Membranous zone –is responsible for thromboxane A2 synthesis. This dense tubular system is connected to the surface platelet membrane to aid thromboxane A2 release.

8. Formation & fate Formed in bone marrow. Smallest blood cells but have largest precursor cell. Precursor cell, called megakaryocyte . Megakaryocyte is a large, multinucleated cell. Size of megakaryocyte is 35-160 µ diameter. Nucleus is multi- loabed in appearance and has chromatin material equivalent to 4-16 nuclei. Nucleus has divided repeatedly and cytoplasm has grown correspondingly but cell has not divided into daughter cells.

9. Mega- karyocytes develop demarcation membranes with in their cytoplasm and then form pseudopodia like processes. These processes may penetrate endothelial lining of bone marrow sinusoids. Then processes split along the demarcation membrane. Each cytoplasmic fragment so formed is a platelet. One mega- karyocyte forms 2000-4000 platelets by this process of fragmentation.

10. Life span of throbocytes is 9-12 days. They are destroyed in the spleen. Spleen serves as both graveyard and reservoir. Death of a platelet may be due to senescence or random destruction. Thrombopoiesis and destruction is balanced process. Number of platelets fluctuates within narrow range. Thrombopoiesis seems to be regulated by a group of humoral factors collectively called thrombopoietin .

11. Functions The functions of platelets are primarily related to haemostasis . Serotonin released by platelets contributes to the vasoconstriction observed immediately after vascular injury. Platelets aggregates to plug the vascular injury. Platelets provide platelet factor 3 which accelerate the clotting process. Contractile proteins of the platelets bring about clot retraction. It has growth factors which stimulate mitosis in vascular wall thus repair the damage of vessel wall.

12. Laboratory tests for platelet function

13. Applied Low platelet concentration is thrombocytopenia and is due to either  decreased production  or  increased destruction . Elevated platelet concentration is  thrombocytosis  and is either  congenital ,  reactive  (to cytokines), or due to  unregulated production : one of the myeloproliferative neoplasms  or certain other myeloid neoplasms .

14. Thrombocytopenia Immune thrombocytopenias (ITP) – formerly known as immune thrombocytopenic purpura and idiopathic thrombocytopenic purpura Splenomegaly Gaucher's disease Familial thrombocytopenia Chemotherapy Babesiosis Dengue

15. Altered platelet function Congenital Gray platelet syndrome ADP Receptor defect Decreased cyclooxygenase activity Storage pool defects, acquired or congenital Acquired Paroxysmal nocturnal hemoglobinuria Asthma Cancer Malaria Decreased cyclooxygenase activity

16. Thrombocytosis and  thrombocythemia Chronic infection Chronic inflammation Malignancy Hyposplenism (post- splenectomy ) Iron deficiency Acute blood loss Myeloproliferative neoplasms – platelets are both elevated and activated Essential thrombocytosis Polycythemia vera Associated with other myeloid neoplasms Congenital