Platelets or thrombocytes in animals
Platelets or thrombocytes in animals are disk-shaped structures, derived from the megakaryocytes of the bone marrow. Mammalian platelets are non-nucleated, whereas they are nucleated in birds and reptiles.
In mammals, platelets originate from the megakaryocytes of the bone marrow and In birds they arise from large mononucleated cells of bone marrow. Thrombocytes are about 3 µm in diameter.
Megakaryocyte
Megakaryocyte are the myeloid cells of the bone marrow, rarely seen in the circulation as giant cells. Its fragments provide the platelets or thrombocytes.
Structure of platelets
The thick glycocalyx of the platelet cell membrane contains clotting factors like fibrinogen, factor V, VII and IX. The phospholipid of the cell membrane is the precursors for prostaglandins and platelet factors. Platelet cytoplasm have two types of granules; the alpha and dense granules. They also have contractile fibrils containing actin and myosin, lysosomes, mitochondria and glycogen.
| a – Granules | Functions |
| Albumin | Maintains colloidal osmotic pressure |
| β – Thromboglobulin | Inhibitor of Prostacycline formation |
| Factor V/Proaccelerin | Involved in blood coagulation |
| Fibronectin | Helps platelet adhesion with extracellular membrane |
| Platelet derived growth factor | Necessary for the growth of endothelial cells & Fibroblast |
| TGF – β | Involved in tissue repair |
| von Willebrand factor (vWF) | Helps platelet adhesion |
| Factor XIII | Fibrin stabilizing factor |
| Dense Granules | Functions |
| ADP | Involved in platelet activation |
| ATP | Provides energy |
| Calcium | Provides principle stimulus to blood coagulation |
| Serotonin | Vasoconstriction |
| Actin & Myosin | Contractile proteins, involved in clot retraction |
| Mitochondria | Powerhouse to synthesis ATP |
| Lysosomes | To digest the damaged tissues |
| Glycogen | Primary source of energy |
Platelets produce thromboxane A2, which produces platelet aggregation and contraction of platelet (Clot retraction). Platelets survive for 8-11 days in circulating blood.
Functions of platelets
- main function of platelets is in blood coagulation and clot retraction.
- They provide platelet phospholipid for coagulation and carry several clotting factors on their surface.
- Platelets prevent hemorrhage when blood vessels are injured by transforming into platelet plugs, which is an important mechanism for closure of minute ruptures in blood vessels.
Normal platelet count in domestic animals
| Dog | Cat | Cow | Sheep | Goat | Horse | Pig |
| 3,00,000 | 4,50,000 | 5,00,000 | 4,00,000 | 4,50,000 | 2,25,000 | 5,20,000 |