The mechanism of thrombocytes (platelets) and how it affects human health

Introduction

Thrombocytes, also called “platelets” are blood cells whose function (along with the coagulation factors) is to stop bleeding. Platelets have no nucleus, they are fragments of cytoplasm which are derived from the megakaryocytes of bone marrow (Machlus, 2014) and enter the circulation. Platelets are found only in mammals; where as in other animals (example birds, amphibian) thrombocytes circulate as intact mononuclear cells. On a stained blood smear, platelets appear as dark purple sports about 20% the diameter of red blood cells. The smear is used to examine platelets for size, shape, qualitative number and clumping. The ratio of platelets to red blood cells in a healthy adult is 1: 10 to 1:20.

The main function of platelets is to contribute to hemostasis; the process of stopping bleeding at the site of interrupted endothelium adhesion, secondly they change shape turn on receptors and secrete chemical messengers’ activation. Thirdly they connect to each other through receptor bridges aggregation, immediately an artery or vein is damaged, the platelet  in the area of injury begins to clump and stick together at the edges of the cut.

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 cytokine) or due to unregulated production. A disorder of platelet function is thrombocytopathy

Discovery, early observation and naming

George Gulliver in 1841 drew pictures of platelets using the twin lens (compound) microscope invented in 1830 by Joseph Jackson Lister. This microscope improved resolution sufficiently to make it possible to see platelet for to the first time Lionel Beale in 1864 was the first to publish a drawing showing platelet. Max Schulteze in 1865 describe what he called “Spherules”, which he noted were much smaller than red blood cells, occasionally clumped and were sometimes found in collections of fibrin material. Willam Osler observed them and in published lectures in 1886, called them a third compuscle and a blood plaque and described them as a colourless protoplasmic disc. James Wrigth 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 (Wrigth, 1910).

The term thrombosis (clot cell) came into use in the early 1900s and is sometimes used as a Synonym for platelet, but not generally in the scientific literature, except as a root word for other terms related to platelets (example thrombocytopenia meaning low platelets).

In some contexts, the word thrombus is used interchangeably with the word clot, regardless of its composition (White, Red or Mixed), thrombus anises from physiologic haemostasis, thrombosis arises from a pathologic and excessive quantity of clot. In other contact, it is used to contrast the result from the process: thrombus is the result to thrombosis is the process. (Furie, 2008)

Platelets production

Platelet is produced from Megakaryocytic stem cell in the bone marrow, Megakaryocytic and platelet production is regulated by thrombopoietin, a hormone produced in the kidneys and liver. Each megakaryocyte produces between 1,000 and 3,000 platelet during it lifetime, an average of 10 “platelets are produced daily in a healthy adult, reserve platelets are stored in the spleen and are released when needed by splenic contraction induced by the sympathetic nervous system. The average life span of circulating platelets is 8 to 9 days (Harker, 2000). Life span of individual platelets is controlled by the internal apoptotic regulating pathway. Old platelets are destroyed by phagocytosis in the spleen and liver.

Functions of thrombocytes

Adhesion

Thrombus formation on an intact endothelium is prevented by nitric oxide and prostacydin (Jones, 2012). Endothelial cells are attached to the sub endothelial collagen by Von Willebrand Factor (VWF) which these cells produced; platelets store VWF in their granules, when the endothelial layer is disrupted. Collagen and VWF anchor platelets to sub endothelium, plate receptor binds with VWF and GPVI receptor bind with collagen.

Activation

Platelet activation begins after adhesion occurs, It is triggered when collagen from subendothelium or tissue factor from the media and adventitia bind with the respective receptors on the platelet. These are G protein coupled receptors and they turn on calcium mediated signalling pathways within the platelet overcoming the baseline calcium efflux. Families of G proteins (Gs, Gi, G12) operate together for full activation.

Aggregation

It begins minutes after activation and occurs as a result of turning on the GPIIb/IIIa receptor which allows these receptor to bind with VWF or Fibrinogen,. These are 50-100 of these receptors per platelet, when any one or more of at least nine different platelet surface receptors are turn on during activation; intra-platelet signalling pathways cause existing GPIIb/IIIa receptors to change shape curled to straight and thus become capable of binding

Disorders of thrombocyte

Thrombocytopenia

This is a condition in which there is low concentration in platelet count, it often occur as a result of separate disorder such as leukaemia or an immune system problem or it can be a side effect of taking certain medication.

Causes of thrombocytopenia

1. Trapped platelet
2. Decreased production of platelet
3. Increased breakdown of platelet

Trapped platelet: This occurs when there is an enlarged spleen which can be caused by a number of disorders and may harbour too many platelets causing decrease in the number of platelet circulation.

Decreased production of platelet: As the production of platelet is from the bone marrow, low production of it will lead to various factors that can cause decreased platelet production such as;

• Leukaemia
• Anaemia
• Viral infection such as hepatitia C
• Chemotherapy drugs
• Heavy alcohol consumption

Increase breakdown of platelet: Some conditions can cause the body to use up or destroy platelet more rapidly than they are produced. And these lead to shortage of platelet in the bloodstream

• Pregnancy
• Immune thrombocytopenia
• Bacteria in the blood
• Medications

Signs and symptoms of thrombocytopenia

• Easy or excessive bleeding
• Prolong bleeding from cuts
• Bleeding from gums or nose
• Blood in urine or stool
• Fatigue
• Enlarge spleen
• Jaundice
• Superficial bleeding into the skin that appears as rash

Diagnosis of thrombocytopenia

Thrombocytopenia is been diagnosed along with complete blood count

Treatment of thrombocytopenia

The treatment for thrombocytopenia depends largely on it severity and the underlying cause. Patient with thrombocytopenia do not require regular platelet transfusion except in causes of surgery or other invasion procedure is planned in a patient with low platelet count less than 50,000 then the transfusion may be necessary to keep the platelet count greater than 50,000 (Nokes, 2010).

Prevention of thrombocytopenia

Thrombocytopenia can be preventable only if it underling cause is known for example a patient with alcohol, induce thrombocytopenia than alcohol avoidance is recommended. If any medication is known to cause low platelet count in an individual then it future use in that person may be discouraged.

Thrombocytosis

This is a condition in which there is high concentration of platelet count, it can be caused by underlying condition such as infection, these disorder is called Reactive Thrombocytosis; it can also be caused by blood and bone marrow disease and this is called Autonomous  Primary or Essential Thrombocythemia.

Causes of thrombocytosis

1. Acute bleeding and blood loss
2. Allergic reaction
3. Cancer
4. Chronic kidney failure
5. Heart attack
6. Infections
7. Iron deficiency anaemia
8. Removal of spleen
9. Haemolytic anaemia: type of anaemia in which the body destroy red blood cells faster than it produces, often due to certain blood disease or autoimmune disorders.
10. Inflammation such as from rheumatoid arthritis, celiac disease.
11. Major surgery
12. Pancreatitis
13. Trauma

Medications that can cause reactive thrombocytopenia

• Epinephrine (Adrenalin Chloride)
• Tretinion
• Vincristine

Sign and symptoms of thrombocytosis

• Headache
• Dizziness or light headedness
• Chest pain
• Weakness
• Fainting
• Temporary vision changes
• Numbness or tingling of hand and feet

Diagnosis of thrombocytosis

Laboratory test might include-full blood count, liver enzymes, renal function and erythrocyte sedimentation rate.

Treatment of thrombocytosis

Often no treatment is required or necessary for reactive thrombocytosis, in cases of reactive thrombocytosis of more than 1,00 x10⁹/L  it may be considered to administer daily low dose aspirin  to minimize the risk. High platelet count in primary thrombocytosis can be treated with hydroxyurea (a cytoreducing agent) or anagrelide (Agrylin) (Harrison, 2005).

Diagnosis of thrombocytes

Platelet concentration is measured either manually using a haemocytometer or by placing blood in an automated platelet analysis using electrical impedance such as coulter counter. The normal range for platelets in healthy causasians is 150,000 to 400,000 per cubic millimeter. The normal range has been confirmed to be the same in elderly and Spanish populations (Ruocco, 2001), Men have slightly higher mean values than women.

Principle of platelet count

Blood is diluted in 1 in 20 in 1% solution of ammonium oxalate, which lyses the red cells. Platelets are counted microscopically using an improved Neubver counting chamber and the number of platelet per litre of blood calculated.

Materials needed for visual platelets count

• Improved Neubauer counting chamber with cover glass
• Test tube
• Pasteur pipette
• Microscope
• EDTA blood sample
• 1% ammonium (reagent)

Procedure

1. Pipette 0.38 ml of filtered ammonium oxalate into a clean test tube
2. Add 0.02 ml of well mixed anticoagulated venous blood and mix
3. Clean the counting chamber and cover glass
4. Fill the counting chamber with its cover glass already in position using Pasteur pipette
5. Place the counting chamber in a moist Petri dish and leave untouched for at least 20 mins to give time for the platelet to settle.
6. Dry the underside of the chamber and place it on the microscope stage.
7. Using the x10 objective, focus the rulings of the grid and bring the central square of the chamber into view.
8. Change to x40 objective and focus the small platelet. They appear as small refractile particles.
9. Count the platelet in the small 5 squares of 0.04 mm² area of the central square.
10. Report the number of platelet in 1 litre of blood.

Conclusion

Therefore since thrombocyte contributes to haemostasis which is the process of stopping bleeding along with coagulation factor, it is therefore concluded that each individual should check and their visual platelets count.

References

Furie, B.  and Furie, B. C. (2003): Mechanism of thrombous formation.  New England Journal of Medicine 359 (9): 938-49.

Harker L. A., Roskes.L.K. and Marzec, U. M. (2000): Effects of Magakaryocyte growth and development factor on platelet production, platelet life span and platelet function in healthy human volunteers. Blood 95 (8): 2514-22.

Harrison, C. N., Campbell, P. J., Buck, G. and Wheatley, K. (2005): Hydroxyurea compared with anagrelide in high-risk essential thrombocythemia. NEJM 353 (1):33-45.

Jones, C.L., Barrett, N. E. and Moraes, L. A. (2012): Endogenous inhibitory mechanisms and the regulation of platelet function. Methods Mol. Biol. 788:341-66.

Machlus, K. R., Thon, J. N. and Haliano, J. E. (2014): Interpreting the developmental dance of the megakaryocyte: A review of the cellular and molecular processes mediating platelet formation. British Journal of Hematology 165 (2): 227-36.

Nokes, T. J., Van Veen, J. J. and Malaris, M. (2010): The risk of spinal haematoma following neuraxial anaesthesia or lumbar puncture in thrombocytopenic individuals. Br. J. Haematol. 148 (1): 15-25.

Ruocco, L. Delcorso, M. and Romanelli, A. M. (2001): New haematological indices in the healthy elderly. Minerva medica 92 (2): 69-73.

Wright, J. H. (1910): The histogenesis of blood platelets. J morphology year 21: 263-278.