10 Steps Around Platelet Activation

10-Step Platelet Activation Process

The Overview

As you may know, platelets form a significant portion in our blood. The three cells that constitute the blood are red blood cells, white blood cells and platelets. Among these three, the platelets act as the supportive agent, when it comes to blood clotting.

Platelets play a cognitive part in blood coagulation. The human body is sensitive to the levels of platelets. And there are certain reasons in which the thrombocyte or platelet count in the human body increases and decreases.

The platelets are mainly formed out of megakaryocytes. The bone marrow is the location where the platelets are initially accumulated and formed. But there are certain other factors that lead to their activation and function.

The megakaryocytes are cells that are present inside our bone marrow. Megakaryocyte plays an important role in blood cell formation. As an example, it helps to yield platelets.

As the megakaryocytes develop into bigger cells, it undergoes fragmentation where 1,000 platelets are released per megakaryocyte.

Thrombopoietin is the dominant hormone that controls megakaryocyte development. It has been shown in a number of research that in the presence certain medical disorders, megakaryocytes and platelet production is usually negatively affected. This can result to an increase or decrease in the amount of platelets.


There is a systemized process that takes place when it comes to platelet activation and formation.

These steps can be categorized in the following, from formation to activation.

1. Formation:

The platelets are formed out of megakaryocytes that underwent a fragmentation process. This makes platelets as cell fragments and devoid of nucleus.

They circulate in the bloodstream for around 10 days.  They certainly don’t have nucleus, but consists of organelles which help keep them inactive in formed shape.

2. Role of prostacyclin:

When the platelets can keep its deactivated form, this is due to prostacyclin. The prostacyclin is a prostaglandin that chiefly prevents formation of the platelet plug that is involved in primary hemostasis by inhibiting platelet activation. It is also known as an effective vasodilator.

In the presence of blood vessel injury, platelets migrate to the breakage and mixes with the endothelium. The endothelium triggers and helps in the activation of platelets.

3. Trigger by thromboxane:

The thromboxane is another factor along with ADP that helps trigger the growth and activation of platelets. It is a vasoconstrictor and a potent hypertensive agent.

Thromboxane triggers platelet aggregation, its role in clot formation.

4. Granule Exocytosis (secretion):

When the platelets get activated by the triggering agents like thromboxane, ADP etc., contents from the granules (alpha granules, lambda granules, and dense granules) contained in the platelets are secreted.

5. Thromboxane A2 formation:

The next step comes the secretion thromboxane A2 by activated platelets.

Thromboxane A2 production eventually increases to stimulate activation of other platelets. It is also a powerful mediator of platelet aggregating response.

The thromboxane A2 is one of the most important stimuli that triggers conversion of platelets to its active form as well as cause vasoconstriction which is important in tissue injury and inflammation.

6. Change in shape (Morphological Changes):

After the above steps, from formation of platelets to successfully being triggered by several stimuli to activation, then happens its taking shape to its activated form.

The original shape of the platelets seem to just be amorphous or disc-like colorless cell fragment. But when activation takes place, it makes an even more prominent shape, where it looks like spiculated spheres with several protruding extensions.

7. Binding:

Next to platelet activation comes the bindings. The binding is the next process wherein platelets clump or binds to each other and work towards the common purpose of blood clotting. The binding results to platelet plug formation.

The binding is mainly done through its receptors. The receptors are present in the outer layer of the activated platelets.

The fibrinogen is the main substance that acts as the receptor which helps in the process of clumping together.

The process involves the release of the alpha granules that contains several clotting mediators.

8. Blood coagulation:

Blood coagulation is the process where the blood changes from liquid to gel in order to stop bleeding.

When platelets successfully achieved its activated form and clumped together, it forms a blood clot. This potentially results to hemostasis, cessation of blood loss from the damaged blood vessel.

Primary hemostasis happens when platelets immediately form a plug at the site of injury.

Secondary hemostasis happens when it does it simultaneously.

The coagulation process happens through the activation, adhesion, and aggregation of platelets.

Moreover, there are several coagulation factors or clotting factors that responds in a complex cascade for the formation of fibrin strands that strengthens the platelet plug.

9. Conversion of inactive blood protein to enzyme:

Along with the clotting process performed by platelets, blood-clotting proteins that circulate in the blood plasma gets poised to participate in blood coagulation in response to the tissue injury.

These inactive proteins gets converted to active proteolytic enzymes.

10. Conversion of Prothombin to Thrombin:

The next and the final step is the conversion of prothombin to the active enzyme thrombin. The sequence is dependent to the platelets.

In combination with platelets, it completes a clot. The enzyme converts the blood protein fibrinogen, also present in plasma, to fibrin.

The resulting fibrin molecules adhere to each other and assemble long fibrils. As the developing fibrils of fibrin is formed, they trap interspersed activated platelets, and form the clot.


After identifying the steps that goes around and within the platelet activation process, you’ll find that each and every steps, is influenced by several factors.

Moreover, it’s clear that the entire process is completed through mutual help from one factor to another. All the platelets, the prothrombin, the regulatory membrane, the blood proteins, the active enzyme transformations, etc. works together in a mutual fashion towards a common goal of stopping bleeding.

It is of utmost importance to keep the entire process function normally, knowing that inability to stop bleeding can result to a life-threatening condition.

There are times when the platelet count may be affected by several factors resulting to abnormal levels. It can be factors that either cause a low platelet count or high platelet count. Or it can simply just be a normal drop in platelets during pregnancy.

Whatever the reason, if you’ve noticed any changes from normal platelet count levels, take into account undergoing diagnostic procedures to identify any abnormal changes.


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