Haemostasis is the process by which blood vessels with a hole in them produce a stable plug to fill in the hole and prevent haemorrhage. Coagulation is the process by which a soluble protein called fibrinogen, produced by the liver and normally present in the blood is converted into insoluble polymers of fibrin monomers. These fibrin polymers are interwoven into a plug to increase its stability. Both endothelial cells and platelets have essential roles in the processes of haemostasis and coagulation. But as we shall see the role of endothelial cells is more in signalling the need for the formation of a haemostatic plug, whilst platelets actually form the haemostatic plug.
The Primary Haemostatic plug
Healthy endothelial cells are incredibly important for preventing the formation of a haemostatic plug in a healthy blood vessel. But if for some reason the endothelial cells are damaged then a primary haemostatic plug forms. When endothelial cells are injured the collagen of the basement membrane of the endothelial cells and the collagen of the extra cellular matrix (depending on how severe the injury is) are exposed on the luminal surface of the blood vessel. In addition the injured endothelial cells begin to produce and secrete a protein called Von Willebrand factor onto the surface of the exposed collagen where it binds. Platelets, also called thrombocytes which are in the circulation possess a surface glycoprotein called gp1B which will bind to the Von Willebrand factor. Hence a layer of platelets will form over the exposed collagen. This is known as platelet adhesion.
In addition the exposed collagen will activate the platelets and cause them to secrete granules which are stored in their cytoplasm. These granules contain many substances but some particularly important ones are Thromboxane A2, Serotonin (also called 5-Hydroxytryptamine) and ADP. Both Thromboxane A2 and Serotonin are vasoactive and produce vasoconstriction of the arterioles leading to the damaged blood vessel. Hence less blood will be reaching the damaged wall and this will help to reduce blood loss due to the injury. Similarly injured endothelial cells also take action to try and reduce blood flow to the affected area, but rather than producing a vasoconstrictor, they instead stop secreting the vasodilator Prostacyclin.
The final of the products that platelets release is ADP. Platelets have on their surface receptors for ADP and when ADP binds to this receptor it activates the platelet. Activated platelets will then bind to other platelets; specifically they will bind to the platelets which have formed a layer over the exposed collagen, to produce a multilayered structure of platelets. This structure is known as the primary haemostatic plug. The antithrombotic drug Clopidogrel works by binding to the receptor for ADP on the surface of platelets and not stimulating it, i.e. it is an antagonist and simply prevents ADP binding and activating the receptor.
Overall with regards to the formation of the primary haemostatic plus we can see that platelets and endothelial cells have vastly different functions. The platelets actually form the structure which will plug the hole. Whilst endothelial cells are essential for preventing the formation of such a structure in healthy blood vessels and ensuring that it is only produced at sites of injury. So their role is more in coordinating haemostasis.
The purpose of the coagulation cascade is to transform the primary haemostatic plug into a secondary haemostatic plug. A primary haemostatic plug as previously described consists of a multilayered structure of platelets. Whilst a secondary haemostatic plug is a multilayered structure of platelets but with a dense meshwork of fibrin between the platelets holding them in position. The secondary haemostatic plug is far more stable than the primary haemostatic plug and prevents haemorrhage more effectively.
In order to produce such a dense meshwork of fibrin it is necessary to turn fibrinogen which is a soluble protein in the blood plasma into these fibrin strands. The enzyme which converts fibrinogen into fibrin monomers is called Thrombin and the inactive precursor of thrombin, called Prothrombin is constitutively present in the blood plasma. Another enzyme called Factor XIIIa then converts the fibrin monomers into Fibrin strands. Hence in order to produce the fibrin strands it is necessary for Prothrombin to be converted into thrombin and the series of reactions by which this conversion occurs is known as the coagulation cascade.
Similarly to in the formation of the primary haemostatic plug it is the damaged endothelial tissue which starts the coagulation cascade. Damaged endothelium does this in two ways. The first way is through the intrinsic pathway, when there is damage the protein HMW kinin, standing for heavy molecular weight kinin is produced. This converts factor XII into XIIa. The activated factor XII then in turn activates XI to XIa and XIa activates IX into IXa. Then comes the important stage which is the activation of factor X. Factor X is activated by factor IXa, but it must be in the presence of factor VIIIa, calcium and phospholipids. This is important because it is the activated platelets which release calcium ions and it is also the platelets which act as a surface of phospholipids on which this reaction can occur. Hence this stage means that the coagulation cascade can only occur on platelets surfaces, i.e. it ensures that the fibrin is going to be produced in the primary haemostatic plug.
Factor X has two stages of activation. The first we have just seen and the second is done by Factor Va. Once factor X is activated to factor Xa, it will catalyse the conversion of prothrombin to thrombin and hence coagulation can begin. Thrombin then further activated factor V to Va to produce a positive feedback loop.
The second pathway by which endothelial cells can activate the coagulation cascade is through the extrinsic pathway. In this pathway, the endothelial cells along with any damaged cells in the interstitum underneath produce a protein called tissue factor. Tissue factor converts factor VII to factor VIIa and factor VIIa can undertake the first portion of factor X activation. Again though this transformation must take place on a phospholipids surface and in the presence of calcium, which helps to ensure that fibrin deposition occurs actually within a primary haemostatic plug and not just in the free circulation.
Overall we have seen that in both the formation of the primary haemostatic plug and its conversion to a secondary haemostatic plug, via the coagulation cascade, endothelial cells have a regulatory function. Indeed they help in both cases to ensure that both processes only occur when there is damage. They do this by preventing exposure of the underlying collagen in the case of platelet adhesion and by expressing surface molecules like Antithrombin which inactivates thrombin in the case of coagulation. Platelets on the other hand play a more active role in the case of primary plug formation, since they are the structures which will actually bind together to form the plug. With regards to coagulation they are more similar to endothelial cells and play a guiding role, i.e. they ensure that fibrin deposition happens at the correct site.