Stages of thrombus morphogenesis:

1 Platelet agglutination.

a. Platelet adhesion to naked collagen at the site of damage to the endothelial lining is carried out using fibronectin on the surface of blood plates, stimulated by type III and IV collagen, the mediator is Willebrand factor produced by the endothelium.

b. Platelet secretion of adenosine diphosphate (ADP) and thromboxane A2, causing vasoconstriction and platelet aggregation (blocking the formation of thromboxane A2 in small doses of acetylsalicylic acid underlies preventive therapy for thrombosis), histamine, serotonin, PDGF, etc.

 

c. Platelet aggregation = formation of primary platelet plaque.

2. Coagulation of fibrinogen with the formation of fibrin (stabilization of the primary platelet plaque).

• Occurs when the blood coagulation system (coagulation cascade) is activated.

but. The internal coagulation system is triggered by the contact activation of factor XII (Hageman) with collagen and is enhanced by the platelet phospholipid (factor III) released by conformational changes in their membrane.

b. The external system is triggered by tissue thromboplastin released from the damaged endothelium (tissues) and activating factor VII.

• Ultimately, both paths lead to the conversion of prothrombin (factor II) into thrombin (factor Pa), which promotes the conversion of fibrinogen to fibrin, and also causes further release of ADP and thromboxane A2 from platelets, contributing to their aggregation.

Electron microscopic picture: in this stage, accumulations of degranulated platelets and fibrin are found near the destroyed endothelium.

3. Agglutination of red blood cells.

4. Precipitation of plasma proteins.

Thrombus morphology. Depending on the structure and appearance, white, red, mixed (layered) and hyaline blood clots are distinguished.

a. The white thrombus consists mainly of platelets, fibrin and leukocytes, it is formed slowly with a rapid current of blood (usually in the arteries).

b. A red thrombus, in addition to platelets and fibrin, contains a large number of erythrocytes, is formed quickly with a slow current of blood (usually in the veins).

с. Mixed thrombus has a layered structure (layered thrombus) and a motley appearance, contains elements of both white and red thrombus (most often it forms in the veins, in the cavity of the aortic and aneurysm of the heart).

The hyaline thrombus is a special type of thrombus: it is formed in the vessels of the microvasculature, rarely contains fibrin, consists of destroyed red blood cells, platelets and precipitating plasma proteins resembling hyaline.

In a mixed thrombus, there is a head (it has the structure of a white thrombus), a body (a mixed thrombus itself) and a tail (it has the structure of a red thrombus). The head is attached to the endothelial lining of the vessel, which distinguishes a blood clot from a posthumous blood clot.

In relation to the vessel lumen, the thrombus can be near-wall or occlusive (occluding).

With the growth of a blood clot talk about a progressive thrombosis.

Macroscopic picture:

a) a thrombus in the aorta is usually represented by a near-wall dryish formation of a gray-red color with a corrugated dull surface fixed to the intima in the region of the head; body and tail lie loose. Aortic intima uneven, with numerous fibrous plaques, areas of ulceration;

b) blood clots in the veins (deep veins of the lower extremities), as a rule, have the appearance of dark-red dryish masses with a dull surface, occlude the lumen of the vessel, are associated with the inner lining of the vessel in the head area.

The difference between blood clots and death clots: death clots elastic consistency, shiny, lie freely in the lumen of the vessels and the cavities of the heart.

Microscopic picture. Mixed thrombus in the vein: the lumen of the vein is occluded by a thrombus consisting of fibrin filaments, between which are seen erythrocytes, platelets, leukocytes.

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