Main Types of Thrombi by Composition and Structure
Classification of thrombi is based on their macroscopic appearance, chemical composition, and conditions of formation within the vascular bed:
- White thrombus: Forms slowly under conditions of rapid blood flow. It is composed of platelets, fibrin, and leukocytes. Typical localization includes arteries, heart valve cusps, and the spaces between endocardial trabeculae.
- Red thrombus: Forms rapidly against the background of sluggish blood flow. It contains platelets, fibrin, and a high number of erythrocytes. The primary site of formation is in veins.
- Mixed thrombus: Combines elements of both white and red thrombi. Hematoxylin and eosin (H&E) staining on histological sections reveals a characteristic layered structure. Anatomically, it is divided into three parts:
- Head: Structurally corresponds to a white thrombus and is firmly attached to the vessel wall.
- Body: The actual mixed zone.
- Tail: Has the structure of a red thrombus and is loosely attached to the intima; the tail is prone to detachment, becoming a thromboembolus.
Hyaline and Other Special Forms of Thrombi
In addition to classic arterial and venous formations, pathological practice distinguishes several specific variants:
- Hyaline thrombus: Forms exclusively within the microvasculature. Causes include shock, burn disease, severe trauma, disseminated intravascular coagulation (DIC), dehydration, and severe intoxications. It consists of precipitated plasma proteins and agglutinated cellular elements, representing a homogeneous, structureless mass with a weakly positive reaction for fibrin.
- Marantic thrombus: A mixed thrombus developing in the setting of general wasting, dehydration, or old age (frequently in superficial leg veins and dural venous sinuses).
- Tumor thrombus: Forms when a malignant tumor invades the lumen of a vein or when microvessels are occluded by tumor cell aggregates.
- Leukemic and septic thrombi: The former consist of clotted blood and atypical cells in leukemias (within microvessels), while the latter represent infected mixed thrombi in veins during sepsis and suppurative vasculitis.
Relationship to the Vessel Lumen and Process Dynamics
The morphological characterization of a thrombus must account for how it is positioned within the vessel lumen:
- Mural (attached) thrombi: Narrow the vessel lumen but do not completely occlude it. They typically have a white or mixed structure. A prime example is a thrombus overlying an atherosclerotic plaque.
- Occlusive thrombi: Completely obstruct the vessel lumen and structurally are most frequently red thrombi.
Based on their dynamic progression, processes are divided into localized (confined to the site of injury) and propagating (growing in the direction of blood flow).