Structure and Growth Dynamics of a Fibrous Plaque
The microscopic architecture of a mature fibrous plaque includes several essential components. In the center lies the lipid core — a zone containing accumulated lipid debris. Overlying this is the fibrous cap (a connective tissue shell) that forms the plaque's roof. This cap faces directly into the vessel lumen and is lined by endothelium, the cells of which show degenerative changes. The cellular composition of the plaque includes smooth muscle cells and specialized foam cells — macrophages that have engulfed lipids.
plaque development is closely linked to neoangiogenesis. Microvessels (vasa vasorum or vasa plaquorum) actively proliferate at the margins of the plaque. Through this capillary network, plasma proteins and lipoproteins are continuously delivered into the pathological focus. This mechanism drives the progressive enlargement of the fibrous plaque, ultimately resulting in a hemodynamically significant stenosis of the affected artery.
Predilection Sites
Fibrous plaques are distributed unevenly throughout the vascular tree. The pathological process has a typical localization, occurring in arterial segments that experience chronically elevated hemodynamic stress (high pulse wave impact).
Such high-risk zones include:
- Anatomical branching points (bifurcations).
- Pronounced bends of major arteries.
- Aspects of arteries directly adjacent to rigid anatomical structures. A classic example is the posterior wall of the abdominal aorta, which rests against the rigid framework of the spine.
Van Gieson's picrofuchsin staining is traditionally used for precise histological diagnosis and visualization of the plaque's connective tissue components.
Unstable Plaque and Pathogenesis of Rupture
A key element in the morphogenesis of acute forms of ischemic heart disease (IHD) is the formation of the unstable atherosclerotic plaque (vulnerable plaque). Its defining characteristic is structural alterations that render the cap highly fragile and prone to sudden rupture.
Pathogenesis of fibrous cap weakening is closely tied to local inflammation. Lymphocytes and macrophages actively infiltrate the plaque tissue. This cellular infiltrate secretes a cascade of inflammatory mediators—proinflammatory cytokines (IL-1, IL-6, IL-12, TNF, CD40L, etc.).
Under the influence of cytokines, enzymatic activity increases sharply: serine proteinases and matrix metalloproteinases (MMPs) are activated. MMPs trigger degradation by damaging the connective tissue cap, causing lipoidosis, swelling, and the ultimate breakdown of collagen fibers.
Note: For a long time, an infectious theory of this inflammation was considered (implicating viruses, H. pylori, Chlamydia). However, attempts to use specific antibiotic therapy to stabilize plaques proved ineffective.
Consequently, the cap becomes critically fragile. Three primary factors contribute to its rupture:
- Loss of structural integrity of the cap itself.
- Presence of a large lipid core.
- Sudden coronary artery vasospasm.
Morphology of Instability and Thrombosis
The morphology of an unstable plaque has distinctive features. Microscopic examination reveals destruction of the collagen framework, with diffusely distributed lymphocytes and macrophages among loosened tissue, alongside massive clusters of foam cells. Visually, a specific "moiré pattern" is formed.
Histochemical analysis (Sudan III staining) reveals significant accumulations of neutral lipids as well as cholesterol crystals within the thinned cap.
Rupture of such a vulnerable plaque inevitably leads to thrombosis. The trigger is coronary vasospasm, which tears the fragile cap. Platelet aggregation is immediately initiated. Specific glycoproteins are activated on the platelet surface, serving as an essential prerequisite for fibrin precipitation. The rupture site is rapidly covered by fibrin strands, forming a thrombus that initiates acute coronary syndrome.