Structure and growth dynamics of fibrous plaque
The microscopic picture of a mature fibrous plaque includes several mandatory components. In the center lies the lipid core — an area containing accumulations of fatty masses. Above it, the fibrous cap (connective tissue sheath) forms, which constitutes the plaque cap. This cap faces directly into the vessel lumen and is lined by endothelium, in the cells of which signs of dystrophic changes can be traced. The cellular composition of the plaque is represented by smooth muscle cells and specific foam cells — macrophages that have accumulated lipids.
The dynamics of development of the formation is closely related to the process of vascularization. At the edges of the plaque, active neoformation of microvessels occurs (vasa plaquorum). It is through this capillary network that plasma proteins and lipoproteins continuously flow into the tissue of the pathological focus. This mechanism ensures the continuous increase in the size of the fibrous plaque, which ultimately leads to a hemodynamically significant narrowing (stenosis) of the lumen of the affected artery.
Preferred localization
Fibrous plaques are distributed unevenly throughout the vascular bed. The pathological process has a typical localization — these are areas of the artery that constantly experience increased hemodynamic impact (a strong pulse wave strike).
Such risk zones include:
- Sites of anatomical branching of vessels (bifurcations).
- Pronounced bends of arterial trunks.
- Sides of arteries that are directly adjacent to dense anatomical structures. A classic example is the posterior wall of the aorta, resting against the rigid frame of the spine.
For accurate histological diagnosis and visualization of the connective tissue structures of the plaque, van Gieson's picro-fuchsin staining is traditionally used.
Unstable plaque and the 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 main characteristic is structural changes that make the cap extremely vulnerable and prone to sudden rupture.
The pathogenesis of the weakening of the fibrous cap is inextricably linked to local inflammation. Lymphocytes and macrophages actively penetrate into the plaque tissue. This cellular infiltrate begins to release a cascade of inflammatory mediators — pro-inflammatory 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 the destruction mechanism: they damage the connective tissue cap, causing lipoidosis, swelling, and the ultimate disintegration of collagen fibers.
Note: For a long time, the infectious theory of such inflammation was considered (involving viruses, H. pylori, chlamydia). However, attempts to use specific antibiotic therapy to stabilize plaques proved ineffective.
As a result, the cap becomes critically fragile. Three main factors contribute to the rupture:
- Impairment of the strength of the cap itself.
- Presence of a large lipid core.
- Sudden spasm of the coronary artery.
Morphology of instability and thrombosis
The morphology of an unstable plaque has striking distinctive features. Microscopic examination reveals the destruction of the collagen framework, lymphocytes and macrophages are diffusely distributed among the loosened tissue, and massive accumulations of foam cells are visible. Visually, a specific "moire pattern" is formed.
Histochemical analysis (staining with Sudan III) makes it possible to determine significant accumulations of neutral lipids in the thinned cap, as well as the presence of cholesterol crystals.
Rupture of such a prepared plaque inevitably leads to thrombosis. The trigger is a spasm of the coronary artery, which tears the fragile capsule. Immediately after this, the process of platelet aggregation is triggered. Specific glycoproteins are activated on the surface of platelets, which is a prerequisite for fibrin precipitation. The rupture surface is rapidly covered with fibrin threads, forming a thrombus that initiates acute IHD.
Briefly: answers to topical questions
What stages of morphogenesis (macroscopic stages) does an atherosclerotic plaque go through?
In the morphogenesis of an atherosclerotic plaque, several successive macroscopic stages are distinguished. The generalized classification includes the following stages:
- Stage of fatty spots and streaks (lipoidosis) — focal infiltration of the intima with lipids; macroscopically, yellow spots and streaks are visible, not protruding above the surface.
- Stage of fibrous plaques (liposclerosis) — proliferation of connective tissue with the formation of a plaque protruding into the vessel lumen.
- Stage of complicated lesions — includes atheromatosis, ulceration, hemorrhage into the plaque, thrombosis, and calcinosis (atherocalcinosis).
The initial pre-lipid stage is also distinguished, in which the vascular wall is macroscopically unchanged.
What complications, besides thrombosis, can an atherosclerotic plaque cause?
In addition to thrombosis, an atherosclerotic plaque can cause a number of other local and systemic complications. These include:
- Stenosis — narrowing of the arterial lumen, leading to progressive ischemia, atrophy, and sclerosis.
- Ulceration — destruction of the cap with the formation of an atheromatous ulcer and the release of atheromatous masses into the lumen.
- Hemorrhage into the plaque — formation of an intramural hematoma upon rupture of the cap.
- Calcinosis (atherocalcinosis) — deposition of calcium salts in fibrous tissue and atheromatous masses.
- Aneurysm — destruction of the muscular-elastic framework of the vessel with the risk of dissection and rupture.
- Thromboembolism — occlusion of distal vessels by detached masses.
How does a stable atherosclerotic plaque differ from an unstable one?
The main difference indicated in the sources is that an unstable atherosclerotic plaque is characterized by structural changes that make it prone to rupture.
An unstable plaque is characterized by:
- impaired cap strength;
- large size of the lipid core;
- destruction of the collagen framework of the cap;
- accumulations of foam cells;
- diffuse tissue infiltration by lymphocytes and macrophages;
- activation of pro-inflammatory cytokines and proteolytic enzymes, including matrix metalloproteinases;
- damage to the connective tissue cap with lipoidosis, swelling, and disintegration of collagen fibers;
- significant accumulations of neutral lipids in the cap when stained with Sudan III;
- presence of cholesterol crystals;
- tendency to rupture during coronary artery spasm followed by platelet aggregation and fibrin deposition.
The clinical significance of an unstable plaque is its involvement in the morphogenesis of acute forms of IHD.
What components make up the lipid core of an atherosclerotic plaque?
The lipid core of an atherosclerotic plaque is the central part of the plaque containing fatty masses.
According to sources, the following are associated with it:
- lipid or lipid-protein masses in the central part of the plaque;
- atheromatous masses;
- atheromatous detritus — a fine-grained amorphous mass formed by the breakdown of lipid masses, collagen, and elastic fibers in the central part of the plaque.
To remember the factors of plaque rupture, use the "Three C's" rule: Compromised cap (destroyed by enzymes), Core of lipids (large lipid core), Coronary spasm (triggering factor).
Class and exam questions
What cells are called "foam cells" and how do they form?
These are macrophages that have migrated into the vessel wall and ingested a large amount of lipids. Due to the accumulation of fat droplets, their cytoplasm appears foamy under a microscope.
Why do plaques form more frequently on the posterior wall of the aorta?
The posterior wall of the aorta is closely adjacent to the spine. In this zone, the vessel experiences increased mechanical impact from the pulse wave, which contributes to the development of atherosclerosis.
What is the role of matrix metalloproteinases (MMPs)?
MMPs are proteolytic enzymes secreted by cells of the inflammatory infiltrate. They degrade the collagen fibers of the fibrous cap, making it fragile and prone to rupture.
What else to study on this topic
- Mechanisms of neoangiogenesis (vasa vasorum) at the edges of the atherosclerotic plaque.
- The role of pro-inflammatory cytokines (IL-1, IL-6, TNF, CD40L) in the destruction of the collagen framework.
- Histochemical methods for detecting lipids (Sudan III staining) and fibrous tissue (Van Gieson staining).
- Pathogenesis of platelet aggregation and fibrin precipitation upon cap rupture.
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