Abdominal aortic aneurysms (AAA) are predominantly infrarenal and associated with atherosclerosis. However, the pathogenesis of AAA involves more than just atherosclerotic narrowing. Which molecular mechanism explains medial degeneration and loss of structural integrity in AAA?

• A Infiltrating macrophages and T cells secrete MMP-2, MMP-9, MMP-12, and elastases that degrade medial elastin and collagen; inflammatory cytokines (TNF-α, IL-1β) from infiltrating cells inhibit TIMP-mediated MMP suppression, causing progressive extracellular matrix destruction, medial thinning, and aneurysmal dilatation; reduced smooth muscle cell survival (via Fas-FasL apoptosis) further depletes the ECM-synthesizing cell population ✓
• B Foam cell-derived cholesterol crystals directly cause physical splitting of elastic lamellae in the aortic media via crystal growth pressure
• C Calcification of elastic lamellae in the media causes brittleness and crack propagation under pulsatile pressure, analogous to stress fracture
• D Loss of vasa vasorum due to atherosclerotic ostial occlusion causes medial ischemia, which depletes SOD activity in SMCs, leading to oxidative elastin loss

Explanation

AAA pathogenesis involves chronic transmural inflammation with macrophages (particularly M1 polarized), CD4+ and CD8+ T cells, and neutrophils in the adventitia and media. These cells secrete matrix metalloproteinases (MMP-2/gelatinase A, MMP-9/gelatinase B, MMP-12/macrophage metalloelastase) that degrade elastin and collagen, the structural proteins of the aortic wall. Inflammatory cytokines downregulate TIMPs (tissue inhibitors of metalloproteinases), further tipping the balance toward ECM degradation. Apoptosis of medial smooth muscle cells (the ECM-synthesizing population) mediated by Fas/FasL and TNF-TRAIL signaling depletes the ECM repair capacity. The net result is progressive medial thinning, loss of elastin, and aneurysm formation.

Reference: Robbins & Cotran Pathologic Basis of Disease, 10th ed.

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Written and medically reviewed by the StethoPrep medical team.