Adventitial elastolysis is a primary event in aneurysm formation

Histologic appearance of elastin-stained normal infrarenal aorta (longitudinal section, original magnification × 40, DBF left to right). Media demonstrates multiple elastin lamellae. Inner layer of adventitia, just beyond border of media, demonstrates dense elastin staining with multiple, closely applied layers of elastin.

Histologic appearance of inner portion of normal adventitia (longitudinal section, original magnification × 500, DBF left to right). Inner layer of normal human aortic adventitia is composed of densely compacted alternating lamellae of elastin (black) and collagen (pink). Direction of elastin fibers is parallel to DBF, but direction of collagen fibers is perpendicular to DBF.

Scanning electron micrograph of adventitial elastin fibers in formic acid—digested normal aortic wall (DBF left to right). Inner layers of adventitia contained lamellae of elastin composed of thick fibers, with axis of orientation parallel to DBF. These fibers appeared to be held together by thinner circumferential elastin fibers.

High-resolution scanning electron micrograph of thin circumferential elastin fibers (DBF left to right). Thinner circumferential fibers appear to arise from thick fibers and course circumferentially to bind thick fibers together.

Histologic appearance of human AAA wall (longitudinal section, original magnification × 40, DBF left to right). All aneurysm specimens demonstrated extensive loss of media. Elastin within inner portion of adventitia was also significantly reduced.

Histologic appearance of inner portion of adventitia in aneurysm wall (longitudinal section, original magnification ×500, DBF left to right). Few remaining elastin elements were surrounded by collagen fibers, which were of finer order than those in normal aortas.

Histologic appearance of residual elastin in adventitia of aneurysm (frontal section, original magnification × 500, DBF left to right). Fibers in remaining lamellae were coiled and without organization.

Scanning electron micrograph of residual elastin in adventitia of aneurysm (frontal section, original magnification × 1000).

Quantitation of adventitial elastolysis in aneurysm. A, On longitudinal sections there was 81.6% ± 2.1% reduction in elastin content compared with that in controls (p < 0.001), which shows significant decrease in elastin lamellae in aneurysmal adventitia (reference area = 1.6 × 10⁴ μm²). B, On frontal sections there was 85.7% ± 4.2% reduction in elastin content compared with that in controls (p < 0.001), which shows significant decrease in number of fibers per lamella in aneurysmal adventitia (reference area = 1.5 × 10³ μm²).

Quantitation of adventitial elastolysis in small, moderate, and large aneurysms. There was diffuse adventitial elastolysis in aneurysms of all diameters. There was no statistically significant difference in extent of elastolysis in adventitia of small, moderate, or large aneurysms (reference area = 1.6 × 10⁴ μm²).