Heterogeneity of the histopathology of human abdominal aortic aneurysm (AAA). Regional heterogeneity within three distinct regions—inflammatory, active, and amorphous—is demonstrated. The order of these distinct regions may correspond to AAA progression from early to advanced phases. ECM: extracellular matrix; VSMC: vascular smooth muscle cell; HE: hematoxylin and eosin; EVG: elastica Van Gieson.