Surgical pathologists frequently examine ascending aortic specimens in the context of aneurysm or dissection. In such cases, the primary diagnostic distinction is often between inflammatory and non-inflammatory disease. The latter includes a spectrum of degenerative changes within the aortic media, the portion of the aorta characterized histologically by repeating lamellar units (1). Lamellar units—consisting of the elastic lamella and the contents of its adjacent interlamellar zone (2)—are altered in various pathological states, including hypertension and connective tissue disorders.

Degenerative abnormalities may include mucoid extracellular matrix accumulation (MEMA), elastic fiber fragmentation, loss, thinning, or disorganization, smooth muscle nuclei loss, laminar medial collapse, and medial fibrosis. While these features of medial degeneration are well defined, it has traditionally been assumed that all regions of the aorta share the same structural and morphological characteristics. However, evidence suggests that different aortic segments have distinct embryologic origins, making this assumption unlikely (3). These developmental differences may, at least in part, account for the regional variation in aortic anatomy and aortic disease patterns.

Wall weakening and aneurysms affecting different aortic regions often present with distinct clinical features and may require tailored treatment approaches. Even within the ascending aorta, clinical significance can hinge on the specific sub-region involved (e.g., sinus vs. tubular segment). Anatomically, the aortic sinus (AS) refers to the portion of the aortic root extending from the valve annulus to the sinotubular junction, encompassing the three sinuses located behind each aortic valve cusp—left, right, and posterior. These sinuses have unique anatomical relationships with the heart and are subject to differing hemodynamic forces. The tubular aorta (TA) refers to region beyond the sinotubular junction to the pericardial reflection or takeoff of the brachiocephalic artery.

It is plausible that variation in histomorphology between aortic regions may help explain patterns of disease distribution. Pathologists should consider this variability when assessing for abnormalities. However, few studies have systematically examined the normal regional histologic variability of the ascending aorta (4,5), and most existing data come from functional assessments or experimental models (6,7).

To address this knowledge gap, this study aims to characterize the normal microscopic anatomy of the aortic sinus and compare it with that of the adjacent tubular aorta. Our goal is to establish a histologic framework that can aid pathologists in distinguishing normal sinus architecture from true medial degeneration.