Extracellular matrix of the human aortic media: An ultrastructural histochemical and immunohistochemical study of the adult aortic media

Aortic distensability is the key to normal aortic function and relates to t he lamellar unit in the media. However, the organization of the extracellul ar matrix components in these lamellar units, which are largely responsible for the distensability, is insufficiently known, especially in the human. We therefore performed a detailed ultrastructural analysis of these compone nts. Thoracic aortas of 56 individuals (age 45-74 years), none of whom suffered from aortic disease, were studied by immunoelectron microscopy of elastin, collagen types I, III, IV, V, and VI, fibronectin, and fibrillin-1, and by ultrastructural histochemistry of proteoglycans, which were further charact erized by enzymatic digestion. The elastic lamellae were closely associated with thick collagen fibers con taining types I, III, and V collagen. Between these collagen fibers, numero us complex, circumferentially oriented streaks of elastin protruded from th e lamellae. In contrast to what is usually reported in the aortas of experi mental animals, the smooth muscle cells preferentially adhered to these ill -defined streaks rather than directly to the solid lamellae. Fibrillin-1- a nd type VI collagen-containing bundles of micro fibrils (oxytalan fibers) w ere also involved in the smooth muscle cell-elastin contact. The smooth mus cle cells were invested by basal lamina-like layers connecting them to each other as well as to the oxytalan fibers. Unexpectedly, these layers were a bundantly labeled by anti-fibronectin, whereas type IV collagen, a specific basement membrane component, was mainly found in larger, flocculent deposi ts. The proteoglycans present were collagen-associated dermatan sulfate pro teoglycan, cell-associated heparan sulfate proteoglycan, and interstitial c hondroitin sulfate proteoglycan. Our observations demonstrate that the extracellular matrix in the human aor ta is extremely complex and therefore differs from most descriptions based on experimental animals. They serve as reference for future studies on aort ic diseases, such as aneurysmas and dissections. Anat Rec 258:1-14, 2000. ( C) 2000 Wiley-Liss, Inc.