The close association of elastin, collagen, and smooth muscle in the mammalian aortic media results in viscoelastic properties that account for many of its static and dynamic mechanical features. The structural components of the media are precisely oriented in concentric layers, or lamellar units, of fairly uniform composition. A comparative study of the adult thoracic aorta in 10 mammalian species, including 15 canine breeds, showed that the number of lamellar units in the media of adult mammalian aortas is very nearly proportional to aortic radius regardless of species or variations in measured wall thickness. Estimated wall tensions ranged from 7,820 dynes/cm in a 28-g mouse to 203,000 dynes/ cm in a 200,000-g sow, but the average tension per lamellar unit of an aortic media was remarkably constant regardless of species, ranging from 1,090 to 3,010 dynes/cm. The findings suggest that the elastin lamella and the contents of its adjacent interlamellar zone represent the unit of structure and function of the mammalian aortic wall.ADDITIONAL KEY WORDS structure-function elastin collagen wall tension comparison of 10 species• The mammalian aortic wall contains smooth muscle cells and the fibrous proteins collagen and elastin. Smooth muscle proteins account for about 20? of the dry weight of the media; collagen and elastin together account for about 60% (1, 2). There is usually more elastin than collagen in the thoracic aortic segment and more collagen than elastin in the abdominal segment. However, the total fibrous protein concentration is nearly the same for both segments and is remarkably constant for adult mammalian aortas, regardless of species (1, 2). At physiological distending pressures, aortic elastin and collagen fibers and smooth muscle cells are precisely

The arrangement and interrelation of the structural components of the rabbit aorta were studied by light and electron microscopy. Segments of abdominal aorta were restored to in vivo length and fixed in formalin or osmic acid while intraluminal pressures ranging from 0 to 200 mm Hg were maintained by a constant pressure perfusion apparatus. Transverse, longitudinal, and tangential sections of vessels fixed at various distending pressures were examined. Micrometric measurements included vessel diameters and wall thickness, thickness and waviness of elastin lamellae and interlamellar distances. With increasing pressures below the diastolic value, aortic radius increased and wall thickness decreased rapidly. Waviness of the tubular elastin lamellae decreased uniformly throughout the wall. Interlamellar distances decreased uniformly and markedly. Lamellar thicknesses decreased uniformly but much less than interlamellar distances. A fine fibrillary elastin network connected the thick lamellae. Collagen fibers showed no definite pattern of orientation. At and above diastolic pressure radius and wall thickness changed little with increasing pressures. Elastin lamellae were straight and interlamellar distances were uniform; the fibrils of the interlamellar elastin net were arranged obliquely. Collagen fibers were arranged nearly circumferentially. Collagen and elastin fibers were closely intermingled in the narrow interlamellar space but no collagen-elastin connections were obserevd. The mechanical properties and organization of the collagen and elastin components of the aortic media indicate that the wall normally functions as a "two-phase" material. At and above physiological pressures, circumferentially aligned collagen fibers of high tensile strength and relatively high modulus of elasticity bear most of the stressing force. Elastin lamellae and fibrils of relatively low modulus of elasticity distribute stressing forces uniformly. Attempts to assess the role of medial pressure and tension gradients in the pathogenesis of aortic disease must take into account the special mechanical properties of this "two-phase" material.

The effects of long-term (16 months) hypertension on the thoracic aorta of male rats were compared to previously reported short-term (2.5 months) changes and to concurrent aging changes. Hypertension was produced by clipping a renal artery. Although short-term hypertension was characterized by a disproportionate increase in noncollagenous alkali-soluble proteins, which have been attributed primarily to vascular smooth muscle, with long-term hypertension there was no further increase in these proteins but instead there were striking increases in mural accumulations of elastin and collagen. Chronically elevated wall tension in hypertensive vessels was associated with a progressive increase in wall thickness which resulted in a value for wall stress no different from that of control vessels. Concurrent aging changes were qualitatively similar to, but much less pronounced than, those seen with hypertension and were attributed to an increase in wall tension in controls resulting from a combination of significant increases in diameter and systolic blood pressure with age. This study of the interaction of vessel structure and function has revealed common features of what appears to be a diverse group of vascular alterations. KEY WORDSaortic wall tension vessel structure and function elastin collagen aortic wall stress vascular disease• Hypertension results in a strikingly increased thickness and stiffness of the entire vascular tree (1, 2); similar changes are seen to a lesser degree in the vessel wall during growth and aging (3, 4). Morphological and chemical studies of hypertensive vessels have shown that the changes in vessel dimensions and properties are due to smooth muscle hypertrophy (1) and hyperplasia (5) and increased amounts of mural mucopolysaccharides (6), elastin, and collagen (7). Elastin and collagen also accumulate in vessels during aging (4), though the relative degree of increase in each of the fibrous proteins is controversial (4,(8)(9)(10). These similarities and others have led some to suggest that hypertension is an accelerated form of aging (3).Although the changes in vascular morphology associated with hypertension have been well described (1) and no qualitative morphological differences between vessels exposed to acute hypertension and chronic hypertension were found (11), a detailed comparison of short-term and long-term effects of hypertension on the dimensions and chemical composition of the vessel wall has not been done. This would be of considerable interest for several reasons. First, the lack of significant effects of treatment on mortality from coronary artery disease in hypertensive patients has been postulated to be due to

Clinical and experimental studies indicate that hypertension accelerates the development of arteriosclerosis. Morphological and chemical studies of the distended rat thoracic aorta were undertaken to define the structural and compositional alterations of the media which accompany hypertension and to relate these changes to increases in calculated medial stress. An 8-week period of hypertension was associated with significandy greater diameter, medial thickness, and cross-sectional area of the media of the thoracic aorta than in nonnotensive animals. Calculated wall tension was significantly higher in hypertensive animals, but the number of medial lamellar units was not greater than that usual in normotensive animals; this resulted in a strikingly elevated value for calculated tension per medial lamellar unit for the aortas of hypertensive animals. A highly significant linear relation was found between total tension and cross-sectional area of the media of the same segment. In addition, the absolute amounts of both medial elastin and collagen were increased in hypertensive animals; however, the percent of these elements remained essentially constant, indicating little change in the composition of aortic tissue. Increments in both fibrous proteins were linearly related to increases in calculated mural stress, and medial accumulations of elastin and collagen proceeded at similar rates. These findings demonstrate a linear relation among vessel dimensions and amounts of medial elastin and collagen and calculated wall tension.ADDITIONAL KEY WORDS structure-function relations elastin collagen smooth muscle vascular disease cross-sectional wall area

Thoracic aortic segments of 12 mammalian species were fixed while distended at normal physiological pressures after the vasa vasorum were filled by a perfusion mixture containing gelatin and carbon. Mammals whose aortas had 29 or fewer medial lamellar units had no demonstrable intramural vascular channels; those whose aortas had more than 29 medial lamellar units, had medial vasa. Aortas with medial vasa vasorum always had a subintimal medial zone devoid of vasa vasorum. In growing animals the width of this avascular zone increased with age; in adults, the width increased slightly with increasing species body weight. However, the number of lamellar units in the avascular zone was independent of both age and species and equal to 29.0 ± 2.5. All of the species with intramural aortic vasa vasorum as adults had 29 or more aortic medial lamellar units at birth; none of the species which had 29 or fewer lamellar units at birth had more than 29 lamellar units at maturity. In aortas with medial vasa vasorum, widening of the avascular zone during growth appeared to be due only to the uniform thickening of each of the approximately 29 lamellar units already present at birth; the vascularized outer zone widened both by enlargement of its lamellar units and the addition of new lamellar units. Species differences in medial distribution of vasa vasorum are due primarily to differences in thickness of the vascularized outer zone.