Structural Basis for the Static Mechanical Properties of the Aortic Media

Abstract

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.