The effect of age on the unfolding of elastin lamellae and collagen fibers with stretch in human carotid arteries.

We measured folding of elastin lamellae and collagen fibers in human carotid arteries and correlated the results with the elastic properties of the vessels. Specimens cut into circumferential strips were stretched to various degrees, fixed, and stained for elastin and collagen. Folding was measured on photographic projections. Elastin lamellae unfolded quickly with initial stretch. In old vessels they did not straighten as much as in the young suggesting that the content of the interlamellar space may interfere with the unfolding. Collagen fibers straightened more during stretch in stiffer older vessels than in the young, already at low degrees of stretch. Young's modulus at extension of 5% appeared to correlate with unfolding of elastin lamellae in young extensible vessels. The modulus correlated significantly with unfolding of collagen fibers at extensions of 15% and greater, and the slope of the regression of the modulus on folding increased with stretch. Our findings provide for the first time morphologic evidence for the role of elastin lamellae in the determination of the elastic properties at low extensions, for the importance of collagen fibers in increasing stiffness with further stretch, and for the relationship between increased stiffness with age and the earlier recruitment of the collagen fibers.