Elastin/collagen ratio

Increased collagen concentration is another matrix modification that has been widely observed in human AAAs [18-20,22,25]. Modifications in collagen organization and deposition have been correlated to rupture in human AAAs [17,19,21,26]. Although there are noticeable differences in the findings of these studies, it is evident that increases in the coUagen-to-elastin ratio are a general observation in AAAs. 

Resorption of subcutaneously implanted SELF films has been evaluated in rats, over the coxu se of 7 weeks (2). A collagen control and SELP-0, with a 4 1 elastin to silk ratio were both resorbed within Iweek. SELP-8 implants, with a 2 1 ratio of elastin to sfik, retained 18% of their initial mass after 7 weeks, while SELP-3 implants, with a 1 1 elastin to silk ratio retained 58% of their initial mass. SELP-4 and SELP-5, each containing eight sfik-fike blocks and 3 2 and 2 1 elastin to silk ratios, respectively, showed no evidence of resorption after 7 weeks. These studies demonstrate that the resorption of SELPs is controlled more by the length of the silk-like blocks (i.e., sequence) than the elastin to silk ratio (i.e., composition). 

In an ideal or perfect elastomer the energy repeatedly invested in extension is repeatedly and completely recovered during relaxation. Ideality increases as the elastic force results from a decrease in entropy upon extension, because this occurs without stressing bonds to the breaking point. Elastin models and elastin itself in water provide examples of such entropic elastomers with about 90% of the elastic force being entropic, that is, the /e//ratio of Equation (4) is about 0.1. This is essential to human life expectancy, because the half-life of elastin in the mammalian elastic fiber is on the order of 70 years. This means that the elastic fibers of the aortic arch and thoracic aorta, where there is twice as much elastin as collagen, will have survived some billion demanding stretch-relaxation cycles by the start of the seventh decade of life. This represents an ultimate in ideal elasticity. 

Table B6.6 lists the constituents of additional arteries (canine), and the the ratio of collagen to elastin [21].

Table B6.9 lists the changes in elastin and collagen contents of canine carotid and iliac arteries due to dietary atherosclerosis [26]. In the iliac site the ratio of collagen to elastin was increased, while the ratio in the carotid site was decreased.

Table B6.6 Arterial Wall Constituents, and Ratio of Collagen to Elastin ... 

These include nearly all the structural proteins which have molecules with a high axial (length/ width) ratio. Most of them are very insoluble and metabolically unreactive. They include the various keratins found in the skin and its appendages, the fibrin of blood clots, and collagen and elastin the protein constituents of connective tissues. Myosin the fibrous protein of muscle contains both fibrous and globular regions and is both metabolically reactive and relatively soluble. 

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