An Integrated Investigation of Cartilage as a Skeletal Material

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This research project is a multi-level, interdisciplinary investigation of the performance, biomechanics, and biochemistry of cartilage as a skeletal material. Cartilage is usually perceived as an articular material, a bearing surface between bony elements, or as contour-filler in the nose and ears - yet sharks and their relatives use cartilage as their skeletal material. The skeletal system is fundamental to the success of vertebrates, providing support, protection, and serving vital physiological roles. It is therefore unsurprising that the biology of bone, the typical skeletal material, is quite well understood. Similarly, cartilage, in its 'usual' role as an articular surface has been well studied because damage and malformations of this type of cartilage are major health issues. It would not be an exaggeration to say that cartilage, in its role as a skeletal material, is almost completely unknown. In fact, there is not a single published value for the stiffness or strength of cartilage from a shark.Despite a cartilaginous skeleton sharks and their relatives evolved an exclusively hard prey diet at least four times. This project compares the cartilaginous skeleton of hard prey crushers and closely related generalists at four levels: whole animal performance; structural mechanics; material properties and biochemistry. Preliminary data indicate that the cartilage of hard prey specialists is different in structure, material, and biochemistry than that of generalists, and also varies from mammalian cartilage. Preliminary data indicate that cartilage is more varied among four species of shark than among all tetrapods, perhaps because the demands placed on a cartilaginous skeletal tissue are more variable than the demands placed on articular cartilage. Performance will be assessed by determining the bite forces in hard prey crushers and generalists, while structural mechanics will be determined through analysis of 3-d CT scans of jaws. The material properties of blocks of cartilage are measured with solid and viscoelastic tests. Three aspects of the biochemical composition - collagen content and cross linking, aggrecan content, and leucine-rich proteoglycan content (decorin, biglycan) will be assessed and compared to values for tetrapods. In addition, correlations between composition and material properties will lead to a clearer understanding of the causal relationship between the constituents of cartilage and its response to load.