This research project is a multi-level, interdisciplinary investigation of the structure and function of the mineralized portion of the cartilaginous skeleton. Cartilage is usually perceived as an articular material, a bearing surface between bony elements, or as contour-filler in the nose and ears. However, sharks and their relatives use cartilage as their skeletal material. This form of cartilage is lightly mineralized, with a surface coating of small, discrete blocks that completely cover the surface. Two factors make this 'tessellated' form of cartilage particularly interesting: 1) sharks abandoned a bony skeleton in favor of this cartilage so we may see the developmental and biochemical traces of early bone formation in the mineralization processes; and 2) since sharks, like human, cannot heal their cartilage it must be particularly resistant to fatigue damage over the life of the animal. Understanding the basis for this fatigue resistance may uncover a new class of biological materials that are both stiff and able to dissipate energy well. Preliminary data shows many differences between the mineralization process in tessellated cartilage and bone. The calcifying cells of tessellated cartilage do not enlarge and die as they do in bone, nor are the cells organized into files, but just as in bone there is a distinct region of organized collagen fibers on the edges of the mineralizing front. This may signal similarities at the developmental level. The researchers will investigate the extent of these similarities with histological techniques that probe for distinctive biochemical signals of developing bone. They will also use cryogenic scanning electron microscopy to examine the shape and arrangement of the cells in the developing mineralizing tissue. This microscopic and developmental investigation will be complemented by a study of the mechanics of the tessellated skeleton. Whole skeletal elements will be tested with a Periometer capable of measuring damping in living tissue to test the hypothesis that this tissue is very good at dissipating strain energy. These studies will be augmented with model-based investigations of the effect of mineralized block size and unmineralized cartilage stiffness on the damping qualities. A rapid prototyper will be used to 'print out' models with different block sizes and shapes, which will then be examined on a conventional material testing system. These experiments will be carried out by a collaboration between labs at the University of California - Irvine, George Washington University, the Max Planck Institute in Stuttgart, Germany and the Institute for Interdisciplinary Marine Sciences in La Paz, Mexico. The scientists, including graduate and undergraduate students, performing the research include developmental biologists, biomechanists, mechanical engineers and surface biologists. Several members of the group are under-represented minorities including one of the principal investigators and four of the graduate students.
