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.
We set out to understand the selective pressures that might have led to the abandonment of bone by the sharks, skates and rays. Though they do not have bone sharks and rays do have a light crust of mineralized tiles on their skeleton. The blocks are tied together with collagen and overlie a soft, unmineralized core of hyaline cartilage. The data we collected over the course of this grant suggest that this unusual architecture allows the skeleton to resist very high numbers of loading cycles without fatigue. This would explain how a lighter skeletal material that is entirely unable to repair itself has utility as a skeletal system in long lived, highly active animals. For example, a great white shark might flex its tail 10 million times over the course of its life, but its skeleton has none of the fatigue repair capability of bone. The skeleton of cartilaginous fishes also serves as inspiration for a new class of high stiffness, high damping materials. Usually these composite materials rely on energy absorption by plastic deformation of a honeycomb material. In the sharks and rays the high modulus elastic component sits over a viscoelastic gel that absorbs energy through movement of water out of the tissue. This is substantially better in that the core can resist many loading cycles instead of a single cycle as with materials that permanently deform. This reinforces the idea that ‘blue sky’ research, which has no envisioned commercial application, always has the potential to lead to applied results. Three Ph.D. students, including one African American who is now in the professoriate, were part of this research. Their projects covered topics ranging from the material properties of shark vertebral cartilage (it is as stiff and nearly as strong as bone) to the selective pressures that led to the evolution of jaws (speed not strength was a key). Three students received M.S. degrees, all of whom were representatives of under represented minorities. Of these students one is now in a Ph.D. program, one is teaching high school science and the last is working in the biomedical industry.
