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First published online September 14, 2007
Journal of Experimental Biology 210, 3319-3327 (2007)
Published by The Company of Biologists 2007
doi: 10.1242/jeb.006189

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The contribution of mineral to the material properties of vertebral cartilage from the smooth-hound shark Mustelus californicus

Marianne E. Porter^1,*, Thomas J. Koob² and Adam P. Summers¹

¹ Comparative and Evolutionary Physiology, Department of Ecology and Evolutionary Biology, 321 Steinhaus Hall, University of California, Irvine, CA 92697-2525, USA
² Department of Chemical Engineering, University of South Florida, Tampa, FL 33612, USA

View larger version (34K): [in this window] [in a new window]   Fig. 1. (Top) Schematic of M. californicus vertebral column. Vertebrae used in these experiments were excised from under the first dorsal fin (boxed) (A). Neural and hemal arches were removed leaving cylindrical centra for materials testing. (B) The mineralized double cone structure is highlighted in this generalized elasmobranch vertebra. Intricate mineralization patterns branch off the double cone structure and the patterns vary extensively among elasmobranch species (Ridewood, 1921). (C) Drawings of 3/4 views of two species of shark vertebrae. The anterior surface is concave, part of the double cone structure, coming to a point in the middle of the centra. Mako (I. oxyrinchus) centra have many plates of mineral surrounding the double cone while the silky shark (C. falciformis) has a crust of mineral extending from the central double cone. (D) Anterior view radiographs of mako and silky vertebral centra with excised neural and hemal arches. The mako shark vertebra mineral is arranged in plates around the centra and relatively unmineralized cartilage fills the gaps between the plates. The silky shark vertebra has a highly mineralized sheath around the centra with less mineralized cartilages appearing where the neural and hemal arches are placed (Porter et al., 2006).

View larger version (13K): [in this window] [in a new window]   Fig. 2. Mineral content as % dry mass (DM) in eight species of elasmobranch, including one axially undulating batoid, the electric ray. There are significant differences among species (F7,159=15.061; P<0.001). We tested material properties of 20 vertebrae each from five gray smooth-hounds M. californicus (boxed). Letters above the box and whisker plot denote significant differences and species are color-coded by order. N=10, except for M. californicus (N=100).

View larger version (24K): [in this window] [in a new window]   Fig. 3. Vertebral mineral content during serial demineralization. (A) Vertebrae from anterior and lateral views. Fully mineralized vertebra show the morphology described in Fig. 2. A partially demineralized vertebra contains approximately 25% mineral content by dry mass (DM). A demineralized vertebra has mineral arranged in disjointed fragments and it consists of approximately 10% mineral by dry mass in the cartilage. (B) Mineral content (%) decreases significantly with prolonged immersion in EDTA (F4,118=108.94; P<0.001). After 279 h in EDTA, vertebrae had approximately 72% of their original mineral content removed. Data is shown in box and whisker plots and letters above the boxes denote significant differences. N=20, except for 0 h (N=100).

View larger version (14K): [in this window] [in a new window]   Fig. 4. Arrangement of mineral (structure) in a cartilaginous matrix contributes more to the material properties of elasmobranch vertebral cartilage than the amount of mineral. (A) Strength increases with both mineral amount (blue; R2=0.112; P<0.001) in M. californicus vertebrae and mineral arrangement (red; R2=0.580; P<0.001) in eight elasmobranch species. Increasing mineral from 40% to 50% will increase strength over a range of morphologies 44% (red) but only 32% over the range of mineral amount (blue). (B) Stiffness only increases with respect to mineral arrangement (red) within the vertebral cartilage (R2=0.604; P<0.001). The natural variation (blue) of mineral contents found in M. californicus vertebrae are presented this regression. Mineral morphology (red) is shown as mean mineral content and strength or stiffness for M. californicus and for each of seven species previously examined (Porter et al., 2006). Regression statistics were calculated using all data points from each species rather than the mean value shown in the figure.

View larger version (14K): [in this window] [in a new window]   Fig. 5. Linear regressions of mineral content on material properties in vertebral cartilage from M. californicus. (A) Strength (MPa) increases significantly as mineral content increases (R2=0.64; P<0.001). (B) Stiffness (MPa) increases significantly with increased mineral content (R2=0.36; P<0.001). The red line is the regression line and the gray lines bounding it are the 95% CI. These regressions include data from control vertebrae and those that were demineralized in EDTA.

View larger version (9K): [in this window] [in a new window]   Fig. 6. Failure strain and yield strength of mineralized M. californicus vertebrae. (A) Failure strain (%) did not vary with strain rate (P=0.20). (B) Yield strength (MPa) in mineralized vertebrae varied significantly among the strain rates tested here (F3,94=4.729; P<0.01). Yield strength of vertebrae tested at strain rates of 1% s–1 was significantly lower than vertebrae tested at higher strain rates (10% s–1 and 20% s–1).

View larger version (9K): [in this window] [in a new window]   Fig. 7. Strength (MPa) of mineralized vertebrae (yellow) (approximately 50% mineral by dry mass) and demineralized vertebrae (blue) (<15% mineral by dry mass) at various strain rates (% s–1). We found mineralized vertebrae were significantly stronger than demineralized vertebrae at all strain rates (P<0.001). Strength of mineralized vertebrae increases significantly with increasing strain rate (F3,96=4.978; P<0.01). Strength does not differ with strain rate in the demineralized vertebrae (P=0.142). Letters above the box and whisker plot denote significant differences.

View larger version (9K): [in this window] [in a new window]   Fig. 8. Stiffness (MPa) of mineralized (yellow) (approximately 50% mineral by dry mass) and demineralized vertebrae (blue) (<15% mineral by dry mass) at various strain rates (% s–1). Mineralized vertebrae are stiffer than demineralized vertebrae (P<0.001). Stiffness does not vary significantly with strain rate in mineralized vertebrae (P=0.818). However, demineralized vertebrae had significantly lower stiffness values at 1% and 5% strain than they did at 10% and 20% strain (F3,30=10.693; P<0.001). Letters above the box and whisker plot denote significant differences.