X-ray microCT study of pyramids of the sea urchin Lytechinus variegatus

This paper reports results of a novel approach, X-ray microCT, for quantifying stereom structures applied to ossicles of the sea urchin Lytechinus variegatus. MicroCT, a high resolution variant of medical CT (computed tomography), allows noninvasive mapping of microstructure in 3-D with spatial resolution approaching that of optical microscopy. An intact pyramid (two demipyramids, tooth epiphyses, and one tooth) was reconstructed with 17μm isotropic voxels (volume elements); two individual demipyramids and a pair of epiphyses were studied with 9-13μm isotropic voxels. The cross-sectional maps of a linear attenuation coefficient produced by the reconstruction algorithm showed that the structure of the ossicles was quite heterogeneous on the scale of tens to hundreds of micrometers. Variations in magnesium content and in minor elemental constitutents could not account for the observed heterogeneities. Spatial resolution was insufficient to resolve the individual elements of the stereom, but the observed values of the linear attenuation coefficient (for the 26keV effective X-ray energy, a maximum of 7.4cm^-1 and a minimum of ∼2cm^-1 away from obvious voids) could be interpreted in terms of fractions of voxels occupied by mineral (high magnesium calcite). The average volume fraction of mineral determined for a transverse slice of the demipyramid near where it joins an epiphysis was 0.46; for a slice 3.3mm adoral it was 0.70. Local volume fractions of mineral approached 1, and, away from resolvable voids, considerable portions of the demipyramids had volume fractions of calcite at or below ∼0.33. MicroCT imaging of a demipyramid before and after infiltration with a high absorptivity fluid (sodium polytungstate) confirmed the determination of the volume fractions of minerals.