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Unusual extracellular matrix organization in elephant ivory.

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Ivory has been used since prehistorical times to produce objects with high symbolic value (figurines, beads, ornaments...) which therefore constitute important archaeological records of socio-cultural habits of past societies. By combining polarized light imaging, electron microscopy and X-ray scattering, the origin of the checkboard pattern visible on elephant ivory has now been elucidated.

One highly specific and intriguing feature of elephant ivory is the observation under visible light of a highly regular radial macroscopic motif resembling a distorted checkboard pattern. This so-called Schreger pattern is best observed on transverse sections of the tusks and is highly correlated with its fracture surface morphology. This tends to indicate that the mechanical properties of elephant tusks are strongly influenced by the material characteristics giving rise to the Schreger pattern. However, despite decades of ivory investigations, the precise origin of this peculiar motif is still unclear.
In a recent set of studies we first investigated whether or not the radial alternating dark and bright trapezoidal patches could result from the presence of microscopic cellular porosity (microtubules hosting ondontoblastic processes). This porosity was found to adopt a more complex 3D geometry than expected, but this was not sufficient to account for the macroscopic observations*. We therefore investigated the organization of collagen microfibrils within the tissue using polarized light microscopy and synchrotron X-ray scattering. Surprisingly, the tissue structure of elephant ivory has common features to both dentin and bone : the collagen microfibrils adopt a twisted plywood packing which is modulated by the helical displacement of odontoblasts during tissue growth**. Altogether, this implies that the Schreger pattern of elephant ivory results from light interactions with an unusual extracellular matrix organization.

* Relation between the Macroscopic Pattern of Elephant Ivory and Its Three-Dimensional Micro-Tubular Network
Marie Albéric, Mason N. Dean, Aurélien Gourrier, Wolfgang Wagermaier, John W. C. Dunlop, Andreas Staude, Peter Fratzl, Ina Reiche

** The three-dimensional arrangement of the mineralized collagen fibers in elephant ivory and its relation to mechanical and optical properties
M. Albérica, A. Gourrierb, c, W. Wagermaiera, P. Fratzla, I. Reiche