Department of Biomaterials, Max-Planck-Institute of Colloids and Interfaces, Potsdam, Germany.
Univ. Grenoble Alpes, CNRS, LIPhy, 38000 Grenoble, France; European Synchrotron Radiation Facility, Grenoble, France.
Acta Biomater. 2018 May;72:342-351. doi: 10.1016/j.actbio.2018.02.016. Epub 2018 Mar 1.
Elephant tusks are composed of dentin or ivory, a hierarchical and composite biological material made of mineralized collagen fibers (MCF). The specific arrangement of the MCF is believed to be responsible for the optical and mechanical properties of the tusks. Especially the MCF organization likely contributes to the formation of the bright and dark checkerboard pattern observed on polished sections of tusks (Schreger pattern). Yet, the precise structural origin of this optical motif is still controversial. We hereby address this issue using complementary analytical methods (small and wide angle X-ray scattering, cross-polarized light microscopy and scanning electron microscopy) on elephant ivory samples and show that MCF orientation in ivory varies from the outer to the inner part of the tusk. An external cohesive layer of MCF with fiber direction perpendicular to the tusk axis wraps the mid-dentin region, where the MCF are oriented mainly along the tusk axis and arranged in a plywood-like structure with fiber orientations oscillating in a narrow angular range. This particular oscillating-plywood structure of the MCF and the birefringent properties of the collagen fibers, likely contribute to the emergence of the Schreger pattern, one of the most intriguing macroscopic optical patterns observed in mineralized tissues and of great importance for authentication issues in archeology and forensic sciences.
Elephant tusks are intriguing biological materials as they are composed of dentin (ivory) like teeth but have mineralized collagen fibers (MCF) similarly arranged to the ones of lamellar bones and function as bones or antlers. Here, we showed that ivory has a graded structure with varying MCF orientations and that MCF of the mid-dentin are arranged in plywood like layers with fiber orientations oscillating in a narrow angular range around the tusk axis. This organization of the MCF may contribute to ivory's mechanical properties and, together with the collagen fibers birefringence properties, strongly relates to its optical properties, i.e. the emergence of a macroscopic checkerboard pattern, well known as the Schreger pattern.
象牙由牙本质或象牙组成，这是一种分层的复合材料，由矿化的胶原纤维（MCF）组成。人们认为 MCF 的特定排列负责象牙的光学和机械性能。特别是 MCF 的组织可能有助于形成在象牙抛光截面观察到的明亮和黑暗棋盘图案（Schreger 图案）。然而，这种光学图案的确切结构起源仍然存在争议。我们使用补充分析方法（小角和广角 X 射线散射、偏光显微镜和扫描电子显微镜）来解决这个问题，对象牙样本进行了研究，结果表明象牙中的 MCF 取向从象牙的外部到内部发生变化。MCF 的外部有向层与象牙轴垂直，包裹着中间牙本质区域，其中 MCF 主要沿象牙轴排列，并以胶合板状结构排列，纤维取向在狭窄的角度范围内波动。MCF 的这种特殊的波动胶合板结构和胶原纤维的双折射特性，可能有助于 Schreger 图案的出现，这是在矿化组织中观察到的最吸引人的宏观光学图案之一，对于考古学和法医学中的认证问题非常重要。
象牙是一种有趣的生物材料，因为它们由类似于牙齿的牙本质（象牙）组成，但具有类似于层状骨的矿化胶原纤维（MCF）排列，并具有骨骼或鹿角的功能。在这里，我们表明象牙具有渐变结构，MCF 取向不同，并且中间牙本质的 MCF 以类似于胶合板的层排列，纤维取向在围绕象牙轴的狭窄角度范围内波动。MCF 的这种组织可能有助于象牙的机械性能，并且与胶原纤维的双折射特性一起，与象牙的光学特性密切相关，即宏观棋盘图案的出现，即众所周知的 Schreger 图案。
