Department of Physics, University of Wisconsin, Madison, Wisconsin 53706, USA.
J Am Chem Soc. 2009 Dec 30;131(51):18404-9. doi: 10.1021/ja907063z.
Sea urchin teeth are remarkable and complex calcite structures, continuously growing at the forming end and self-sharpening at the mature grinding tip. The calcite (CaCO(3)) crystals of tooth components, plates, fibers, and a high-Mg polycrystalline matrix, have highly co-oriented crystallographic axes. This ability to co-orient calcite in a mineralized structure is shared by all echinoderms. However, the physico-chemical mechanism by which calcite crystals become co-oriented in echinoderms remains enigmatic. Here, we show differences in calcite c-axis orientations in the tooth of the purple sea urchin ( Strongylocentrotus purpuratus ), using high-resolution X-ray photoelectron emission spectromicroscopy (X-PEEM) and microbeam X-ray diffraction (muXRD). All plates share one crystal orientation, propagated through pillar bridges, while fibers and polycrystalline matrix share another orientation. Furthermore, in the forming end of the tooth, we observe that CaCO(3) is present as amorphous calcium carbonate (ACC). We demonstrate that co-orientation of the nanoparticles in the polycrystalline matrix occurs via solid-state secondary nucleation, propagating out from the previously formed fibers and plates, into the amorphous precursor nanoparticles. Because amorphous precursors were observed in diverse biominerals, solid-state secondary nucleation is likely to be a general mechanism for the co-orientation of biomineral components in organisms from different phyla.
海胆牙齿是一种显著而复杂的方解石结构，在形成端不断生长，并在成熟的研磨尖端自锐化。牙齿成分的方解石（CaCO3）晶体、板、纤维和高镁多晶基质具有高度共取向的结晶轴。这种在矿化结构中使方解石共取向的能力是所有棘皮动物所共有的。然而，方解石晶体在棘皮动物中变得共取向的物理化学机制仍然是个谜。在这里，我们使用高分辨率 X 射线光电子能谱显微镜（X-PEEM）和微束 X 射线衍射（muXRD）显示了紫色海胆（Strongylocentrotus purpuratus）牙齿中方解石 c 轴取向的差异。所有的板都共享一个晶体取向，通过支柱桥传播，而纤维和多晶基质则共享另一个取向。此外，在牙齿的形成端，我们观察到 CaCO3 以无定形碳酸钙（ACC）的形式存在。我们证明，多晶基质中纳米粒子的共取向是通过固态二次成核发生的，从先前形成的纤维和板中向外传播，进入无定形前体纳米粒子。因为在不同的生物矿化中都观察到了无定形前体，所以固态二次成核很可能是不同门生物体内生物矿化成分共取向的一般机制。
