Department of Biology, University of Hildesheim, Hildesheim, Germany.
PLoS One. 2013 Sep 6;8(9):e74597. doi: 10.1371/journal.pone.0074597. eCollection 2013.
We studied the structural characteristics and periodicities of regular incremental markings in sheep enamel using fluorochrome injections for vital labeling of forming enamel and backscattered electron imaging in the scanning electron microscope. Microscopic analysis of mandibular first molars revealed the presence of incremental markings with a daily periodicity (laminations) that indicated successive positions of the forming front of interprismatic enamel. In addition to the laminations, incremental markings with a sub-daily periodicity were discernible both in interprismatic enamel and in enamel prisms. Five sub-daily increments were present between two consecutive laminations. Backscattered electron imaging revealed that each sub-daily growth increment consisted of a broader and more highly mineralized band and a narrower and less mineralized band (line). The sub-daily markings in the prisms of sheep enamel morphologically resembled the (daily) prisms cross striations seen in primate enamel. Incremental markings with a supra-daily periodicity were not observed in sheep enamel. Based on the periodicity of the incremental markings, maximum mean daily apposition rates of 17.0 µm in buccal enamel and of 13.4 µm in lingual enamel were recorded. Enamel extension rates were also high, with maximum means of 180 µm/day and 217 µm/day in upper crown areas of buccal and lingual enamel, respectively. Values in more cervical crown portions were markedly lower. Our results are in accordance with previous findings in other ungulate species. Using the incremental markings present in primate enamel as a reference could result in a misinterpretation of the incremental markings in ungulate enamel. Thus, the sub-daily growth increments in the prisms of ungulate enamel might be mistaken as prism cross striations with a daily periodicity, and the laminations misidentified as striae of Retzius with a supra-daily periodicity. This would lead to a considerable overestimation of crown formation times in ungulate teeth.
我们使用氟化物注射对形成中的牙釉质进行活体标记，并在扫描电子显微镜下进行背散射电子成像，研究了绵羊牙釉质中规则递增标记的结构特征和周期性。对下颌第一磨牙的微观分析显示，存在具有每日周期性（纹层）的递增标记，表明棱柱间牙釉质形成前沿的连续位置。除了纹层之外，在棱柱间牙釉质和牙釉质棱柱中还可以辨别出具有亚日周期性的递增标记。在两个连续的纹层之间存在五个亚日增量。背散射电子成像显示，每个亚日生长增量由一个更宽且更高度矿化的带和一个更窄且矿化程度较低的带（线）组成。绵羊牙釉质棱柱中的亚日标记在形态上类似于灵长类牙釉质中的（每日）棱柱交叉条纹。在绵羊牙釉质中未观察到具有超日周期性的递增标记。根据递增标记的周期性，记录到颊侧牙釉质的最大平均每日附着率为 17.0 µm，舌侧牙釉质的最大平均每日附着率为 13.4 µm。牙釉质延伸率也很高，在颊侧和舌侧牙釉质的上冠区域，最大平均值分别为 180 µm/天和 217 µm/天。在更靠近牙颈的冠部区域，数值明显较低。我们的结果与其他有蹄类物种的先前发现一致。使用灵长类牙釉质中存在的递增标记作为参考可能会导致对有蹄类牙釉质中递增标记的误解。因此，有蹄类牙釉质棱柱中的亚日生长增量可能会被误认为是具有每日周期性的棱柱交叉条纹，而纹层可能会被错误识别为具有超日周期性的 Retzius 纹。这将导致对有蹄类牙齿冠形成时间的极大高估。
