Shellis R P
MRC Dental Group, Dental School, Lower Maudlin Street, Bristol, BS1 2LY, U.K.
J Hum Evol. 1998 Oct-Nov;35(4-5):387-400. doi: 10.1006/jhev.1998.0260.
Enamel retains two fundamental periodic markings: the prism cross-striations, marking a diurnal rhythm of prism elongation, and the striae of Retzius, a longer-term marker of formation of the enamel surface. It is argued that, while artefacts superficially resembling prism cross-striations can be observed, these should not mislead an experienced histologist and that true cross-striations are reliable markers of the daily incremental apposition of enamel. The incremental markings can be used to elucidate several aspects of tooth crown growth. (1) Rate of enamel apposition. In some teeth, e. g., hominoid permanent teeth, the enamel apposition rate increases progressively between the inner and outer enamel, while in others the apposition rate appears to be uniform. (2) Crown formation time. This can be estimated on intact teeth, using counts of perikymata (surface traces of the striae), together with information on the time interval between successive striae. Much better estimates can be obtained from sections, using counts of cross-striations. (3) Pattern of crown formation. The rate at which new enamel formation extends over the presumptive enamel-dentine junction (extension rate) can be estimated from knowledge of the cross-striation interval together with angular measurements of prisms and striae. Multiple measurements of the extension rate within a tooth can be used to provide an estimate of crown formation time. The main application of extension rate measurements is to give quantitative information on the pattern of crown formation. In large primate teeth, the extension rate is high early in crown formation and slower later in crown formation. In smaller, more rapidly-formed teeth, a gradient in extension rate appears to be absent. Enamel thickness is negatively correlated with extension rate among primates. This seems to be a mechanism ensuring that there is sufficient time for enamel maturation to be completed before eruption takes place. Among primates it appears that variation in the prism pattern (i.e., the extent to which the outline of the pattern 3 prisms is closed or open) is correlated with the extension rate. It is postulated that the extension rate controls prism shape by influencing the morphology of the Tomes process pits during their initial formation.
棱柱横纹，标志着棱柱伸长的昼夜节律；以及芮氏线，它是牙釉质表面形成的一种长期标志。有人认为，虽然可以观察到表面上类似棱柱横纹的人工痕迹，但这些不应误导经验丰富的组织学家，真正的横纹是牙釉质每日增量沉积的可靠标志。这些增量纹路可用于阐明牙冠生长的几个方面。（1）牙釉质沉积速率。在某些牙齿中，例如类人猿恒牙，牙釉质沉积速率在内釉质和外釉质之间逐渐增加，而在其他牙齿中，沉积速率似乎是均匀的。（2）牙冠形成时间。这可以在完整的牙齿上通过计算釉面横纹（芮氏线的表面痕迹），并结合连续横纹之间的时间间隔信息来估计。从切片中通过计算横纹可以获得更好的估计值。（3）牙冠形成模式。根据横纹间隔的知识以及棱柱和纹路的角度测量，可以估计新牙釉质形成在假定的釉质 - 牙本质交界处扩展的速率（扩展速率）。在一颗牙齿内对扩展速率进行多次测量可用于估计牙冠形成时间。扩展速率测量的主要应用是提供有关牙冠形成模式的定量信息。在大型灵长类动物牙齿中，牙冠形成早期扩展速率高，后期较慢。在较小、形成更快的牙齿中，似乎不存在扩展速率梯度。在灵长类动物中，牙釉质厚度与扩展速率呈负相关。这似乎是一种确保在牙齿萌出之前有足够时间完成牙釉质成熟的机制。在灵长类动物中，棱柱模式的变化（即模式3棱柱的轮廓闭合或开放的程度）似乎与扩展速率相关。据推测，扩展速率通过在其初始形成过程中影响托姆斯突凹的形态来控制棱柱形状。
