Polychronis Georgios, Halazonetis Demetrios J
Department of Orthodontics, School of Dentistry, National and Kapodistrian University of Athens, Athens, Greece.
J Anat. 2014 Aug;225(2):220-31. doi: 10.1111/joa.12202. Epub 2014 Jun 10.
The occurrence of mutual genetic loci in morphogenesis of the face and teeth implies shape covariation between these structures. However, teeth finalize their shape at an early age, whereas the face grows and is subjected to environmental influences for a prolonged period; it is therefore conceivable that covariation might modulate with age. Here we investigate the extent of this covariation in humans by measuring the 3D shape of the occlusal surface of the permanent first molars and the shape of the craniofacial complex from lateral radiographs, at two maturations stages. A sample of Greek subjects was divided into two groups (110 adult, 110 prepubertal) with equally distributed gender. The occlusal surfaces of the right first molars were 3D scanned from dental casts; 265 and 274 landmarks (including surface and curve semilandmarks) were digitized on the maxillary and mandibular molars, respectively. The corresponding lateral cephalometric radiographs were digitized with 71 landmarks. Geometric morphometric methods were used to assess shape variation and covariation. The vertical dimension of the craniofacial complex was the main parameter of shape variation, followed by anteroposterior deviations. The male craniofacial complex was larger (4.0-5.7%) and was characterized by a prominent chin and clockwise rotation of the cranial base (adult group only). Allometry was weak and statistically significant only when examined for the sample as a whole (percent variance explained: 2.1%, P = 0.0002). Covariation was statistically significant only between the lower first molar and the craniofacial complex (RV = 14.05%, P = 0.0099, and RV = 12.31%, P = 0.0162, for the prepubertal and adult groups, respectively). Subtle age-related covariation differences were noted, indicating that environmental factors may influence the pattern and strength of covariation. However, the main pattern was similar in both groups: a class III skeletal pattern (relative maxillary retrusion and mandibular protrusion), hyperdivergency, forward rotation of the posterior cranial base and upward rotation of the anterior cranial base were associated with mesiodistal elongation of the lower molars and height reduction of their distal cusps. This pattern mimics phylogeny in humans, where flexion and counterclockwise rotation of the cranial base, considered advantageous to survival, co-occur with tooth reductions that cannot be easily explained in evolutionary terms. The similarity of the phylogenetic and covariation patterns seems to support the pleiotropic gene hypothesis.
面部和牙齿形态发生过程中共同遗传位点的出现意味着这些结构之间存在形状协变。然而，牙齿在幼年时就确定了其形状，而面部则会持续生长并长期受到环境影响；因此可以想象，协变可能会随年龄而变化。在这里，我们通过测量两个成熟阶段恒牙第一磨牙咬合面的三维形状以及侧位X线片上颅面复合体的形状，来研究人类中这种协变的程度。一组希腊受试者被分为两组（110名成年人，110名青春期前儿童），性别分布均匀。从石膏模型上对右侧第一磨牙的咬合面进行三维扫描；分别在上颌和下颌磨牙上数字化了265个和274个地标点（包括表面和曲线半地标点）。相应的侧位头影测量X线片用71个地标点进行数字化。使用几何形态测量方法来评估形状变异和协变。颅面复合体的垂直维度是形状变异的主要参数，其次是前后偏差。男性颅面复合体更大（4.0 - 5.7%），其特征是下巴突出和颅底顺时针旋转（仅成年组）。异速生长较弱，仅在对整个样本进行检查时具有统计学意义（解释的方差百分比：2.1%，P = 0.0002）。协变仅在下颌第一磨牙和颅面复合体之间具有统计学意义（青春期前组和成年组的RV分别为14.05%，P = 0.0099和12.31%，P = 0.0162）。注意到了与年龄相关的细微协变差异，表明环境因素可能会影响协变的模式和强度。然而，两组的主要模式相似：III类骨骼模式（相对上颌后缩和下颌前突）、高度离散、后颅底向前旋转和前颅底向上旋转与下颌磨牙的近远中伸长及其远中尖高度降低相关。这种模式与人类的系统发育相似，在人类中，被认为有利于生存的颅底屈曲和逆时针旋转与难以用进化术语解释的牙齿减少同时出现。系统发育和协变模式的相似性似乎支持了多效基因假说。
