Patterns of morphological variation in enamel–dentin junction and outer enamel surface of human molars

Abstract

Tooth crown patterning is governed by the growth and folding of the inner enamel epithelium (IEE) and the following enamel deposition forms outer enamel surface (OES). We hypothesized that overall dental crown shape and covariation structure are determined by processes that configurate shape at the enamel-dentine junction (EDJ), the developmental vestige of IEE. This this hypothesis was tested by comparing patterns of morphological variation between EDJ and OES in human permanent maxillary first molar (UM1) and deciduous second molar (um2). Using geometric morphometric methods, we described morphological variation and covariation between EDJ and OES, and evaluated the strength of two components of phenotypic variability, canalization and morphological integration, in addition to the relevant evolutionary flexibility, i.e. the ability to respond to selective pressure. The strength of covariation between EDJ and OES was greater in um2 than in UM1, and the way that multiple traits covary between EDJ and OES was different between these teeth. The variability analyses showed that EDJ had less shape variation and a higher level of morphological integration than OES, which indicated that canalization and morphological integration acted as developmental constraints. These tendencies were greater in UM1 than in um2. On the other hand, EDJ and OES had a comparable level of evolvability in these teeth. Amelogenesis could play a significant role in tooth shape and covariation structure, and its influence was not constant among teeth, which may be responsible for the differences in the rate and/or period of enamel formation.

Morphological comparison of the enamel–dentine junction and outer enamel surface of molars using a micro-computed tomography technique

“…b: buccal; m: mesial; l: lingual; d: distal. Data are from Morita et al [11]. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)…”

“…(For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) Morita et al [11] used a geometric morphometric analysis of anatomical landmarks collected along the occlusal table of the EDJ and OES to capture the complex dental crown topography in more detail. The correlation coefficient and percentage of covariance between EDJ and OES were examined using a partial least-squares (PLS) analysis.…”

“…Therefore, it determines the potential range or distribution of morphological variations and ultimately affects the evolutionary change tempo and mode. The difference in phenotypic variability (i.e., canalization and morphological integration and relevant evolutionary flexibility) was examined for EDJ and OES because of the distinct developmental differences between them [11]. Canalization is generally considered the property of an organism that limits phenotypic variation by buffering developmental processes against both environmental and genetic perturbations [16,18], while morphological integration refers to the tendency for different characteristics to vary as a result of common underlying developmental factors, which constrains the production of phenotypic variation and evolvability [16,17].…”

“…The slope and intercept of the regression line for UM1 are 0.863 and À 0.002, respectively (r ¼0.92, P o0.001), while those of the regression line for UM2 are 0.918 and 0.007, respectively (r¼ 0.95, Po 0.001). Data are from Morita et al[11].…”

Analytical methods and interpretation of variation in tooth morphology

“…Nondestructive morphological studies of the EDJ were conducted after the year 2000, when μCT was developed [111][112][113]. While the earlier studies had been based on the evaluation of crown traits, the differences between EDJ and OES could be analyzed quantitatively in studies using μCT.…”