Tooth size is, in part, predicated on the amount of enamel that is laid down over a dentine core during amelogenesis (the process of enamel formation where cells of the inner epithelium differentiate into ameloblasts, Nanci & Smith, 1992; Smith & Nanci, 2003). We can expect that teeth with larger metrical dimensions differ in their underlying development in some way compared to teeth with smaller metrical dimensions. Between individuals, this may be due to different body sizes, enamel cap sizes, or amelogenesis processes. However, within an individual, antimeres (left and right versions of the same tooth) have also been found to differ in morphology and dimensions (Kieser et al., 1997, O’ Donnell and Moes, 2020, Palmer et al., 1994). Here, we seek to better understand whether there is evidence of differential ameloblast behavior (i.e., daily secretion rates) that could contribute to asymmetry in antimeric tooth size.

Symmetry refers to balanced development and proportions of the right and left sides of bilateral structures that have the same genetic blueprint (Palmer & Strobeck, 1992). The failure of these structures to develop as mirror images of each other creates phenotypic asymmetry, classified as directional asymmetry, fluctuating asymmetry, and antisymmetry (Graham and Özener, 2016, van Valen, 1962). Asymmetry and its directionality can only be determined at the population level with sufficiently large sample sizes. Indeed, Smith et al. (1982) found that several hundred samples were necessary to detect fluctuating dental asymmetry, while Kieser (1990) argued that samples sizes of 75 are sufficient to prevent Type II errors.

In the dentition, morphological asymmetry has most commonly been attributed to the inability of individuals to buffer disruptions during development, whether environmental or through small random differences at the cellular level (Dempsey & Townsend, 2001; Fraser & Markow, 1994; Graham & Özener, 2016; Lens et al., 2002; Matabuena Rodríguez et al., 2017; Palmer, 1994). Dental asymmetry has important implications in clinical settings (Matabuena Rodríguez et al., 2017; Rehmani & Fida, 2012; Shah et al., 2020; Škrinjarić et al., 2018; Sprowls et al., 2008) and is used frequently in population level studies in anthropology as a measure of environmental stress (Bailit et al., 1970; Barrett et al., 2012; DiBennardo & Bailit, 1978; Kieser & Groeneveld, 1988; Marado, 2017; Milella et al., 2018; Sciulli, 2018; Townsend & Brown, 1980). Here, we investigate whether metrical asymmetry in antimeric tooth size may correspond to underlying asymmetries in enamel developmental rates within an individual.

To form enamel, ameloblast cells differentiate along the enamel-dentine junction (EDJ) as it lengthens, and move perpendicularly to the EDJ, secreting ∼3–5 µm of enamel matrix per day until they reach what will be the outer surface of the tooth (Aris et al., 2020, Nanci, 2018). In histological thin sections of enamel, daily cross striations appear as small constrictions in the enamel prism and represent 24-hours of enamel secretion and ameloblast movement (Desoutter et al., 2019, FitzGerald, 1998, Mahoney, 2011, Smith et al., 2003, Smith et al., 2004). The volume of enamel that is secreted between cross striations increases from the EDJ towards the outer enamel surface and decreases from the cuspal to the cervical enamel (Aris et al., 2020, Birch and Dean, 2009, Beynon, 1992, FitzGerald, 1998, FitzGerald and Hillson, 2009, Lacruz and Bromage, 2006, Mahoney, 2008, Smith et al., 2006). Because crown formation is not consistent throughout the enamel forming front, DSRs are typically calculated for inner, middle, and outer regions of cuspal (enamel located at the cusp of the tooth) and lateral enamel (enamel on the side or wall of the tooth); in this paper (Fig. 1).

Ultimately, the size, shape and thickness of tooth crowns are determined developmentally by a number of factors, including the rate of enamel matrix secretion, the duration of secretion by each cohort of ameloblasts, the ameloblast secretory lifespan, the number of secretory ameloblasts along the forming front and their rate of differentiation (the extension rate), and the angle relative to the EDJ of the forming front and its structural elements, the prisms or rods, (reviewed in Dean, 2000; Emken et al., 2021; Hogg, 2018; O’Meara et al., 2018; Simmer et al., 2010; Smith et al., 2007). In this study, we compare daily secretion rates (DSRs) in specific areas of the enamel crown between left and right premolars from the same individual.

Here, we examine 16 pairs of modern human premolars to identify whether observed metric differences between left and right teeth from the same individual may be matched by secretion rate differences in enamel. Because this study has a small sample size too small to represent a population and is a mix of first and second premolars, true fluctuating asymmetry cannot be determined (Garn et al., 1966, Guatelli-Steinberg et al., 2006, Harris and Nweeia, 1980, Kieser, 1990, Noss et al., 1983, Smith et al., 1982). Nevertheless, a relationship between metric differences between antimeres and corresponding local DSRs can be determined.

We hypothesize that localized variation in crown dimension will occur with localized variation in daily secretion rate in the underlying enamel. We predict that if size differences are localized, those areas of tooth crown that are larger will have higher enamel daily secretion rates. In other words, the distance between cross striations will be greater in areas that appear to be larger in one tooth than in the antimere.