Patterns of variation in canal and root number in human post-canine teeth

Abstract

Descriptive morphology of tooth roots traditionally focuses on number of canals and roots. However, how or if canal and root number are related is poorly understood. While it is often assumed that canal number is concomitant with root number and morphology, in practice canal number and morphology do not always covary with external root features. To investigate the relationship between canal and root number, fully developed, adult post-canine teeth were examined and quantified from medical computerized tomography scans from a global sample of 945 modern humans. We tested the hypotheses that root and canal number do not follow a 1:1 ratio, that canal to root ratios differ between teeth, and that canal to root ratios differ across populations. Results indicate that not only is root number dependent on canal number, but that this relationship become more variable as canal number increases, varies both between individual teeth and by population, and changes as populations increase in distance from Sub-Saharan Africa. These results show that the ratio of canal number to root number is an important indicator of variation in dental phenotypes.

Canal number and configuration are predictors of external root morphology

“…The first is that a wider root is the product of multiple canals, and the relationship between canal number and root morphology is a spatial one. Canal formation precedes external root formation (Miller, 2013), and canal number has been shown to predict root number (Gellis & Foley, 2022). However, this interpretation is problematic.…”

“…However, phenotypic variation in organismal development is biased towards certain phenotypes (Arthur, 2004; J. M. Smith et al, 1985; Wilkins, 2007). While canal and root formation are comprised of a series of reciprocal cellular interactions (Jernvall & Thesleff, 2000), clusters of blood vessels entering the developing tooth coincide with the positions where roots will eventually form (Miller, 2013); and there is a clear relationship between canal and root number, in which canal number is either equal to or exceeds root number (Gellis & Foley, 2022). These observations of root development suggest that the structures that help determine root number and position are present early in tooth morphogenesis and play some part in the developmental bias of roots.…”

“…However, it is unknown if this is due to adaptive pressures of dietary differences within and between the geographic groups used in this study, some other selective pressure. The linear relationship between canal and root number has been shown to significantly vary between populations (Gellis & Foley, 2022). Further work on tooth root trait expression and frequency, and how these correlate with masticatory behaviours will need to be carried out in order to determine how and what traits are adaptive versus which ones are population specific.…”

“…In the case when two canals bifurcate from a single canal, if the degree of bifurcation is >= 33% of the total length of the canal structure, the canal count is two. The full method and its application are discussed in Gellis and Foley (2021, 2022), but are visualized in Figure 5.…”

“…Tooth germ size also effects molecular signaling governing the development of interradicular processes which can lead to differing degrees of fusion and bifurcation (ibid). While canal number has been shown to predict root number (Gellis & Foley, 2022), explanations for external tooth root morphology recorded in the literature (Table 1) are almost non-existent.…”