Enamel apatite crystallinity significantly contributes to mammalian dental adaptations

Anna Kallistová,Irena Matulková,Petr Čejchan,Roman Skála,Miroslav Šlouf,Ivan Horáček

doi:10.1038/s41598-018-23826-0

Abstract

The monophyodont molar teeth, prismatic enamel and the complexity of enamel microarchitecture are regarded as essential dental apomorphies of mammals. As prominent background factors of feeding efficiency and individual longevity these characters are crucial components of mammalian adaptive dynamics. Little is known, however, to which degree these adaptations are influenced by the crystallographic properties of elementary hydroxyapatite crystallites, the only inorganic component of enamel. In a miniature pig where individual molars differ significantly in duration of their development and in enamel resistance to attrition stress, we found highly significant differences between the molars in the size of crystallites, amount of microstrain, crystallinity and in enamel stiffness and elasticity, all clearly scaled with the duration of tooth calcification. The same pattern was found also in red deer bearing different molar type. The results suggest that the prolongation of tooth development is associated with an increase of crystallinity, i.e. the atomic order of enamel hydroxyapatite, an obvious component of micromechanical property of mature enamel. This relation could contribute to prolongation of dental development, characteristic of mammals in general. The aspects of enamel crystallinity, omitted in previous studies on mammalian and vertebrate dental evolution, are to be taken in account in these topics.

Highlights

Enamel, the hardest tissue of vertebrate bodies and proper agent of teeth function, is exclusively a mineral matter: it is composed of compact aggregates of hydroxyapatite (HAp) crystallites while the non-mineral components, proteins and water, form less than 5 wt.% only[1]
Two of them are to be emphasized: prismatic enamel and the monophyodont multicuspidate distal teeth called molars[7,8,9]
Our results suggest that apatite crystallinity increases with length of enamel maturation and influences the final mechanical quality of the enamel coat in an essential way

Summary

Introduction

The hardest tissue of vertebrate bodies and proper agent of teeth function, is exclusively a mineral matter: it is composed of compact aggregates of hydroxyapatite (HAp) crystallites while the non-mineral components, proteins and water, form less than 5 wt.% only[1]. The crystalographic properties of enamel apatite and microarchitecture of enamel coat are the essential factors of functional quality of teeth and, in consequence, feeding efficiency and life-expectancy of the individual This is valid for mammals whose diphyodont dentition provides no chance for reparative dental rearrangements during adult age. With functional specialization of the deciduous dentition (such as molarization of deciduous premolars), the developmental time of the distal teeth can be further prolonged, enabling them to grow larger to respond functional demands of even very large adult body size Such a kind of developmental heterochrony is strongly selected in the clades whose feeding depends upon processing of large amounts of food in molariform dentition, namely in herbivores[10,11]. Our results suggest that apatite crystallinity increases with length of enamel maturation and influences the final mechanical quality of the enamel coat in an essential way

Methods

Results

Discussion

Conclusion