Abstract
For humans and other mammals to eat effectively, teeth must develop properly inside the jaw. Deciphering craniodental integration is central to explaining the timely formation of permanent molars, including third molars which are often impacted in humans, and to clarifying how teeth and jaws fit, function and evolve together. A factor long-posited to influence molar onset time is the jaw space available for each molar organ to form within. Here, we tested whether each successive molar initiates only after a minimum threshold of space is created via jaw growth. We used synchrotron-based micro-CT scanning to assess developing molars in situ within jaws of C57BL/6J mice aged E10 to P32, encompassing molar onset to emergence. We compared total jaw, retromolar and molar lengths, and molar onset times, between upper and lower jaws. Initiation time and developmental duration were comparable between molar upper and lower counterparts despite shorter, slower-growing retromolar space in the upper jaw, and despite size differences between upper and lower molars. Timing of molar formation appears unmoved by jaw length including space. Conditions within the dental lamina likely influence molar onset much more than surrounding jaw tissues. We theorize that molar initiation is contingent on sufficient surface area for the physical reorganization of dental epithelium and its invagination of underlying mesenchyme.
Highlights
Introduction iationsThe proper development, growth, and evolution of animal bodies require multiple body parts to change together in a coordinated way over time
We suggest that the propagation of oral and subsequently dental epithelium is most vital to molar initiation
We hypothesize that only marginal elongation of the jaw primordium and, later, the jawbone, is important for molar initiation. We propose that this marginal growth allows the oral/dental epithelium and dental lamina to achieve a minimum length required for the ribbon of tissue to buckle, catalyzing invagination of the epithelium into the underlying mesenchyme [39]
Summary
The proper development, growth, and evolution of animal bodies require multiple body parts to change together in a coordinated way over time. Explaining the mechanisms underpinning these coordinated changes is foundational to understanding animal evolution and development (evo-devo). A powerful case study of coordination—or “integration”—among body parts is a dentate jaw. Dentitions and jaws appear to have evolved separately from each other, at different times, in prehistoric fossil fishes [1]. Stringent selection pressure for a kinetic jaw outfitted with cutting and grinding tools is surely why the majority of vertebrates living today retain this ancient and revolutionary phenotype [2]. The jaw skeleton and the dentition are both impressively adaptable to a broad range of diets and feeding strategies [3]. Teeth and the jaws that house them show substantial developmental lability: they may develop, and evolve, in very different ways
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