Comparison of dentin/enamel boundary for human, living and extinct animals

Objectives: The dentin-enamel junction (DEJ) is common and a universal kind of coupling uniting the stronger external protective enamel with dentin, the living and softer internal part of a tooth. We searched for comparisons of any DEJ structures that were reported in the literature with our own studies performed on humans, African buffalo, and the extinct shark, Squalicorax pristodontus, from the upper Cretaceous, found in Ouled Abdoun Basin, Morocco. The latter were used since their teeth were exceptionally well-preserved. It was difficult to find studies on fossils, because the remnants of the original organic material, e.g., collagen, had to have been preserved to allow for a full chemical comparison. The studies were aimed to determine both optical and chemical profiles of the DEJ.

Methods: The investigations were carried out by light microscopy, Scanning Electron Microscope (with an attached Si[Li) detector], and micro-Raman spectrometry. The linear scans were selected for comparison of the results.

Results: The uniform linear optical scans were performed for humans and several species of animals reported in the literature. The optical scans showed surprising uniformity and size of the DEJ structure for such different animals as the horse, extinct horse, beaver, dinosaur, and alligator. The only significant difference was the inversion of the colors between enamel and dentin for living and extinct animals. Micro-Raman analysis showed robust growth of the phosphate signal, and the amine/carbon ratio was obvious for the layers of enamel at a distance of some 15-20 µm inside the enamel. Rather less clear was the increase in the C-H signal inside the same distance on the dentin side.

Conclusions: The DEJ structures for such different organisms as humans, randomly selected mammals, and the very ancient fossil shark proved to be surprisingly similar, in terms of both size and chemical arrangement.