On the systematic documentation of the structural characteristics of bovine enamel: A critic to the protein sheath concept
The common structural description of bovine enamel used in materials science studies – nano-sized hydroxyapatite crystallites form micron-sized prisms surrounded by protein sheaths, which in turn build a complex decussation pattern – overlook many important morphological information. This hampers the correct interpretation of the data determined by mechanical analysis. For a profound structural description of enamel morphology, the visualization of its building blocks by high-resolution electron microscopy and focused-ion beam tomography technique, which reveals their form, orientation and configuration at different regions of a tooth (cut in different directions), is undertaken in this work. We adapted here the paleontological classification system and terminology developed for the description of enamel microstructures seen in different species, and accordingly documented the morphological singularities of bovine incisor enamel. The appearance of the boundary regions between crystallites and prisms contradicts to the well-known protein sheath concept. Neighboring crystallites and prisms are not separated by prominent gap zones but they are largely in contact with each other. Proteins might exist within the pores of 20–30 nm in size, which are distributed inhomogeneously through the boundary regions, rather than as protein sheaths covering each crystallite and prism.