Unveiling the Remarkable Architecture of Rodent Incisors—Iron-Rich Enamel for Superior Mechanical and Chemical Resilience

Teeth are a perfect example of biogenic composites, consisting of optimally arranged simple inorganic and organic compounds. Continuously growing elongated rodent incisors show adaptations and optimizations that are superior when compared to human teeth. They are recognizable by their orange-brown color and their specific construction, in which hard enamel selectively covers only the labial side of the incisors, making them a self-sharpening device [1]. When fully formed, enamel consists of approximately 96 wt% of elongated hydroxyapatite (HA) crystals, with the remainder being organic material and water [2]. The complexity of the enamel structure depends on the region; in the outer radial enamel, the rods are arranged in parallel, while in the inner enamel, the rods within a layer are parallel to each other, while adjacent rows are inclined in opposite directions. Only a finite outer part of the outer radial enamel is occupied by iron-rich material, resulting in the formation of acid-resistant enamel with exceptional physical, chemical and mechanical properties [3-5].<br/>In this study [6], we have followed the complete structural and chemical development of rodent incisors from seven species (beavers, coypus, marmots, squirrels, voles, rats, mice) from the macro to the nano scale.<br/>First, we studied ameloblasts during their pigmentation stage while they were filled with ferritin nanoparticles. These nanoparticles showed a crystalline structure with a high Fe/P ratio being consistent with the values for mammalian ferritin. Their crystalline ferrhydrite core with Fe in the 3+ state suggests an iron storage mechanism that protects cells from potential toxicity.<br/>Next, as enamel matures, iron-rich material penetrates the outer layer of radial enamel and occupies the empty spaces between the elongated hydroxyapatite crystals. We call these tiny and irregularly shaped structures pockets. The outer radial enamel is filled with iron-rich material to a certain depth, forming an acid-resistant, iron-rich enamel. The infiltrated pockets are arranged in a 3D network and form a secondary phase with a ferrihydrite-like composition and iron in the 3+ oxidation state. Although they make up less than 2% of the volume of the iron-rich enamel, they contribute to improved mechanical properties and increased resistance to acid attack.<br/>Finally, an organic/inorganic surface layer formed parallel to the surface of the incisors was observed at different stages of development. The variable iron-calcium-phosphorous composition is observed not only between different species but also within the incisors of the same individual. The final thickness of the surface layer is variable and can be influenced by mechanical abrasion during the gnawing. Our observations suggest that there is a strong correlation between the color of the incisors and the thickness of the surface layer. Moreover, we observed a striking effect of color propagation only through the thickness of iron-rich enamel, although its color resembles that of regular enamel. Consequently, we propose to redefine the existing nomenclature and to rename ``pigmented enamel`` to iron-rich enamel.<br/>Our findings have significant consequences and implications for human health, the development of dental materials, and restorative dentistry.<br/><br/>References:<br/>[1] Chen P-Y et al., Prog. Mater. Sci. 57 (2012) 1492-1704.<br/>[2] Nanci A, Ten Cate Histology. Development, Structure and Function, 7^th ed., Mosby Elsevier: St. Louis, MO (2008) 411 pp.<br/>[3] Gordon et al., Science 347 (2015) 746-750.<br/>[4] Srot et al., ACS Nano 11 (2017) 239-248.<br/>[5] Madsen et al., Hyperfine Interact. 29 (1986) 1431-1434.<br/>[6] Srot et al., ACS Nano 18 (2024) 11270-11283.