that the formation of tooth enamel in animals is almost identical to formation in humans. The enamel organ, including the dental papilla, and ameloblasts function similarly. The variations of enamel that are present are infrequent but sometimes important. Differences exist, certainly, in the morphology, number, and types of teeth among animals. Dogs are less likely than humans to have tooth decay due to the high pH of dog saliva, which prevents an acidic environment from forming and the subsequent demineralization of enamel which would occur. If tooth decay does occur (usually from trauma), dogs can receive dental fillings just as humans do. Similar to human teeth, the enamel of dogs is vulnerable to tetracycline staining. Consequently, this risk must be accounted for when tetracycline antibiotic therapy is administered to young dogs. Enamel hypoplasia may also occur in dogs. The mineral distribution in rodent enamel is different from that of monkeys, dogs, pigs, and humans. In horse teeth, the enamel and dentin layers are intertwined with each other, which increases the strength and wear resistance of those teeth. Other organisms Enamel or enameloid is found in the dermal denticles of sharks and many early vertebrates, and it appeared there before gnathostome teeth evolved. The ganoin that covers the scales of many actinopterygians is probably derived from enamel. Enamel-like substances also coat the jaws of some crustacea, but this is not homologous with vertebrate enamel. Enameloid covers some fish scales. Mechanical properties Due to the unique structure of enamel, the mechanical properties are very interesting. Enamel is the hardest material in the body and is one of the most durable load-bearing biological materials. It has been shown to have a fracture toughness three times greater than that of geological hydroxyapatite, the main mineral in enamel. Consisting of rod and interrod regions within the microstructure of enamel, the mechanical properties of enamel vary depending on the location within the microstructure. The rod and interrod structure causes anisotropy in enamel as both components have different mechanical properties. The interrod enamel has around 53% and 74% decreased hardness and elastic modulus when compared to the rod structures. This leads to a composite like hierarchical structure of enamel. The hardness and stiffness parallel to the rod axis results in high hardness and modulus, shown to have modulus values of 85-90 GPa and hardness values of 3.4-3.9 GPa. The hardness and modulus in the direction perpendicular to