but it can also cause enamel hypoplasia and green staining of enamel. enamel hypoplasia is broadly defined to encompass all deviations from normal enamel in its various degrees of absence. the missing enamel could be localized, forming a small pit, or it could be completely absent. erythropoietic porphyria is a genetic disease resulting in the deposition of porphyrins throughout the body. these deposits also occur in enamel and leave an appearance described as red in color and fluorescent. fluorosis leads to mottled enamel and occurs from overexposure to fluoride. tetracycline staining leads to brown bands on the areas of developing enamel. children up to age 8 can develop mottled enamel from taking tetracycline. as a result, tetracycline is contraindicated in pregnant women. celiac disease, a disorder characterized by an auto-immune response to gluten, also commonly results in demineralization of the enamel. other mammals for the most part, research has shown 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. teeth of a rottweiler 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 the rod directions are lower values, shown to have an elastic modulus between 70 and 77 gpa and a hardness value of 3.0-3.5 gpa. the anisotropy between the two directions can be as high as 30%. this is due partially to the structure of the material and the directionality of the rods in the c-direction. the structure of enamel is also composite in nature between the interrod and rods which also leads to anisotropy. single crystallite hydroxyapatite, the mineral enamel is based on, is also anisotropic. single crystallite hydroxyapatite has a higher hardness and young's modulus, which may be due to the defects present in enamel, such as substitutional ions as well as the presence of organic materials. the mechanical properties of enamel not only are anisotropic due to the structure of the rods and interrods. they are also varying across the length of enamel from the enamel at the surface of the tooth, the outer enamel, to the junction between the dentin and enamel, dej. the elastic modulus increases as the distance between the dentin-enamel junction (dej) increases within enamel. the fracture toughness is also anisotropic. the fracture toughness can vary by up to a factor of three due to the orientation of the rods. also in enamel, cracks do not easily penetrate the dentin, which may lead to the higher fracture toughness. overall, enamel is a highly anisotropic material due to its microstructure, which leads to properties needed for the effective use of our teeth.