and wear. it is of two types, either reactionary, where dentin is formed from a pre-existing odontoblast, or reparative, where newly differentiated odontoblast-like cells are formed due to the death of the original odontoblasts, from a pulpal progenitor cell. tertiary dentin is only formed by an odontoblast directly affected by a stimulus; therefore, the architecture and structure depend on the intensity and duration of the stimulus, e.g., if the stimulus is a carious lesion, there is extensive destruction of dentin and damage to the pulp, due to the differentiation of bacterial metabolites and toxins. thus, tertiary dentin is deposited rapidly, with a sparse and irregular tubular pattern and some cellular inclusions; in this case, it is referred to as "osteodentin". osteodentin is seen in vit.a deficiency during development. however, if the stimulus is less active, it is laid down less rapidly with a more regular tubular pattern and hardly any cellular inclusions. the speed at which tertiary dentin forms also varies substantially among primate species. defect and conditions dentinal sclerosis dentinal sclerosis or transparent dentin sclerosis of primary dentin is a change in the structure of teeth characterized by calcification of dentinal tubules. it can occur as a result of injury to dentin by caries or abrasion, or as part of the normal aging process. dentinal sclerosis animal dentin elephant ivory is solid dentin. the structure of the dentinal tubules contributes to both its porosity and its elasticity. elephant tusks are formed with a thin cap of enamel, which soon wears away, leaving the dentin exposed. exposed dentin in humans causes the symptom of sensitive teeth. dentin is best known for its occurrence in teeth, but in early vertebrates, it was an important part of the dermal skeleton that covered most of the body, and it persists today in a few taxa such as the coelacanth. because dentin is softer than enamel, it wears away more quickly than enamel. some mammalian teeth exploit this phenomenon, especially herbivores such as horses, deer or elephants. in many herbivores, the occlusal (biting) surface of the tooth is composed of alternating areas of dentin and enamel. differential wearing causes sharp ridges of enamel to be formed on the surface of the tooth (typically a molar), and to remain during the working life of the tooth. herbivores grind their molars together as they chew (masticate), and the ridges help to shred tough plant material. in xenathrans, enamel is generally absent, with the tooth instead consisting of alternating orthodentine and vasodentine. a material similar to dentin forms the hard material that makes up dermal denticles in sharks and other cartilaginous fish.