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Pulp acts as a security and alarm system. Slight decay in tooth structure not extending to the dentin may not alarm the pulp, but as the dentin gets exposed, due either to dental caries or trauma, sensitivity starts. The dentinal tubules pass the stimulus to the pulp's odontoblastic layer, triggering the response. This mainly responds to cold.
Dentin is classified into three types: primary, secondary, and tertiary. [24] [25] Secondary dentin is a layer of dentin formed after the tooth's root has fully formed. Tertiary dentin develops as a result of a stimulus, such as a carious attack or wear. [26]
Nutrition for odontoblasts within the dentin comes through the dentinal tubules from tissue fluid that originally traveled from the blood vessels located in the adjacent pulp tissue. Within each dentinal tubule is a space of variable size containing dentinal fluid, an odontoblastic process, and possibly an afferent axon (see next discussion).
Tertiary dentin on the surface of a gorilla tooth. The darker area indicated by the white arrow is tertiary dentin and has formed as a response to tooth wear. Wear on the surface of a tooth can lead to the exposure of the underlying dentin. When wear is severe tertiary dentin may form to help protect the pulp chamber. [4]
Dentin formation, known as dentinogenesis, is the first identifiable feature in the crown stage of tooth development. The formation of dentin must always occur before the formation of enamel. The different stages of dentin formation result in different types of dentin: mantle dentin, primary dentin, secondary dentin, and tertiary dentin. [31]
This tissue contains blood vessels and nerves that enter the tooth from a hole at the apex of the root. [23] Along the border between the dentin and the pulp are odontoblasts, which initiate the formation of dentin. [16] Other cells in the pulp include fibroblasts, preodontoblasts, macrophages and T lymphocytes. [24]
Reactionary dentin is formed by odontoblasts when the injury does not damage the odontoblast layer. Reparative dentin is formed by replacement odontoblasts when the injury is so severe that it damages a part of the primary odontoblast layer. Thus a type of tertiary dentin forms in reaction to stimuli, such as attrition or dental caries.
Secondary dentin is darker and more opaque than primary dentin. This gives the dentin an overall darker appearance. [11] At the same time, the enamel layer is gradually thinned by tooth wear processes such as attrition and acid erosion, a degree of which is considered normal. [11] Enamel also becomes less porous and phosphate-deficient.