Structure and Composition of the Mammalian Tooth

The component tissues of teeth include an outer enamel layer and an underlying
layer of dentin. These tissues surround and protect the inner pulp cavity, which

Structure of the Human Tooth
https://biologydictionary.net/teeth/

is filled with the nerves and blood vessels that enter our teeth.

To begin, the outer layer enamel is composed almost entirely of inorganic material. It is developed from the epithelium of the mouth (Lucas, 2006). The proteins belonging to the enamel fall in one of two classes - the amelogenins or enamelins. They are found in the extracellular matrix and their role is to control the mineralization of the enamel crystals; they compose the material that will make up the enamel. These proteins are secreted by cells called ameloblasts. Interestingly, enamel is the hardest substance found in the human body and is 85-92% mineral in a permanent human tooth (Lucas, 2006). A major component of the enamel is the mineral hydroxyapatite (HA) which gives our teeth the rigidity necessary to chew tough foods. The major function of the enamel is to protect the dentin (Ferguson et al., 2006). However, as we will see later, it is susceptible to degradation. Enamel is only found on areas of the dentin that are exposed to the outside, referred to as the crown of the tooth (see the diagram). The area surrounded by the gums, referred to as the root of the tooth, has an outer layer called the cementum - which is similar in composition to enamel but less mineralized and more closely resembles bone (Ferguson et al., 2006).

Relative Compositions of Enamel and Dentin Layers of the Teeth
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Underneath the enamel is the dentin layer. It is the barrier between the sensory and vascular supply of the pulp and the functional surface of the tooth (Ferguson et al, 2006). It too is highly mineralized, composed of roughly 45% hydroxyapatite, but not nearly as strong as the enamel. Since the dentin is in close contact with nerves and blood vessels, it differs from enamel in the sense that it is sensitive. Contrarily to enamel, it can be formed throughout life and repaired when it is worn down (Lucas, 2006). The living tissue inside of the pulp cavity is responsible for this maintenance. The organic matrix of the dentin is based on collagen fibres with scattered regions of crystals, called calcospherites (Lucas, 2006). It has a critical role in the support and function of the teeth.

Finally is the dental pulp, the living component of our teeth. It serves to provide nutrition and functional requirements to the odontoblasts (tooth forming cells) so they can continue to form dentin throughout our lifetime (Lucas, 2006). As a result of the continuous dentin formation, our pulp cavity gradually becomes more narrow as our dentin layer thickens. Interestingly, the nerves that enter and exit our teeth are not necessary for the survival or functioning of our teeth (Ferguson et al., 2006). They have a sensory role - they provide sensation to hot or cold, which of course is helpful but definitely not essential. They are also involved in altering relative amounts of blood flow into the tooth (Abd-Elmeguid et al., 2009). It is because of these nerve fibres that we can sometimes feel pain in our teeth, referred to as toothache.