Species: mammal.
Technique: decalcification, paraffin embedding, and hemaotxylin-eosin staining.
Teeth are found in the oral cavity and are needed to tear and crush food. They are fixed to the upper (maxilla) and lower jaws, inserted into the alveolar processes by the teeth roots. Position and shape determine the name given to each tooth. In adult humans, there are 2 central incisors, 2 lateral incisors, 2 canines, 4 premolars, and 6 molars in each jaw. They perform different roles during feeding. For example, incisors are used for tearing, while molars are utilized for crushing. In humans, 6 months after birth, milk teeth appear with 20 teeth, which are replaced by permanent teeth at 6 to 13 years old.
Macroscopically, the root is the hidden part of the tooth, inserted into the bone alveoli, while the crown is the visible part. Incisors, canines, and premolars, with the exception of the first upper premolar, have a single root. Molars have three roots and occasionally have four. The region where the root connects to the crown is known as the neck.
At the microscopical level, teeth consist of several layers containing a special set of specific molecules that provide a gradient of hardness.
The enamel is a mineralized layer that covers the tooth crown. It cannot be repaired by tooth tissues, as it is formed only once and it is not renewed. It is the only mineralized structure not produced by the connective tissue but by the epithelium. Enamel is regarded as the hardest structure of the tooth, primarily due to its composition of 99% crystalline calcium phosphate, known as hydroxyapatite. Enamel is thicker in those areas of the tooth with crushing functions, up to 2.5 mm in humans. There is no enamel in the tooth root.
The cementum is a mineralized layer, similar to bone, covering the tooth root. However, unlike the bone, it lacks blood vessels. Cementum fixes the tooth to the alveolar wall by a network of collagen fibrils, known as Sharpey's fibers, that anchor the root to the bone. These collagen fibers are oriented obliquely from the cementum to their inserting point in the bone. The fibrous area where these fibers are inserted forms a layer referred to as the periodontal membrane, which is strongly irrigated by blood vessels and innervated by nerves coming from several sources.
Dentin is a calcified material constituting most of the inner part of the tooth. It is found immediately below the enamel and the cementum and it is composed of about 80% hydroxyapatite, which is a lower proportion than in the enamel but higher than in the cementum and bone. Dentin leaves an inner cavity containing connective tissue, referred to as pulp. Dentin does not contain cells and it is synthesized by the odontoblasts, columnar cells that form a one-cell-thick layer separating dentin and pulp. The striations observed in the dentin in histological sections are caused by the material secreted in waves by odontoblasts. At higher magnifications, dentin reveals many little channels arranged radially, which are the spaces where the odontoblast cytoplasm extensions were located during the release of dentin material. Adittionally, there is an inner region where the odontoblasts start releasing non-mineralized organic material known as predentin.
Pulp is loose connective tissue highly supplied with blood vessels and extensively innervated by nerves. Pulp is confined in a tooth cavity delimited by dentin. Both blood vessels and nerves enter the tooth through openings located in the tips of tooth roots known as root-end openings. Pulp looks like embryonic mesenchyme with many star-like fibroblasts and abundant ground substance.