Text I. Tooth structures formation

Dentinogenes is the name given to the origin and formation of dentin. Dentin is the 1^st mineralized tissue to appear in any developing tooth. It is composed of a fibrillar matrix, which mineralizes, and odontoblastic processes which remain unmineralized.

The 1^st dentin is formed at the incisal or cusp area of a tooth, and formation progresses in a rootward direction. Several cytoplasmic extensions of each odontoblast join to make a single dentinal process. The cells remain connected with their point of origin by the cytoplasmic extension, which are branched at their peripheral ends.

Dentin matrix mineralizes progressively. The newest nonmineralized dentin is called predentin. Dentin may be produced along the pulpal wall in a tooth of any age as long as the pulp is intact.

Amelogenesis is the name given to the process of enamel formation. The ameloblasts produce an organic matrix in which mineral salts later crystallize out of solution thereby making enamel a hard tissue. The formation of enamel is completed when submicroscopic crystals develop. Tooth enamel, being about 96% mineral substance, is the hardest of all body tissues.

As the root dentin starts to form, the already formed crown moves occlusally. If the tooth has a single root, the dentin wall encloses a single pulp canal. If the tooth is multirooted, the division of the pulp cavity into 2 pulp canals is apparent after the completion of the crown.

Root length is not complete until 1 or 4 years after the tooth emerges into the oral cavity. A newly emerged tooth has a short root and a large apical opening. As teeth become older, the root length is completed.

Text II. Three periods of cementum deposition

The thin layer of cementum which covers the whole surface of the root is an important tissue for the maintenance of periodontal health as it attaches the fibers of the periodontal ligament to the tooth.

Two types of cementum are recognized: cellular cementum, which contains cells called cementocytes, and acellular, which does not. The first formed cementum is usually of the acellular type and that formed later is cellular. Therefore, acellular cementum is found covering the cervical two-thirds of the root, and cellular cementum tends to cover the apical third and normally overlaps the acellular type.

Cementum is softer than dentine and is similar in many ways to bone. Just over 55% is a thin organic matrix of collagen fibres lying parallel to the root surface and mucopolysaccharide ground substance. As well as the thin fibres of the matrix, there are larger Sharpey’s fibres, which are embedded portions of periodontal fibres.

Cementum is not a static tissue, but alters in response to the functional requirements of the tooth. Throughout life there is a threefold increase in thickness of cementum as more is deposited on the root surface by the cementoblasts:

 Newly formed periodontal fibres can be embedded in the cementum replacing those which have aged.

 Alteration of the inserted fibres allows the tooth to move bodily through the alveolar bone. This occurs during eruption, growth and orthodontic treatment.

 The deposition of more cementum at the apex of the root helps to compensate for wear of the occlusal surface or incisal edge during the functional life of the tooth.