2.7. Dentinogenesis

The odontoblasts lining the pulp chamber produce the dentine. The dentine is a highly permeable tissue, because densely packed dentinal tubules radiate from the pulp throughout all the layers. 15000 tubules/mm² are present in the outer dentine and 55000 tubules/mm² near the pulp. The dentine contains more minerals than the bone: 70% of weight consists of the minerals. Type I collagen is predominate in the organic and hydroxyapatite in the inorganic portion. (Linde & Goldberg 1993)

The dentine may be divided into intertubular dentine and peritubular dentine. The former is the main product of the odontoblasts constituting the largest volume of the dentine. The intertubular dentine consists of a fibrous network of collagen with deposited mineral crystals. The peritubular dentine forms a highly mineralized sheath around the dentinal tubule (0.5-1 micrometers thick in humans). The peritubular dentine gradually (partly or completely) fills up the dentinal tubules at some distance away from the pulp chamber. (Linde & Goldberg 1993)

The first stage of the dentinogenesis forms mantle dentine on the dentine-enamel junction during the early stages of the tooth development. In human, the mantle dentine is 5-30 micrometers thick (Linde & Goldberg 1993). It is rich in proteoglycans and more irregular and less mineralized than the following layers. (Jenkins 1978, Linde & Goldberg 1993).

The second stage, forming the next layer and consisting the most of the tooth structure, is called primary dentinogenesis. Some confusion exists in the literature concerning the ending time of this stage (Cox et al. 1992). The primary dentinogenesis is considered to be finished and secondary dentinogenesis started at different phases in different publications, such as when the crown is fully formed, when the tooth erupts (Cox et al. 1992), when the tooth becomes functional (Linde & Goldberg 1993) or when the root is fully formed (Torneck 1994). The latter seems most reasonable, because (in humans) the tooth metabolism becomes slower after the root apex is formed.

In rat molars, the dentine formation slows down gradually during both the primary and the secondary dentinogenesis with no apparent transition from the former to the latter (Johannessen 1961, Hietala & Larmas 1992, Kortelainen & Larmas 1994). The structure of the secondary dentine is supposed to be slightly more irregular than that of the primary dentine (Torneck 1994).

The next stage, tertiary dentinogenesis, occurs as a tooth response to irritations. Tooth preparation made by a dentist, dentinal caries, attrition, abrasion and/or erosion are the most common irritating factors (Cox et al. 1992). The tertiary dentine may also be named according to the quality of the irritation: The dentine formed as a response to attrition, abrasion or erosion is called "reactional dentine" to separate it from caries and preparation induced "reparative dentine". The tertiary dentinogenesis may be absent even in a fully matured tooth. The quality of the tertiary dentine seems to be dependent on the speed of its formation: the faster it is formed, the more irregular it appears (Linde & Goldberg 1993, Torneck 1994).

Predentine is the innermost layer of the dentine, right next to the odontoblasts and the pulp. It is a thin layer of unmineralized organic matrix, mostly collagen. This layer is present also in an old tooth, in which the dentinogenesis is slowed down. (Linde & Goldberg 1993)

During all the stages of the dentinogenesis, permanent layers of dentine are formed. Thus, a disturbance in any stage leaves persisting marks in the structure. This is different from the bone, in which constant turnover exists.

Because of the confusing terminology concerning the different stages of the dentinogenesis and the lack of apparent zones of transition from one stage to another, the names of the stages are partly ignored in this study. Considering the age of the rats during the experiments (3-10 weeks), however, the dentine formed was mostly primary.