Formation of dentin (dentinogenesis)

Dentin is the largest mass in the tooth and is formed and later supplied by the odontoblasts. In the bell stage of tooth germs, the odontoblasts differentiate from the cells of the dental papilla. These cells come from the embryonic connective tissue and the neural crest. The differentiation into odontoblasts can be seen from the increase in cell volume and the cell organelles contained therein. The initially star-shaped (stellate) cells become narrow and columnar, and a thick cytoplasmic process develops, which protrudes from the outer end of the cell. The odontoblasts thus formed lose their ability to divide and become highly specialized secretory cells.

The thick odontoblast process contains mitochondria, secretory granules, and long microtubules as its typical structural characteristics. It constantly lengthens and gives off short cytoplasmic side branches that can join with each other. Pro-collagen building blocks and mucopolysaccharides are synthesized in the odontoblasts, migrate through the odontoblast process, and are secreted as predentin at the stem of the process. The pro-collagen building blocks polymerize into collagen fibrils, while the mucopolysaccharides form an organic ground substance. Dentin formation therefore goes through an organic precursor phase, namely predentin, followed by mineralization of the dentin.

The odontoblasts differentiate at the interface with the internal enamel epithelium, where the dentinoenamel junction later develops. The differentiation of odontoblasts and the formation of predentin start first in the incisal edges and tips of the cusps and gradually move cervically as the dentin layer gets thicker coronally.The first, outer layer of dentin is known as mantle dentin and differs from the main mass of dentin in its structure and characteristics. Mantle dentin contains numerous collagen fibers that are distributed with irregular density. This is why mineralization of the mantle layer is not as great as in the rest of the dentin.

The mass of dentin between the mantle and the pulp cavity is known as circumpulpal dentin. Unlike mantle dentin, the main mass contains no von Korff fibers and has only minimal branching of odontoblast processes. The circumpulpal dentin develops after the mantle dentin as the odontoblasts increasingly withdraw to the dental papilla. Mineralization of the circumpulpal dentin, unlike that of enamel, only starts at some distance from the odontoblasts. It is not until there is a certain thickness (about 20 |jm) to the cell layer, due to the secretion of predentin, that the odontoblasts initiate the mineralization process by secreting granules with high levels of phosphates and calcium; ie, the odontoblasts (like the ameloblasts) actively organize the mineralization. Dentin is mineralized layer by layer, with a nonmineralized layer of predentin always being retained between odontoblasts and the mineralization front.

The formation of dentin does not take place continuously but periodically in mineralization and resting phases. As the thickness of the dentin layer increases, the pulpal cavity gets narrower, and the border between pulp and dentin becomes smaller (Figs 3-11 and 3-12). The odontoblasts crowd together increasingly closely, but they do not die; they retain their ability to produce new dentin throughout their lives.

However, space for the odontoblast processes and their branches is always left clear during dentin formation so that narrow dentinal tubules develop. The tubule walls are made of far more highly mineralized dentin; this is also produced throughout life, and therefore its thickness also increases.The odontoblast processes and branches become very narrow as well.

The odontoblasts lie on the inner surface of the dentin and form the boundary layer with the pulp. The odontoblast processes permeate the dentin as far as the enamel layer, which means that a process can be as long as 5 mm. Cross-linkages are formed with branches from neighboring processes. The quantity of odontoblast processes and branches accounts for 10 times as much mass as the volume of pulp tissue. The volume of the dentinal tubules and branches is 250 mm3 for the mandibular molars, while the volume of the pulp is only 70 mm3; in the canines, the ratio of pulp volume to dentinal tubules is 9:90 mm3.