Aclose relationship exists between the formation of enamel, dentin, and pulp. Enamel develops from the enamel organ, which is derived from ectoderm, whereas dentin and pulp develop from the dental papilla, which is derived from mesoderm. The mesoderm of the dental papilla determines the shape of the developing crown of the tooth. Animal experiments have shown that if you transplant the dental papilla from an anterior tooth beneath the enamel organ of a posterior tooth, the posterior tooth will assume the shape of the anterior tooth from which the dental papilla came. Although there is an interrelationship between enamel and dentin, it is the dental papilla that seems to have genetic control over tooth shape.
During the bud stage, the cells of the embryonic connective tissue deep to the bud resemble large multipointed cells called mesenchymal cells. As the enamel organ goes into the cap stage, the mesenchymal cells adjacent to the cap become more rounded and condensed and are then called dental papilla cells. The enamel organ is also enlarging at this time (Fig. 20-1). This condensation continues into the bell stage, during which further changes occur. It is at this point that the relationship between enamel and dentin formation becomes more obvious. Following is a list of events that occur during their formation:
1. During the bell stage the inner enamel epithelial cells (IEE) become taller. They increase from 12 to 40 μm in length. These taller cells are then referred to as preameloblasts. As these cells enlarge, preparing to become secretory cells, their organelles are multiplying to prepare for this. We could expect to see more mitochondria and endoplasmic reticulum, and a more obvious Golgi apparatus.
4. The secretion of this dentin matrix causes the preameloblast to change its polarity (the nucleus moves from the center of the cell to the end nearest the stratum intermedium). This is attributable to the change in the route of nourishment of the preameloblasts. Until this time the nourishment had been going from the dental papilla directly to the adjacent cells. However, after the dentin matrix is laid down, it acts as a barrier between the dental papilla and the preameloblasts, and it becomes necessary for the preameloblasts to find a new route for nourishment. Vascular channels begin to penetrate the enamel organ through the outer enamel epithelium. Fluid nutrients pass to the stellate reticulum, the stratum intermedium, and then the preameloblasts. The nuclear shift of a preameloblast cell relates to the nucleus’s need to be closer to the nutrient supply. With the change in polarity the cell is called an ameloblast and is ready to begin the secretion of enamel matrix.
5. The ameloblast lays down a matrix of mucopolysaccharide and organic fiber next to the dentin matrix, and the future dentinoenamel junction (DEJ) is formed. As the ameloblast secretes the matrix, it moves away from the dentin toward the outer enamel epithelium (OEE) (Fig. 20-3).
This process is identical for all developing teeth. It is seen first in developing anterior teeth and later in posterior teeth. It tends to be seen first in the mandibular arch slightly before the maxillary arch. Within any single tooth this type of interrelationship is first seen at the tip of the cusp of a tooth and later spreads toward the cervical line. As you look at a developing tooth, you may see enamel and dentin formation at the tip of a cusp, and yet near the cervical line the odontoblasts have not yet differentiated, and the cells of the enamel organ may still be at the IEE stage.
This development takes place in each tooth, whether primary or permanent. The permanent molars develop as a budding off of a posterior extension of the dental lamina, whereas the anterior permanent teeth and the permanent premolars develop from a budding off of the dental lamina of the primary teeth developing in that position. Regardless of the tooth, the steps of development are always the same.
Enamel is the hardest structure of the body. It is generally white but at times appears yellowish because of the reflection of the color of the underlying dentin. Enamel is about 96% inorganic in composition. This inorganic structure is composed of many millions of crystals of hydroxyapatite, the chemical formula of which is Ca10 (PO4)6 · (OH)2. The other 4% of enamel is composed of water and a fibrous organic material.
The enamel structure comprises two parts: the rod sheath and the enamel rod. The rod sheath outlines the rod and contains most of the fibrous organic substance (Fig. 20-4). However, the rod, which is made up of hydroxyapatite crystals, is the primary unit of enamel’s structure. The rod is a column of enamel that runs all the way from the dentinoenamel junction to the surface of the tooth. It is somewhat perpendicular to the dentinoenamel junction and to the surface of the crown (Fig. 20-5).
There is much debate about how the enamel rod develops. However, it is generally believed that at least three or four ameloblasts act together to form one enamel rod by laying down fibers and a matrix composed of a gluelike material called ground substance and depositing millions of hydroxyapatite crystals into the matrix.
The shape of enamel rods is also debatable. The enamel rods, which fit together tightly because of a cementing substance called interrod substance, are usually described as “keyhole shaped.” However, they may have different appearances in various areas of enamel or because of species differences. For example, enamel rods have also been described as being round to oblong.
A “typical” enamel rod with the keyhole shape (Fig. 20-6, A) has a wide upper end and a narrowed bottom end. The reason for the different appearances of the upper and lower parts of the keyhole is that the crystals in those parts are oriented in different directions. When the ameloblasts produce the matrix and then deposit the crystals into the matrix, the secreting ends of the ameloblasts bulge outward in the direction of the DEJ. This asymmetrical bulge is called the Tomes’ process of an ameloblast. Each ameloblast does not lie parallel to the long axis of the enamel rod but is canted at an angle. As the crystals are secreted, they emerge from the pointed part of the ameloblast in an arrangement perpendicular to the cell membrane. The result is that the crystals in the upper part of the keyhole are arranged in a pattern longitudinal to the long axis of the rod, while the crystals in the lower end are angled almost 60 degrees off of the long axis.