Enamel is the hard, mineralized tissue that covers the anatomic crowns of the teeth. Although the color of enamel generally appears to be white, enamel is relatively translucent and its appearance can be affected by the color of the underlying dentin or adjacent restorations. The formation, composition, and structure of mature enamel are unique. Unlike the other mineralized tissues of the body, bone, dentin, and cementum, which are formed by cells of mesenchymal origin, enamel is formed by epithelial cells. Whereas the formation of dentin and cementum continues after tooth eruption, enamel is completely formed before eruption. Mature enamel is acellular, and its formative cells are lost upon eruption of the tooth. This dictates that enamel repair (except in the case of initial caries lesions; see “Dental Caries” later in this chapter) or replacement, at least at present, must be achieved through the use of non-biological substances.

Enamel structure, composition, and properties

Enamel is the hardest substance in the vertebrate body. Enamel contains about 96% mineral, 2% organic material, and 2% water, by weight. The mineral component, as in all other vertebrate mineralized tissues, is hydroxyapatite, Ca₁₀(PO₄)₆(OH)₂. Like other mineralized tissues, some of the calcium, phosphate, and hydroxyl ions of the enamel hydroxyapatite crystals may be replaced by other ions. These include strontium, magnesium, carbonate, and fluoride. Technically, the mineral of enamel is a substituted hydroxyapatite.

Hydroxyapatite forms hexagonal crystals, thus enamel crystals have six sides when seen in cross-section (Fig. 4.1). Enamel crystals are much larger than the hydroxyapatite crystals in other tissues, whose dimensions typically are 5 to 10 nm in width, 1.5 to 3.5 nm in thickness, and 40 to 50 nm in length. Enamel crystals may be up to 70 nm in width and 30 nm in thickness, and are much longer than the crystals in bone, dentin, and cementum. Although their exact length has not been determined, some investigators believe that individual enamel crystals may extend from the dentinoenamel junction to the enamel surface. The difference in size of hydroxyapatite crystals found in enamel and those in other mineralized tissues most likely can be attributed to the properties of their matrix components.

The organic content of mature enamel consists mainly of residual enamel matrix proteins or fragments of these proteins, mostly coating the surfaces of the enamel crystals. These proteins are produced by ameloblasts, the cells that make the enamel, and they have critical functions in the formation and mineralization of the enamel. During the maturation of the enamel, most of the matrix proteins are degraded, and along with most of the water in the matrix removed to create space for the mineral. The surface layers of enamel also may contain some salivary proteins that are adsorbed to the enamel crystals or incorporated during the repair of initial caries lesions.

The enamel crystals are organized into enamel rods (or prisms) that extend from the dentinoenamel junction to the surface of the tooth (Fig. 4.2). Within the rods the crystals are oriented predominantly parallel to the rod. Each rod is about 4 to 5 μm in diameter, with a more or less circular or slightly oval cross-section. The rods may be up to 2.5 to 3 mm in length, depending on the tooth and the location on the tooth. Additional enamel crystals, located in the interrod enamel, surround each rod. The interrod enamel crystals are oriented at an angle to those of the rod (Fig. 4.3). Around the occlusal or incisal three-quarters of each rod, the differing orientation of the rod and interrod crystals creates a slight space. This space contains an increased amount of residual matrix proteins and is called the rod sheath (Fig. 4.4). Along the cervical one-fourth of each rod, the divergence of the rod and interrod crystals is more gradual, and the rod sheath is absent. In scanning electron micrographs (Fig. 4.5), or in sections cut perpendicular to the rods, the cervical interrod enamel appears to form a “tail” attached to the “body” of the rod just occlusal to it. The observation of this pattern gave rise to the initial interpretation that the enamel rods were shaped like keyholes. It is now generally accepted that the rod-interrod organization described above reflects the true structure of enamel.

In ground sections of enamel, individual enamel rods exhibit a pattern of cross-striations along their length, at regular intervals of about 4 μm (Fig. 4.6). These cross-striations are thought to represent the daily rhythm of enamel formation and mineralization. Because ground sections of enamel may be up to 100 μm thick, the orientation of the rods within the section is difficult to determine. If the rods are sectioned obliquely, the interrod enamel between adjacent rods may appear as cross-striations along a single rod. Also, when viewed in the scanning electron microscope, the rods show periodic variations in diameter, which may contribute to the appearance of cross-striations in ground sections.

Additional longer-term variations in enamel formation result in incremental lines, or striae of Retzius, representing the position of the forming enamel surface at that point in time (Fig. 4.7). The striae of Retzius occur at approximately 7- to 11-day intervals during enamel formation and are thought to arise from the coincidence of two separate biological rhythms. In an erupted tooth, the intersection of the striae of Retzius with the enamel surface results in shallow horizontal grooves, perikymata, that encircle the crown of the tooth. Systemic disturbances that occur during tooth development, such as fevers, will cause the formation of additional incremental lines. The physiological changes that occur at birth also cause a prominent incremental line, the neonatal line (Fig. 4.8). Because enamel is not remodeled over its lifespan, the cross-striations and the striae of Retzius are permanent indicators of the incremental pattern of enamel growth.

The course of each rod from the dentinoenamel junction to the surface of the tooth is not straight, but shows a slight curvature. Additionally, each horizontal row of rods is oriented at a slight angle to the adjacent occlusal and cervical rows, creating an undulating pattern of rod orientations from the occlusal/incisal edge of the tooth to the cervical margin (Fig. 4.5). This pattern is evident in ground sections of mature enamel viewed in reflected light, where adjacent bright and dark bands, the Hunter-Schreger bands, can be seen extending from the dentinoenamel junction to the enamel surface (Fig. 4.9). This varying orientation of the rods and matrix also is visible in sections of immature enamel stained with hematoxylin and eosin. In regions of sharp surface curvature, such as cusp tips and incisal edges, the curved path of the rods is accentuated; in ground sections this results in the appearance of gnarled enamel (Fig. 4.10).

The dentinoenamel junction is the interface between dentin and enamel. It is established when the matrices of these two tissues are first deposited and mineralization begins. This is not a smooth interface; in sections it presents a scalloped profile (Fig. 4.11). The irregular profile of the dentinoenamel junction is thought to increase adherence of the enamel to the dentin, especially in areas of occlusal stress. The region of enamel within a few hundred micrometers of the dentinoenamel junction has a higher content of organic matter, mainly protein, and a lesser hardness than enamel further from the dentinoenamel junction. The different composition of this region results in slightly different physical properties, which are thought to be important in arresting the progression of cracks through enamel and into dentin.

Two structural features of enamel associated with the dentinoenamel junction are frequently observed in ground sections of enamel. Enamel spindles are short extensions of dentinal tubules across the dentinoenamel junction into the enamel (Fig. 4.12). During early dentin formation, prior to the deposition of enamel matrix, odontoblasts may extend their distal process across the future dentinoenamel junction, between adjacent presecretory ameloblasts. When the ameloblasts begin to deposit and partially mineralize the enamel matrix, the odontoblast process is surrounded by matrix and becomes trapped, resulting in an unmineralized space, the enamel spindle. Enamel tufts are bush-like structures that extend from the dentinoenamel junction for a short distance into the innermost enamel (Fig. 4.13). They are hypomineralized regions containing increased residual enamel matrix proteins, thought to be due to changes in direction of adjacent enamel rods originating from different areas of the scalloped dentinoenamel junction.

Another structural feature of enamel, enamel lamellae, are longitudinal hypomineralized defects that originate at the enamel surface and extend part way or in some cases all the way to the dentinoenamel junction (Fig. 4.14). The lamellae contain an increased amount of organic material, either remnants of the enamel organ or connective tissue that has penetrated into the defect. No clinical significance is presently attributed to either tufts or lamellae.

Enamel formation

The process of enamel formation is called amelogenesis. The formation and mineralization of enamel occurs in stages, beginning at the cusp tips or incisal edges and progressing along the sides of the crown toward the cervical margin (Fig. 4.15). The cells that form the enamel are ameloblasts, which differentiate from the inner enamel epithelium of the enamel organ, as described in Chapter 3. The stages of amelogenesis (Fig. 4.16) are:

• presecretory, when the cells prepare to begin enamel matrix formation;
• secretory, during which the ameloblasts deposit and initially mineralize the full thickness of the enamel matrix;
• transition, a short stage in which the cells prepare to become maturation ameloblasts;
• maturation, during which the ameloblasts complete the removal of matrix proteins and the addition of mineral; and
• protective, during which the ameloblasts cover the enamel surface until the tooth erupts.

The function as well as the morphology of the ameloblasts changes as they progress through each stage.

Presecretory stage

During the presecretory stage, the inner enamel epithelial cells induce the peripheral cells of the dental papilla to differentiate into odontoblasts. Shortly after that, the inner enamel epithelial cells begin their differentiation to ameloblasts (Figs. 4.16, 4.17). The cells elongate, the nucleus relocates to what was the apical end of the cell (now called the proximal end), the Golgi complex migrates to the supranuclear cytoplasm, between the nucleus and the former basal end of the cell (now called the distal end), and the rough endoplasmic reticulum increases in amount and forms a cylindrical sleeve around the Golgi complex. The junctional complexes are maintained at the proximal end of the cell, and new junctional complexes are formed at the distal end. These cells are now called presecretory ameloblasts; they have not yet begun to secrete enamel matrix. The distal cell membrane of the presecretory ameloblasts has a relatively smooth outline, and the basal lamina between these cells and the early odontoblasts is still intact (Fig. 4.18a).

Secretory stage

Prior to the initial secretion of enamel proteins, the basal lamina disintegrates and disappears, and the distal ends of the presecretory ameloblasts develop small projections (Figs. 4.16, 4.18b). The cells increase their synthesis of enamel matrix proteins, package them in small secretory granules in the Golgi complex (Fig. 4.19), and release them by exocytosis from the distal ends of the cells, adjacent to the dentin. At this point the cells are early secretory ameloblasts (Fig. 4.16). The initial enamel matrix and forming crystals are closely integrated with the matrix and mineral of dentin. As more enamel matrix is deposited, the distal membrane of the ameloblast again becomes smooth. This early enamel adjacent to the dentin is structureless, i.e., the rod-interrod structure described earlier has not yet been established. As the ameloblasts deposit more enamel matrix and continue to move away from the dentinoenamel junction, an elongated process, Tomes’ process, is formed on the distal end of each cell. The cells are now fully differentiated secretory ameloblasts (Figs. 4.16, 4.20, 4.21). Release of matrix proteins from secretory granules at the tip of Tomes’ process creates the enamel rod, which is oriented at an approximately 45° angle to the ameloblast cell body (Fig. 4.22). The matrix proteins of the interrod enamel are released along the sides of Tomes’ processes, nearer the junctional complexes between adjacent cells. Thus, the rod is created by a single ameloblast, whereas the interrod enamel is created by that ameloblast and all of its immediate neighbors (Figs. 4.22, 4.23). As the cells retreat from the dentinoenamel junction and deposit additional enamel matrix, the interrod enamel is formed first, then the rod enamel is deposited, filling up the spaces formerly occupied by Tomes’ processes (Fig. 4.24). The secretion of the rod and interrod enamel at two different sites on the ameloblast is responsible for the differing />