Enamel is a unique tissue because, unlike bone, which is formed from connective tissue, it is a mineralized material derived from epithelium. Enamel is more highly mineralized and harder than any other mineralized tissue in the body; it consists of 96 to 98% of calcium hydroxyapatite. The enamel that is exposed and visible above the gum line is called the clinical crown; the anatomic crown describes all of the tooth that is covered by enamel, some of which is below the gum line. Enamel varies in thickness over the crown and may be as thick as 2.5 mm on the cusps (biting and grinding surfaces) of some teeth. The enamel layer ends at the neck, or cervix, of the tooth at the cementoenamel junction (Fig. 16.7); the root of the tooth is then covered by cementum, a bonelike material.
Enamel is composed of enamel rods that span the entire thickness of the enamel layer.
The nonstoichiometric carbonated calcium hydroxyapatite enamel crystals that form the enamel are arranged as rods that measure 4 m wide and 8 m high. Each enamel rod spans the full thickness of the enamel layer from the dentin showing dentinal tubules interglobular spaces odontoblasts gingival sulcus epithelium of gingiva pulp chamber granular layer of Tomes fibers of periodontal membrane alveolar bone with marrow pulp canal cellular cementum apical foramen
FIGURE 16.7 • Diagram of a section of an incisor tooth and surrounding bony and mucosal structures. The three mineralized components of the tooth are dentin, enamel, and cementum. The central soft core of the tooth is the pulp. The periodontal ligament (membrane) contains bundles of collagenous fibers that bind the tooth to the surrounding alveolar bone. The clinical crown of the tooth is the portion that projects into the oral cavity. The anatomic crown is the entire portion of the tooth covered by enamel.
dentinoenamel junction to the enamel surface. When examined in cross section at higher magnification, the rods reveal a keyhole shape (Fig. 16.8); the ballooned part, or head, is oriented superiorly, and the tail is directed inferiorly toward the root of the tooth. The enamel crystals are primarily oriented parallel to the long axis of the rod within the head, and in the tail they are oriented more obliquely (Figs. 16.8 and 16.9). The limited spaces between the rods are also filled with enamel crystals. Striations observed on enamel rods (contour lines of Retzius) may represent evidence of rhythmic growth of the enamel in the developing tooth. A wider line of hypomineralization is observed in the enamel of the deciduous teeth. This line, called the neonatal line, marks the nutritional changes that take place between prenatal and postnatal life.
Although the enamel of an erupted tooth lacks cells and cell processes, it is not a static tissue. It is influenced by the secretion of the salivary glands, which are essential to its maintenance. The substances in saliva that affect teeth include digestive enzymes, secreted antibodies, and a variety of inor
FIGURE 16.8 • Diagram showing the basic organization and structure of enamel rods. The enamel rod is a thin structure extending from the dentinoenamel junction to the surface of the enamel. Where the enamel is thickest, at the tip of the crown, the rods are longest, measuring up to 2,000 m. On cross section, the rods reveal a keyhole shape. The upper ballooned part of the rod, called the head, is oriented superiorly, and the lower part of the rod, called the tail, is directed inferiorly. Within the head, most of the enamel crystals are oriented parallel to the long axis of each rod. Within the tail, the crystals are oriented more obliquely.
ganic (mineral) components.