Enamel, an almost entirely inorganic tissue of ectodermal origin, forms the outer covering of the tooth crown. It is the hardest, most dense, and most highly calcified of the mammalian tissues (Robinson et al., 1986). It is composed of: 96-97% inorganic material (hydroxyapatite), 0.4-0.9% organic matrix (amelogenin), and 2.1-3.6% water (Hillson, 1986).
Formed by millions of calcified prisms, the microscopic structure of enamel is highly characteristic. The prism is the structural unit of enamel, and can be defined as a rod formed by crystallites which are covered occlusally by prism sheaths.
Prisms are very regular structures, tightly packed and separated with interprismatic enamel. They radiate outwards from the dentino-enamel junction to the external surface of the tooth, in the direction of growth . Ranging between a few micrometers and 3 to 4 mm at the occlusal plane, prism length largely depends on the position assumed by each structural unit within the tooth and on the enamel thickness at that specific point. Conversely, prism diameter is relatively constant (about 4 µm).
In a human tooth there are about 12 million prisms and an equivalent number of ameloblasts, but with no correspondence between the formation of one ameloblast and one prism (Goodman and Rose, 1990). In fact, while the rounded head of an enamel prism is produced by one ameloblast, its tail is produced through the contribution of up to three adjacent ameloblasts.
The ameloblasts are hexagonal in cross section, whereas most enamel prisms are keyhole-shaped. The specific shape of an enamel prism and the peculiar relationship between prisms and ameloblasts are a result of the unique shape of the Tome's process of a human ameloblast. The Tome's process is shovel-shaped, with a wide, hexagonal base and a thin, horseshoe-shaped, knife-edge tip. Hydroxyapatite crystallites are deposited during enamel matrix formation with their long axes perpendicular to the cell membrane of the Tome's process (Moss-Salentijn and Hendricks-Klyvert, 1985).
During the very early phases of embryo formation in intrauterine life, the cells of the inner enamel epithelium of the tooth germ induce a layer of cells to mature into odontoblasts. At the same time, in the opposite direction, the ameloblasts start to form from the inner enamel epithelium. Odontoblasts produce dentine under the cusp tips, moving inwards along the dentino-enamel junction. In contrast, ameloblasts produce enamel moving outwards from the dentino-enamel junction, and secrete enamel until the full thickness is attained at that point . They then become reduced in size and die (Aiello and Dean, 1990).
Amelogenesis takes place in two stages: matrix production and maturation.
The first step (matrix production) is the secretion of an organic matrix as an extracellular product and the seeding of the crystallites into it. This takes place at the dentino-enamel junction of cusp tips at a characteristic time for each tooth type, and proceeds towards the neck of the tooth. The second phase (matrix maturation) is the process of removing the amelogenin and replacing it with more apatite in order to produce the heavily mineralised mature enamel (Skinner and Goodman, 1992).
Each ameloblast is responsible for a single segment of enamel. It produces matrix for the entire thickness at that point and brings about its maturation. Different areas of enamel begin formation at different times, with ameloblasts at the coronal tip of the crown starting first. The ameloblasts then continue the sequence progressively down the sides of the crown, so that the cells at the cervix start last. All cells are active for roughly the same length of time after they start matrix production (Hillson, 1986).
Amelogenesis is an appositional, or additive, process resulting from cellular secretion, which produces layers of new enamel deposited on the old ones (Massler et al., 1941). Enamel is deposited in successive increments that, at first, completely cover the previously deposited layer. Later, the increments are deposited only on the sides of the crown as overlapping sleeves that extend cervically. In horizontal sections, this appositional growth results in the formation of concentric layers, or rings, like those found in the trunk of trees, reflecting the rhythmicity of ameloblast activity. In longitudinal sections, these rings correspond to the so-called incremental lines of Retzius (Retzius, 1837; Rose, 1973, 1977) and appear brownish in ground sections.
These growth lines run in an oblique direction from the dentino-enamel junction towards the occlusal surface. Each line represents the cross-cut profile of a surface curved in three-dimensional space, thus reflecting the condition of the enamel formation at a particular point during amelogenesis (Schroeder, 1991). The physiological interruptions of growth, related to changes in prism bending, are necessary to give the tooth its functional structure (Gustafson, 1959; Wilson and Shroff, 1970). If the prisms do not curve, there are no Retzius lines.
In deciduous teeth, striae of Retzius are more difficult to observe than in permanent teeth. Enamel formed prenatally rarely contains many striae and those present are usually faint (Hillier and Craig, 1992).
Incremental lines showing a repeating pattern throughout the entire enamel mantle are regularly spaced by 20-80 m m, thus becoming progressively fewer near the neck of the tooth. Usually, striae of Retzius are best seen in the outer one-third of the enamel. They vary greatly in prominence; some are very thin, while others form broad bands up to 10 m m thick. The series of ridges and grooves on the buccal surface of enamel formed where Retzius lines reach the surface of the tooth are named perikymata.
Another rhytmicity is evident in the enamel microstructure, as each prism is segmented by cross striations, undulations along the length of the enamel. Two adjacent cross striations are situated at intervals of 4 m m, and this regular pattern is considered to represent a daily rhythm of growth, or circadian markings, along the length of the prisms (Levine et al., 1979; Hillson, 1986; Aiello and Dean, 1990).
The perikymata, which are visible in histological thin sections separated by 7-8 cross striations, have been used to determine crown formation times in modern humans and fossil hominids (see Bromage and Dean, 1985; Beynon and Wood, 1987; Dean, 1987a, b; Dean and Beynon, 1991; Mann et al., 1991; Beynon, 1992; but see Schroeder, 1991; Skinner and Goodman, 1992; Wolpoff, 1996-97 for criticism).
