Tooth enamel
Tooth enamel is one of the four major tissues that make up the tooth in humans and many animals, including some species of fish. It makes up the normally visible part of the tooth, covering the crown. The other major tissues are dentin, cementum, and dental pulp. It is a very hard, white to off-white, highly mineralised substance that acts as a barrier to protect the tooth but can become susceptible to degradation, especially by acids from food and drink. In rare circumstances enamel fails to form, leaving the underlying dentin exposed on the surface.
Features
Enamel is the hardest substance in the human body and contains the highest percentage of minerals, with water and organic material composing the rest. The primary mineral is hydroxyapatite, which is a crystalline calcium phosphate. Enamel is formed on the tooth while the tooth develops within the jaw bone before it erupts into the mouth. Once fully formed, enamel does not contain blood vessels or nerves, and is not made of cells. Remineralisation of teeth can repair damage to the tooth to a certain degree but damage beyond that cannot be repaired by the body. The maintenance and repair of human tooth enamel is one of the primary concerns of dentistry.In humans, enamel varies in thickness over the surface of the tooth, often thickest at the cusp, up to 2.5 mm, and thinnest at its border with the cementum at the cementoenamel junction.
The normal color of enamel varies from light yellow to grayish white. It has been suggested that the color is determined by differences in the translucency of enamel, yellowish teeth having a thin, translucent enamel through which the yellow color of the dentin is visible and grayish teeth having a more opaque enamel. The translucency may be attributable to variations in the degree of calcification and homogeneity of the enamel. At the edges of teeth where there is no dentin underlying the enamel, the color sometimes has a slightly blue or translucent off-white tone, easily observable on the upper incisors. Since enamel is semitranslucent, the color of dentin and any material underneath the enamel strongly affects the appearance of a tooth. The enamel on primary teeth has a more opaque crystalline form and thus appears whiter than on permanent teeth.
The large amount of mineral in enamel accounts not only for its strength but also for its brittleness. Tooth enamel ranks 5 on Mohs hardness scale and has a Young's modulus of 83 GPa. Dentin, less mineralized and less brittle, 3–4 in hardness, compensates for enamel and is necessary as a support. On radiographs, the differences in the mineralization of different portions of the tooth and surrounding periodontium can be noted; enamel appears lighter than dentin or pulp since it is denser than both and more radiopaque.
Enamel does not contain collagen, as found in other hard tissues such as dentin and bone, but it does contain two unique classes of proteins: amelogenins and enamelins. While the role of these proteins is not fully understood, it is believed that they aid in the development of enamel by serving as a framework for minerals to form on, among other functions. Once it is mature, enamel is almost totally without the softer organic matter. Enamel is avascular and has no nerve supply within it and is not renewed, however, it is not a static tissue as it can undergo mineralization changes.
Structure
The basic unit of enamel is called an enamel rod. Measuring 4–8 μm in diameter, an enamel rod, formally called an enamel prism, is a tightly packed mass of hydroxyapatite crystallites in an organized pattern. In cross section, it is best compared to a keyhole, with the top, or head, oriented toward the crown of the tooth, and the bottom, or tail, oriented toward the root of the tooth.The arrangement of the crystallites within each enamel rod is highly complex. Both ameloblasts and Tomes's processes affect the crystallites' pattern. Enamel crystallites in the head of the enamel rod are oriented parallel to the long axis of the rod. When found in the tail of the enamel rod, the crystallites' orientation diverges slightly from the long axis.
The arrangement of enamel rods is understood more clearly than their internal structure. Enamel rods are found in rows along the tooth, and within each row, the long axis of the enamel rod is generally perpendicular to the underlying dentin. In permanent teeth, the enamel rods near the cementoenamel junction tilt slightly toward the root of the tooth. Understanding enamel orientation is very important in restorative dentistry, because enamel unsupported by underlying dentin is prone to fracture.
The area around the enamel rod is known as interrod enamel. Interrod enamel has the same composition as enamel rod, however a histologic distinction is made between the two because crystallite orientation is different in each. The border where the crystallites of enamel rods and crystallites of interrod enamel meet is called the rod sheath.
Striae of Retzius are incremental lines that appear brown in a stained section of mature enamel. These lines are composed of bands or cross striations on the enamel rods that, when combined in longitudinal sections, seem to traverse the enamel rods. Formed from changes in diameter of Tomes' processes, these incremental lines demonstrate the growth of enamel, similar to the annual rings on a tree on transverse sections of enamel. The exact mechanism that produces these lines is still being debated. Some researchers hypothesize that the lines are a result of the diurnal, or 24-hour, metabolic rhythm of the ameloblasts producing the enamel matrix, which consists of an active secretory work period followed by an inactive rest period during tooth development. Thus, each band on the enamel rod demonstrates the work/rest pattern of the ameloblasts that generally occurs over a span of a week.
Perikymata which are associated with the Striae are shallow grooves noted clinically on the nonmasticatory surfaces of some teeth in the oral cavity. Perikymata are usually lost through tooth wear, except on the protected cervical regions of some teeth, especially the permanent maxillary central incisors, canines, and first premolars, and may be confused as dental calculus. Darker than the other incremental lines, the neonatal line is an incremental line that separates enamel formed before and after birth. The neonatal line marks the stress or trauma experienced by the ameloblasts during birth, again illustrating the sensitivity of the ameloblasts as they form enamel matrix. As one would expect, the neonatal line is found in all primary teeth and in the larger cusps of the permanent first molars. They contain irregular structures of enamel prisms with disordered crystallite arrangements basically formed by the abrupt bending of the prisms towards the root; usually, the prisms gradually bent back again to regain their previous orientation.
Gnarled enamel is found at the cusps of teeth. Its twisted appearance results from the orientation of enamel rods and the rows in which they lie.
Development
Enamel formation is part of the overall process of tooth development. Under a microscope, different cellular aggregations are identifiable within the tissues of a developing tooth, including structures known as the enamel organ, dental lamina, and dental papilla. The generally recognized stages of tooth development are the bud stage, cap stage, bell stage, and crown, or calcification, stage. Enamel formation is first seen in the crown stage.Amelogenesis, or enamel formation, occurs after the first establishment of dentin, via cells known as ameloblasts. Human enamel forms at a rate of around 4 μm per day, beginning at the future location of cusps, around the third or fourth month of pregnancy. As in all human processes, the creation of enamel is complex, but can generally be divided into two stages. The first stage, called the secretory stage, involves proteins and an organic matrix forming a partially mineralized enamel. The second stage, called the maturation stage, completes enamel mineralization.
In the secretory stage, ameloblasts are polarized columnar cells. In the rough endoplasmic reticulum of these cells, enamel proteins are released into the surrounding area and contribute to what is known as the enamel matrix, which is then partially mineralized by the enzyme alkaline phosphatase. When this first layer is formed, the ameloblasts move away from the dentin, allowing for the development of Tomes' processes at the apical pole of the cell. Enamel formation continues around the adjoining ameloblasts, resulting in a walled area, or pit, that houses a Tomes' process, and also around the end of each Tomes' process, resulting in a deposition of enamel matrix inside of each pit. The matrix within the pit will eventually become an enamel rod, and the walls will eventually become interrod enamel. The only distinguishing factor between the two is the orientation of the calcium phosphate crystallites.
In the maturation stage, the ameloblasts transport substances used in the formation of enamel. Histologically, the most notable aspect of this phase is that these cells become striated, or have a ruffled border. These signs demonstrate that the ameloblasts have changed their function from production, as in the secretory stage, to transportation. Proteins used for the final mineralization process compose most of the transported material. The noteworthy proteins involved are amelogenins, ameloblastins, enamelins, and tuftelins. How these proteins are secreted into the enamel structure is still unknown; other proteins, such as the Wnt signaling components BCL9 and Pygopus, have been implicated in this process. During this process, amelogenins and ameloblastins are removed after use, leaving enamelins and tuftelin in the enamel. By the end of this stage, the enamel has completed its mineralization.
At some point before the tooth erupts into the mouth, but after the maturation stage, the ameloblasts are broken down. Consequently, enamel, unlike many other tissues of the body, has no way to regenerate itself. After destruction of enamel from decay or injury, neither the body nor a dentist can restore the enamel tissue. Enamel can be affected further by non-pathologic processes.
Enamel is covered by various structures in relation to the development of tooth: