Structure

The basic unit of enamel is called an enamel rod. Measuring 4–8 μ m in diameter an enamel rod, formerly called an enamel prism, is a tightly packed mass of hydroxyapatite crystals 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 crystals within each enamel rod is highly complex. Both ameloblasts (the cells which initiate enamel formation) and Tomes' processes affect the crystals' pattern. Enamel crystals 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 crystals' 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 (CEJ) 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 crystal orientation is different in each. The border where the crystals of enamel rods and crystals of interrod enamel meet is called the rod sheath.

Striae of Retzius are stripes that appear on enamel when viewed microscopically in cross section. Formed from changes in diameter of Tomes’ processes, these stripes demonstrate the growth of enamel, similar to the annual rings on a tree. Perikymata are shallow furrows where the striae of Retzius end. Darker than the other stripes, the neonatal line is a stripe that separates enamel formed before and after birth.

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.