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 (circadian), 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,