Editing Tooth enamel (section)

== Structure ==
[[File:Labeledandfulltooth.jpg|thumb|left]]
The basic unit of enamel is called an [[enamel rod]].<ref name="johnson" /> Measuring 4–8&nbsp;[[Micrometre|μm]] in diameter, an enamel rod, formally called an enamel prism, is a tightly packed mass of [[hydroxyapatite]] crystallites in an organized pattern.<ref name="ross485" /> 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 [[ameloblast]]s (the cells which initiate enamel formation) and [[Tomes' process]]es affect the crystallites' pattern. Enamel crystallites in the head of the enamel rod are oriented parallel to the long axis of the rod.<ref name="ross485" /><ref name="cate219" /> When found in the tail of the enamel rod, the crystallites' orientation diverges slightly (65 degrees) from the long axis.<ref name="ross485" />

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.<ref name="cate224">Ten Cate's Oral Histology, Nanci, Elsevier, 2013, pp. 122–128</ref> 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.<ref name="cate224" />
[[File:Enamel and dentine - ground section.jpg|thumb|right]]
The area around the enamel rod is known as [[interrod enamel]]. Interrod enamel has the same composition as enamel rod, however a [[histology|histologic]] distinction is made between the two because crystallite orientation is different in each.<ref name="cate219" /> The border where the crystallites of enamel rods and crystallites of interrod enamel meet is called the [[rod sheath]].<ref name="cate224" />

[[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.<ref name="cate224" /> 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.<ref>Fehrenbach, Popowics, p. 168</ref>

[[Perikymata]] which are associated with the Striae are shallow grooves noted clinically on the nonmasticatory surfaces of some teeth in the oral cavity.<ref name=":3" /> 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.<ref>Fehrenbach, Popowics, p. 168</ref> Darker than the other incremental lines, the [[neonatal line]] is an incremental line that separates enamel formed before and after birth.<ref>Ten Cate's Oral Histology, Nanci, Elsevier, 2013, p. 156</ref> 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.<ref>Fehrenbach, Popowics, p. 168</ref>

[[Gnarled enamel]] is found at the cusps of teeth.<ref name=":4" /> Its twisted appearance results from the orientation of enamel rods and the rows in which they lie.