In a tooth the outermost layer of the crown is found by enamel. Beneath it lies the dentin, which forms the bulk of the tooth and surrounds the pulp. Enamel is 96% mineralized and its basic structural unit is enamel rod. Enamel rod in cross section have a keyhole pattern and in longitudinal section have a rod and inter rod pattern. Individual rods run from the dentino-enamal junction to the enamel surface. But its path is not straight, but tortuous and undulating. Moreover, these adjacent enamel rods have dissimilar local orientations, even if the general direction is the same. That is, enamel rods run from the dentino-enamel junction to the enamel surface and have a tortuous course, but that tortuous course is not same for individual enamel rods. Now enamel rods having this tortuous course and not a straight course is thought to be a functional adaptation against masticated forces, to minimize the risk of cleavage and cracks. This complex orientation of enamel rods refracts the path of light differently and thus presents with different microscopic structures. Moreover, in enamel, there are few hypocalcified areas which also refract light and form microscopic structures. So there are various microscopic structures in the enamel. Let us start with Hunter Schreger bands. Hunter-Schreger bands are alternating bands of light and dark areas of varying width. They originate from the dentino-enamel junction and end at some distance from the outer surface. The dark zones are called diazones and light zones are called parazones. They are formed due to the tortuous course of enamel rods. In places where enamel rods get cut longitudinally, appear dark and in places where they get cut transversely appear light. Hunter Schreger bands can be easily visualized under oblique reflected light. Gnarled enamel. Gnarled enamel is an optical appearance of enamel seen in the cusp or incisal ridge area. It originates from the dentino-enamel junction and runs towards the enamel surface. They too appear due to the tortuous path of enamel rods. However, in cuspal areas, as the masticatory forces are maximum, so the enamel rods show more pronounced undulations which again is a functional adaptation. Here, rods are almost intertwined with each other. Incremental lines. Enamel is formed incrementally, with periods of activity alternating with periods of rest. This variation in deposition results in rhythmic structural markings, which are of two types cross striations formed at short periods and incremental lines or striae of Retzius formed at long periods. Cross striations. In the longitudinal section of enamel rods, Cross striations are dark and light bands running perpendicular to the long axis of individual enamel rods. They are about 4 microns wide. These striations represent a diurnal rhythm in enamel deposition. Incremental lines of Retzius. When enamel is viewed longitudinally, incremental lines of Retzius appear as brownish bands. In cervical half, they appear running obliquely from the dentino-enamel junction to the enamel surface in coronal direction. However, in cuspal areas they appear to surround the tip of the dentin. Here, few striae do not reach the enamel surface. When the enamel is viewed in a transversely cut section, these striae are seen forming concentric circles, just like the growth rings in the cross section of a tree. These striae illustrate the incremental pattern of enamel deposition. That is, successive oppositional pattern. These lines may reflect variations in structure and mineralization. Though the exact reason for their formation, is not known. They may be due to the periodic bending of enamel rods, variation in basic organic structure or physiologic calcification rhythms. The enamel deposition rate from dentino-enamal junction to the outer surface of enamel increases. That is why, we see an increase in the gaps between successive striae. However the enamel deposition rate from cusp area to the cervical line decreases, which results in the decrease in gaps between successive striae. The evenly spaced striae on average represent a weekly rhythm. That is, on an average the striae are about 25 to 30 microns apart. Which means between two striae of Retzius there are 7 to 10 cross striations. Incremental lines of retzius when reach the enamel surface produce elevations and depressions called perikymata. Presence of regular striae of retzius is considered normal. Though any variation in their intensity or their frequency represent an alteration in enamel deposition, which may be due to some metabolic disturbances. Neonatal line. In a section of enamel when striae of Retzius are visible neonatal line is just an accentuated straie of Retzius. That is, it represents a variation in the enamel deposition. In this case the alteration is due to the birth of child. A fetus, which gets its nutrition through the placenta, transitioning to a new born, which gets its nutrition from the mouth, brings about an abrupt change in the nutrition. This line divides the enamel developmentally into two parts, prenatal enamel, which is formed during the fetal stages of the development and postnatal enamel, which is formed after the child is born. The junction between these two is clearly depicted by the neonatal line. The prenatal enamel is usually better developed than postnatal enamel. In deciduous dentition, neonatal line is present in all teeth, but in permanent dentition neonatal line is present only in first molars, as development of all other teeth begins after birth. And so have no prenatal enamel. Enamel lamellae. Enamel lamellae are thin leaf-like structures extending from the enamel surface towards the dentino-enamel junction. Depending on their length they may end before reaching the dentino-enamel junction or may end at the dentino-enamal junction or may cross the dentino-enamel junction and end in the dentin. Enamel lamellae form at areas of tension and are hypo-calcified structures. These hypo-mineralized areas may act as pathway for bacteria and development of caries. Depending on their stage of formation they are of three types. Type-A. They are formed during the stages of tooth development. That is, after ameloblast differentiation and during deposition of enamel. And so, they are only restricted in enamel and never cross the dentino-enamel junction. Type-A lamellae are composed of poorly calcified enamel rods. Type-B. They are formed after enamel formation is complete, but before eruption of tooth into the oral cavity. The enamel lamellae now formed may extend up to the dentino-enamel junction or may cross it to reach dentin. Type-B lamellae consists of degenerated cells of enamel organ or cells of dental sac. Type-C. They are formed after the eruption of tooth in to the oral cavity. The enamel lamellae now formed may extend up to the dentino-enamal junction or may cross it to reach dentin. Type-C lamellae consists of organic matter originating from saliva. Enamel tuft. Enamel tuft are ribbon-like structures extending from the dentino-enamel junction to one-fifth to one-third distance to the enamel surface. They are so termed as collectively, they appear similar to a tuft of grass. They represent hypo-calcified enamel rod sections. The term tuft actually gives a wrong picture as these hypo-calcified structures are actually adjacent to each other, but when viewed in a thick section, from a different angle they overlap each other and appear to originate from a single point like a tuft of grass. Enamel spindle. Enamel spindle are individual straight vertical lines originating from the dentino-enamel junction, running perpendicularly for a short distance. They may be slightly thickened at their end. They are more common in cusp areas. Enamel spindles are formed during the tooth development. When amelogenesis and dentinogenesis are occurring simultaneously, as odontoblast deposit dentin, they move backwards and leave an extension of odontoblastic processes. If any odontoblastic process crosses the dentino-enamel junction, it gets incorporated into the developing enamel. Hence, enamel spindle is a hypocalcified area, which contains odontoblastic process. They are perpendicular to the dentino-enamel junction because odontoblastic processes are perpendicular to the dentino-enamel junction.