2
Histology and Embryology of the Teeth and Periodontium
2.1 Tooth Histology
2.1.1 Enamel
Enamel is the strongest structure in both humans and animals, and consists of minerals (96%) and water (4%). Microscopic views reveal that it consists of “enamel rods” that are formed of bundles of hydroxyapatite crystals. The area of the crystals adjacent to other crystals is known as the “rod sheath.” In longitudinal sections of enamel, the crystals closer to the enamel surface are more divergent from the long axis of the rod, and get close to 90°. The external border of the two orientations appears in the shape of a key hole.
2.1.1.1 Striae of Retzius
These are a series of lines representing the stages of growth in cross‐sectional dimensions, and appearing as dark bundles. These lines are representative of the intermittent production of enamel while the dentine is being formed. They are more common in permanent teeth than in primary, and are least common in natal and neonatal teeth. Incremental lines also appear as the result of disturbances in the development cycle – for example, owing to fever, which directly affects amelogenesis.
2.1.1.2 Hunter‐Schreger Bands
These are a lighting phenomenon formed by changes in the direction of enamel rods. These bands are best seen following light reflection on longitudinal dried sections.
2.1.1.3 Gnarled Enamel (Spiral Enamel)
These consist of wavy rods and are mostly seen at the cusp areas.
2.1.1.4 Enamel Tufts and Lamella
These originate from the dentine enamel junction and extend a short way into the enamel. These highly mineralized structures are divided into branches at their enamel ends, and they contain higher protein levels as compared to other parts of the enamel. Lamellae and the proteins between the tufts are formed from the tufts.
2.1.1.5 Enamel Surface
Striae of Retzius lines are continued to the external surface, ending in deep surface fissures known as “perikymata.” The surface of an unerupted tooth is covered with layers of 0.5–1.5 μm cuticles without a specific structure. Small, poorly bonded crystalline pieces are formed immediately beneath the cuticles. The surface layer and cuticles will vanish due to attrition and abrasion as soon as the tooth erupts. Figure 2.1 a and b shows the ground and demineralized sections of a sound maxillary canine tooth as a representation of enamel, dentin, and pulpal space.
Figure 2.1 (a) Ground section of the crown of the maxillary canine enamel; (b) demineralized section of the same tooth, where almost all the enamel tissue has disappeared leaving the remaining collagenous structure.
2.1.2 Dentine
The bulk of the tooth structure is known as dentine. It consists of more water (10%) and collagen (20%) as compared to enamel, and the level of mineral drops to almost 70%. The orientation of the dentine components is different in coronal and radicular dentine (Figure 2.2a and b). Both areas contain tubular odontoblastic processes that extend from the pulp into the surrounding dentine structure providing sensitivity, thereby protecting against stimuli and hazards. The odontoblasts located at the pulpo‐dentinal junction are the source of dentine secretion and production.
Stay updated, free dental videos. Join our Telegram channel
VIDEdental - Online dental courses
