Dental histology

• Calcified tissues are secretory products of cells; formative cells secrete an organic matrix where hydroxyapatite crystals are deposited
• Enamel covers the dentine and forms the crown of the tooth
• Dentine makes up the bulk of the tooth and supports enamel
• On the surface of the root is a thin layer of cementum
• Alveolar bone lines the socket of the tooth and supports it
• When preparing ground sections of the tooth, the specimen is usually cut with a saw cutter and ground down until they are sections of 30-50um, preserves the enamel
• When preparing decalcified sections, the specimens are places in weak acid for several weeks so that the mineral components are dissolved; the decalcified specimens are cut to around 4um and stained with haematoxylin
• Enamel is a protective and resistant covering of variable thickness that covers the entire surface of the crown, rendering it suitable for mastication
• Enamel is usually quite thin at the cervical margin, thins down to a knife-down edge but can be up to 2.5mm on the occlusal/incisal edge
• Enamel is the hardest calcified tissue in the human body but it is relatively weak in terms of compressive strength, i.e. it is brittle and subject to fracture
• Enamel is semi-permeable
• Enamel has a grayish-white color but can appear yellowish-white because it is translucent and the color of the underlying dentine will show through
• Since the underlying dentine has a higher compressive strength, it supports the overlying enamel and protects it from fracture
• Enamel composition by weight: 88-90% hydroxyapatite crystals, 2% water, 1-2% organic materials
• Enamel composition by volume: 96% hydroxyapatite crystals, 5-10% water, 2% organic materials
• The organic matrix of enamel is mainly composed of amelogenins and some non-amelogenins
• During enamel formation, 90% of the organic matrix is composed of amelogenins but in the adult matrix it only makes up 10%; they are absorbed by ameloblasts during enamel maturation
• Hydroxyapatite crystals have a lattice appearance, constitutes of several ions, e.g. magnesium, calcium, hydroxyl groups
• When fluoride ions are introduced to enamel, it can replace hydroxyl groups to confer greater stability and resistance to acidic dissolution
• Enamel prisms/rods are the basic structural unit of enamel, has a general keyhole shape and runs from the DEJ to surface of the tooth
• The enamel rods located at the cusps are longer than those located at the cervical areas of the teeth (thicker enamel); diameter of the rods also increases from the DEJ towards the surface at a ratio of 1:2
• There are three prisms patterns in the human enamel: circular, prisms aligned in parallel rows, and arranged in staggered rows so the tail of a prism lies in between two heads in the next row (keyhole appearance, most prominent pattern)
• The enamel rods run in a sinusoidal/wavy direction from the DEJ to the surface of the tooth, so in longitudinal sections there are alternating regions with different crystal orientations and thus different degrees of polarisation. This forms Hunter-Schreger bands (alternating light/dark bands).
• Enamel rods are not always straight; in the region of the cusps and incisal edges, rods intertwine and form an irregular appearance → gnarled enamel
• Enamel is formed from the DEJ onwards towards the surface, while dentine is forced towards the pulp
• Ameloblasts are phasic and appositional; organic matrix is secreted first and then becomes calcified, and is secreted incrementally
• Ameloblasts show intervals of active formation of enamel (~24 hours) and rest periods in between; following the rest periods the enamel formed has a different mineral density/organic matrix, leading to incremental lines
• Enamel formation can only occur during tooth development and before tooth eruption starts
• Once enamel is formed, the ameloblasts become the reduced enamel epithelium, which covers the enamel before tooth eruption, and after eruption they become combined with the oral epithelium
• Enamel will usually get worn away with age, depending on diet and masticatory habits (e.g. grinding, acidic foods); can also get darker in color (due to its permeability and accumulation of stains)
• Interrod/interprismatic enamel is found in between enamel rods
• Incremental lines: appear because the enamel is formed in an incremental manner with changes in secretory rhythm, chemical position, and/or position of the developing enamel front
• Two types of incremental lines: cross-striations and enamel striae/striae of Retzius
• Cross-striations indicate a short period (24 hours), while enamel of striae represent more long term changes (usually weekly) and runs obliquely across the enamel; represents the successive positions of the enamel forming front
• In some teeth there is a neonatal line, which indicates the physiological changes that happen during birth and demarcates pre and post natal enamel; can only be seen in the deciduous teeth
• Enamel lamellae: cracks in the surface of the enamel that extend from the DEJ to the surface of the tooth
• Enamel spindles: structures at the DEJ where odontoblast processes have extended into the enamel and become trapped; they are spaces and thus appear black in ground sections
• Enamel tufts: developmental defect extending from the DEJ towards the tooth surface but doesn’t run throughout the thickness of the enamel (shorter than lamellae but longer than spindles); forms between groups of enamel rods and filled with organic material called enamelin
• DEJ is usually less mineralised to provide a certain cushioning; at certain areas it is also scalloped (curves towards dentine) to help transmit forces to the dentine in a controlled manner
• Scalloped DEJ usually appears under the cusps/incisal surfaces and is crucial to limit propagation of cracks through the tooth