Lecture 2- Enamel (Part 1).pdf
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Enamel (Part 1) Enamel structure, histology, function By the end of this lecture, you should be able to: Describe the chemical and structural composition of enamel Identify the major hierarchical structures that make up dental enamel Recognise and define major enamel features/incremental lines Ident...
Enamel (Part 1) Enamel structure, histology, function By the end of this lecture, you should be able to: Describe the chemical and structural composition of enamel Identify the major hierarchical structures that make up dental enamel Recognise and define major enamel features/incremental lines Identify major features of enamel along the EDJ Enamel- the basics Why is it called “enamel”? Enamel was named by Charles Allen in the first English dentistry textbook (1685-87) The Operator for the Teeth, later renamed Curious observations in that difficult part of chirurgery to the teeth (1687) Detective work by Michael J. Trenouth (2014) Enamel- the basics “The exposed part of the tooth consists also of two different parts. To wit, its stony cover or case, and its inward substance; the first is as it were a hard periosteum, that invests the head of the tooth on all sides, lying on it much after the same manner that Enamel does upon Gold, or any other thing…” “This natural Enamel which I call the gloss of the tooth is of a far harder, whiter, more dense and lucid nature than the inward substance lying under it” -Charles Allen, 1685 Enamel- the basics Enamel is the hardest tissue in our bodies. Mature enamel is acellular and avascular. Enamel is extremely mineralised: Mature enamel consists of 96% inorganic material (hydroxyapatite), 4% organic material and water. Enamel is harder than underlying dentine, creating some mechanical challenges (will discuss later) In radiographs, enamel is more radiopaque (lighter) than dentine and bone due to its high mineral content. Ten Cate’s Oral Histology (8th Edition); Fig. 8-6 Enamel- the basics Enamel’s primary function is to mitigate tooth wear and protect the underlying dentine. https://commons.wikimedia.org/wiki/File:Teeth_by_David_Shankbone.jpg Enamel Healthy enamel is translucent, grey-white to light yellow in colour. 2.5mm thick over cusps of permanent teeth (1.3mm in deciduous teeth), tapers to a thin layer along the cervical edge. Dentine Oral Anatomy, Histology and Embryology (5th Edition); Fig. 7.2; 7.3 Enamel- the basics Enamel Enamel gap Due to its high mineral content, need ground thin sections to study its structure. Demineralised sections won’t work! Why? Think about mature enamel’s mineral content. Ground section Deminerlised (paraffin) section Enamel- the basics Mature enamel is hierarchically organised (multiple levels of crystal groupings). Hydroxyapatite crystallites Prism/interprism Prism patterns Scanning Electron Microscope (SEM) image Enamel- crystallites Basic building block: hydroxyapatite (HA) crystallites Ca10(PO4)6(OH)2 Can include Mg, F, CO32- Sometimes called hydroxyapatite nanorods (because of their elongate structure) à unsure how long these are, but could be as long as the whole tissue! Oral Anatomy, Histology and Embryology (5th Edition); Fig. 7.5; 7.6; 7.8 Enamel Dentine Enamel- prisms HA crystallites are grouped together into enamel prisms and interprismatic enamel. Prisms (sometimes called “rods”): clusters of millions of HA crystallites coalesced into a ~5-7μm wide “thread”, spanning the entire thickness of enamel. Prisms are separated from surrounding interprismatic enamel by the prism sheath Interprismatic enamel Prism Prism sheath Ten Cate’s Oral Histology (8th Edition); Fig. 7-1 Enamel- prisms HA crystallites are grouped together into enamel prisms and interprismatic enamel. Prisms (sometimes called “rods”) are clusters of millions of HA crystallites coalesced into a ~5-7μm wide “thread”, spanning the entire thickness of enamel. Interprismatic enamel Prisms are separated from surrounding interprismatic enamel by the prism sheath Prism Prism sheath Oral Anatomy, Histology and Embryology (5th Edition); Fig. 7.13 Enamel- prisms Prisms are large enough to see under a light microscope, especially using crosspolarised light. The directions/orientations of the enamel prisms will appear as darker or brighter regions under cross-polarised light. Prisms Dentine Enamel Enamel- prisms The growth of each prism is regulated by a single ameloblast (enamelproducing cell). More on that in the next enamel lecture. Ten Cate’s Oral Histology (8th Edition); Fig. 7-17 Oral Anatomy, Histology and Embryology (5th Edition); Fig. 7.26 Enamel- aprismatic layers Enamel prisms in relatively unworn teeth do not extend all the way to the surface of the enamel. The outer 20–100μm of deciduous enamel and outer 20–70μm of permanent enamel is aprismatic and more highly mineralised. HA crystallites here are all oriented perpendicular to the outer enamel surface. Enamel- prism patterns Prisms are not all oriented in the same direction through the enamel. In the outer third of enamel, the prisms are roughly parallel to each other and are at high angles relative to the outer enamel surface. This is called radial enamel. Enamel- prism patterns In the inner two thirds of the enamel, the prisms are arranged into groups (10-13 layers of prisms) that extend at different angles to neighbouring groups, and even cross each other. The crossing of neighbouring groups of prisms is called prism decussation. These bundles of differently oriented prisms occur in bands called Hunter-Schreger bands. Within the Hunter-Schreger bands, prisms that were cut longitudinally in section form parazones (dark bands), prisms cut transversely are called diazones (light bands). Pa ra zo n e Di az on e Enamel- prism patterns In the inner two thirds of the enamel, the prisms are arranged into groups (10-13 layers of prisms) that extend at different angles to neighbouring groups, and even cross each other. The crossing of neighbouring groups of prisms is called prism decussation. These bundles of differently oriented prisms occur in bands called Hunter-Schreger bands. Within the Hunter-Schreger bands, prisms that were cut longitudinally in section form parazones (dark bands), prisms cut transversely are called diazones (light bands). Hunter-Schreger bands Enamel- prism patterns Extreme example of prism decussation: rodent enamel! Enamel- prism patterns Why do we have Hunter-Schreger bands and radially oriented prisms within our enamel? Radial enamel A mix of resistance to mechanical failure and the deflection of cracks. Radially oriented prisms maximise the strength of each prism relative to occlusal load. Hunter-Schreger bands mitigate and deflect the paths of expanding cracks. Hunter-Schreger banded enamel Enamel- other structures Mature enamel contains several other distinctive features, the development of which can be clinically and even forensically important. These are: Enamel cross-striations Striae of Retzius Perikymata Neonatal line* Enamel-Dentine Junction Enamel tufts Enamel lamellae Enamel spindles Incremental lines Enamel cross-striations Incremental variations in prism thickness that occur from daily variations in enamel production. Enamel Dentine Give the prisms a ladder-like appearance under the light microscope. Under an electron microscope, these are the result of changes in the widths of the prisms. Oral Anatomy, Histology and Embryology (5th Edition); Fig. 7.28; 7.30 Striae of Retzius Sometimes called “Retzius lines”, these mark the successive positions of the layer of ameloblasts as they form enamel. Striae result from variations in prism structure that affect the whole crown during enamel formation. There are 6–12 (usually 7–9) enamel cross-striations between each stria of Retzius, suggesting that there is an almost weekly variation in enamel formation. Causes: internal physiological rhythm? Recruitment of new cohorts of ameloblasts as the developing crown enlarges? Oral Anatomy, Histology and Embryology (5th Edition); Fig. 7.34 Striae of Retzius In longitudinal sections, the striae of Retzius extend obliquely around the enamel. Enamel Because they are so widely spaced, easy to see them under a light microscope, ground thin section. The lines are interrupted where they meet the outer enamel surface and the enameldentine junction Ten Cate’s Oral Histology (8th Edition); Fig. 7-51 Dentine Ten Cate’s Oral Histology (8th Edition); Fig. 7-63 Striae of Retzius In transverse sections, the striae of Retzius are concentric rings within the enamel. Why? This is because of the threedimensionality of these incremental lines à think of the 2 ways to cut an onion… Perikymata (pl.); Perikyma (s.) A groove along the surface of the enamel where a single stria of Retzius has reached the surface. Perikymata appear as horizontal lines along the surface of a newly erupted tooth crown (these wear away over time). Ten Cate’s Oral Histology (8th Edition); Fig. 7-63 Oral Anatomy, Histology and Embryology (5th Edition); Fig. 7.37 Neonatal line A particular type of stria of Retzius that corresponds to the physiological stress of being born. Results from a subtle change in thickness and direction of enamel prisms that occurs at birth. Is found within the enamel (and dentine) of all teeth that were at the appropriate stage of development when a person is born. These include: all deciduous teeth and the permanent first molars. Neonatal line A particular type of stria of Retzius that corresponds to the physiological stress of being born. Results from a subtle change in thickness and direction of enamel prisms that occurs at birth. Is found within the enamel (and dentine) of all teeth that were at the appropriate stage of development when a person is born. These include: all deciduous teeth and the permanent first molars. Neonatal line Forensically important: as a marker of a date of birth, it can be used determine age (in days) when an infant passed away, or to determine if a body belongs to a still-born infant. Dr. Tanya M. Smith Can also be used in archaeological and anthropological studies of growth rates. Before birth After birth Enamel-Dentine Junction (EDJ) The interface between the enamel (a hard, brittle tissue) and dentine (a softer, more elastic tissue). The EDJ is usually not a straight boundary, but a scalloped one in thin section, with the convex surfaces of the scallops facing the dentine. This increases surface area for attachment of the enamel to the dentine Enamel Dentine Enamel spindles Enamel spindle Along the EDJ, there are occasions where the dentine tubules appear to cross into the enamel a short distance. These extensions from the dentine into the enamel are enamel spindles. Enamel How do dentine tubules end up crossing into the enamel? Remember: dentine forms first, followed by enamel. Odontoblast processes become “stuck” in the developing enamel, and may even be pulled some distance through the enamel as the ameloblasts begin to pile up enamel matrix (more on this next lecture). Dentine Enamel spindles Along the EDJ, there are occasions where the dentine tubules appear to cross into the enamel a short distance. These extensions from the dentine into the enamel are enamel spindles. Enamel Enamel spindle How do dentine tubules end up crossing into the enamel? Dentine Remember: dentine forms first, followed be enamel. Odontoblast processes become “stuck” in the developing enamel, and may even be pulled some distance through the enamel as the ameloblasts begin to pile up enamel matrix (more on this next lecture). Enamel tufts Along the EDJ, there are short, branched spaces within the innermost layers of enamel. These are developmental features that may arise from the sharp divergences of neighbouring prisms along the scalloped EDJ These regions contain proportionately higher protein within them than the surrounding enamel. One of the proteins found inside is unique to this region of the enamel à tuftelin Tufts Enamel lamellae Unlike enamel tufts, lamellae are spaces in the enamel that start from the outer enamel surface and extend for different depths into the enamel. Unlike cracks, enamel lamellae contain organic material and are formed within the enamel prior to tooth eruption (they are not cracks!). Ten Cate’s Oral Histology (8th Edition); Fig. 7-52 By the end of this lecture, you should be able to: Describe the chemical and structural composition of enamel Identify the major hierarchical structures that make up dental enamel Recognise and define major enamel features/incremental lines Identify major features of enamel along the EDJ
