Lectures 1 & 2 Enamel Flashcards
Composition of Enamel?
96% mineral & 4% organic material (matrix) & water
Enamel is the hardest, most mineralized tissue in the body.
True
Enamel mineral?
Crystalline calcium hydroxyapatite
Enamel may also include variable amounts of other minerals?
Fluoride, strontium, lead
Hydroxyapatite is present in which other mineralized tissues?
Dentine, cementum, bone & calcified cartilage
Enamel crystals are larger than those in other mineralized tissues
True
Dark regions in ground sections means?
Less mineral
Light regions in ground sections means?
More mineral
Mineral component is laid down on the matrix ______
Extracellularly
Which cells produce the mineral component of the enamel?
Ameloblasts
Ameloblasts are lost during development so that enamel has no ability to regenerate unlike dentine, cementum & bone where viable producing cells are retained.
True
Describe the physical characteristics of enamel
- Translucent, yellowish to grayish white in colour
- Hard but brittle
- susceptible to enamel etching
- Permeable: allows absorption and permits passage of certain molecules
Variable thickness of enamel ranges from?
Maximum of 2.5 mm on the cusps to a knife edge at the cementoenamel junction
Function of dentine?
Maintains integrity of enamel during mastication, since dentine is more resilient.
Enamel will fracture easily if dentine is lost.
True
What is enamel etching?
Dissolved enamel crystals, which can be controlled in a clinical sense to prepare enamel surfaces for a variety of dental treatments.
Fluoride ion incorporation into hydroxyapatite renders it less susceptible to acid etching and caries.
True
Excess fluoride helps reduce further susceptibility to caries and acid etching
False, too much fluoride can damage ameloblasts and result in mottling (white patches in the enamel)
Enamel crystals of hydroxyapatite are highly organized to form enamel subunits called _______?
Enamel rods & Interrods
Describe the rod
Long cylinder about 4um in diameter (slightly wider closer to the surface of the tooth) and extends from close to the tooth surface to close to the dentinoenamel junction.
How are enamel crystals oriented?
They are oriented approximately parallel to the long axis of the enamel rod.
The regions between rods are called ______ regions and the crystals are _____ oriented (angled at about _________________).
Interrod, obliquely, 70 degrees to the long axis of the interrod.
What are interrods seen as on light micrographs?
Dark bands between light bands in the micrographs.
Interrods form first, rods slightly later.
True
What are rods surrounded by?
Thin line of more highly organic material called ROD SHEATHS
When do enamel with rod and interrod structure only forms when?
When ameloblasts have grown a single cytoplasmic process called a TOMES PROCESS.
Tomes processes are _______________________ so enamel adjacent to the dentinoenamel junction & at the enamel surface lacks a regular rod structure (crystals here simply oriented at right angles to the surface).
Absent at the start and at the end of enamel formation
Course of enamel rods?
From dentinoenamel junction to tooth surface. This course appears to be oblique in longitudinal ground sections.
What sort of course do enamel rods take?
They do not take a straight course but twist & turn in various ways
Describe incremental lines
Mineral is laid down in a rhythmic manner resulting in short and long term incremental lines which represent fairly regular variations in the amount of mineral
Rate of enamel formation?
4um per day
Each ameloblast has a diurnal rhythm in production of organic enamel matrix which in turn results in a diurnal variation in the amount of mineral laid down.
True
Diurnal rhythm in enamel production results in?
light bands, followed by a fine dark line, which represents resting phase of the ameloblast.
One light band plus one dark band represents a single day’s growth.
True, which becomes 4um
What are long term incremental lines/striae of retzius
These are a series of dark bands crossing the enamel obliquely
The dark bands in long term incremental lines are separated by light bands
True
One long term light band plus one long term dark band represents growth over about?
6-11 days
Short term lines are less regular than long term incremental lines
False, long term incremental lines are less regular than short term lines.
How are Striae of Retzius described as?
They are described as being closer together in regions of the enamel that form more slowly e.g. cervical enamel
The striae roughly follows the surface of the crown as it develops.
True
How do the Striae of Retzius run in the lateral enamel?
they run from DEJ to surface of tooth rather than following the contours of the entire crown as they do in the cusps of unworn teeth.
Define ‘perikymata’?
Striae that extend to the surface of the tooth form wavelike ridges
How do Striae of retzius/long term incremental lines appear in transverse sections?
Concentric parallel lines. Some striae are particularly pronounced and represent systemic disturbances such as illness that disrupt ameloblast function. The neonatal line is particularly prominent.
Striae of retzius are more obvious in permanent teeth.
True
Describe appearance of Hunter Schreger bands
Diffuse alternating light and dark bands in the inner four fith of enamel and oriented roughly at right angles to the surface.
How are hunter schreger bands formed?
they result from whole bundles of rods following a twisted rather than parallel course through the enamel.
Twisting in adjacent regions of enamel and hunter schreger bands are different
True
The rods forming Hunter Schreger bands are not fully mineralized, like the hypocalcified rods forming the much smaller enamel tufts.
False, they are fully mineralized, unlike the rods.
What is the dentinoenamel junction
This is a scalloped rather than straight junction with the concavities of the scallops facing the enamel. This is a stronger junction than a straight joint.
What are enamel spindles
Distinct dark structures that extend from the dentinoenamel junction into the enamel. They are the results of the ends of odontoblasts growing into the enamel and becoming trapped.
where are enamel spindles most common?
Variable in number, but most common at the cusps of the teeth.
Where do enamel tufts arise?
At the dentinoenamel junction and extend into approximately the inner one third of enamel.
What is the appearance of enamel tufts?
Tassel-like appearance and are the result of groups of hypocalcified rods that twist in a different direction to rods in the surrounding enamel. They are larger and more diffuse than enamel spindles but much smaller than hunter schreger bands.
Clinical significance of tufts?
None, and do not appear to be sites of increased vulnerability to caries or fracture.
Where to enamel lamellae extend from?
They extend from enamel surface inward toward the dentinoenamel junction. They may extend the full thickness of the enamel or only part of the way but do not extend into the dentine.
Enamel lamellae contain more organic material than adjacent enamel.
True
Cracks in the enamel arising may appear similar to true lamellae
True
Cracks in enamel are not restricted to enamel and extend everywhere
Sort of false, it may be restricted to enamel or extend into dentine.
How are enamel lamellae definitively recognized?
With electron microscope or with special staining techniques in the light microscope
Describe the cementoenamel junction
In about 60^ of the ground sections the cementum overlaps the enamel, in 30% cementum & enamel meet in a sharp line & in 10% a gap occurs exposing the root dentine
Describe gnarled enamel
Enamel over the cusps of the teeth may show a particularly complex twisting of rods (more random in pattern than the Hunter Schreger bands).
Function of gnarled enamel
Probably helps the enamel to resist shearing forces during mastication.
What are perikymata?
Wavy lines formed by Striae of Retzius at the surface of the tooth
What is the enamel cuticle?
Used to describe various structures, one being the remnant of the basement membrane derived from the enamel organ covering the crown of the tooth at eruption. Usually regarded as a temporary structure that is lost quickly during mastication.
When is the enamel/salivary pellicle formed?
Formed post eruption after the loss of the cuticle from salivary proteins & glycoproteins which adhere to the tooth surface.
What is the bacterial plaque?
Microorganisms embedded in a matrix of bacterial products & salivary components. Plaque may calcify.
Describe age changes in enamel
- Wears due to masticatory attrition
- Discolouration, a darkening of the underlying dentine which is seen through the translucent enamel especially if the enamel has thinned.
- Reduction in permeability possibly due to a slow increase in the size of crystals (with concurrent loss of water and matrix)
- Reduction in the incidence of caries
- Changes most pronounced at the enamel surface. In particular, there is an increase in the fluoride content (which can be enhanced by topical fluoride treatments).
Odontoblasts are restricted to?
To the outer surface of the pulp & only cytoplasmic processes (odontoblast processes
