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UKCD Histo Enamel

learning objectives for enamel

QuestionAnswer
Which material is harder-bone, enamel or dentin? enamel
Give the reason for enamel being the hardest substance. Enamel contains no collagen so the entire tissue (or 96%) of it is composed of HA crystals.
What is the structural unit of enamel? The enamel rod.
Enamel rods extend from? To where? Enamel rods extend from the DEJ to the tooth (enamel) surface.
An enamel rod is composed of what? Hydroxyapatite crystals.
How much of enamel is collagen? None.
What gives enamel rods their striated appearance? Enamel cross-striations, the diurnal markings of enamel deposition (at roughly 4 micrometers apart).
What is the morphological appearance of an enamel rod in X-section? Roughly key-hole shaped.
What three regions can be discerned in each X-section of enamel rod? Head, tail, and interrod space.
Is there a difference in the inorganic component of the head, tail and interred space of an enamel rod? The interrod sheath contains less HA and more organic material forming a region called the enamel rod sheath.
How are the hydroxyapatite crystals orientated? In the head the HA crystals are oriented in a longitudinal fashion corresponding to the long axis of the enamel rod. In the tail the HA crystals are oriented obliquely.
What is the difference between the enamel rod and the rod sheath? The enamel rod is the head, tail, and interrod space (where tail and rod head lie adjacent to each other). The rod sheath is the interface between the rod head and the overlying interrod enale. In is convex in shape and very thin (0.2 micrometers).
What is the composition and density of the rod and/or sheath? The enamel rod is densely packed HA crystals, the rod sheath has HA crystals but they are less densely packed and additional organic matrix is found in this region.
What is the morphological appearance of an hydroxyapatite crystal? A longitudinal prism.
What regions exist in a specific crystal? The exterior in: the hydration shell, the crystal surface and the crystal interior.
What ions may affect the hardness of the enamel? How? A variety of elements may substitute for the Ca or PO ions. Some enter the crystal, others lodge in the hydration shell. The fluoride ion (F) increases the stability and hardness of the HA crystal while selenium weakens it.
Which specific aspect of the ameloblast secretes enamelins and amelogenins? The secretory face of the Tome’s process - a shovel-shaped concave surface.
What does enamel maturation involved? Removal of the amelogenins allowing HA crystals of the enamelins to grow in thickness and width (but not in number or length).
Where might prismless enamel be located. At the DEJ as well as at the surface of the enamel.
What is prismless enamel? Enamel secreted while Tome’s process either forming (DEJ) or withdrawing from enamel surface (ameloblast enters maturation phase). doesn't have typical rod-shaped appearance due to loss of morphology. It “smoothes” the surface of the formed enamel layer.
Which aspect of the enamel rods exhibit a curvature? The initial 1/3 of the enamel rod - as it leaves the DEJ.
What are Hunter-Schreger bands? Groups of enamel rods curving in different directions as they leave the DEJ.
How do enamel cross-striations differ from incremental lines (of Retzius)? Cross striations are diurnal deposition markings (at roughly 4 micrometers spacing) while incremental lines are pauses in enamel deposition due to illness or other metabolic changes during enamel deposition.
State the corresponding features in dentin for enamel cross-striations, incremental lines and Hunter-Schreger bands. Cross striations of enamel = imbrication lines of von Ebner in dentrin. Incremental lines (of Retzius) in enamel = contour lines (of Owen) in dentin.
What is a neonatal line? A thicker incremental line (of Retzius) that demarcates the amount of enamel deposited at the time of birth. All primary dentition and first molars of the secondary teeth exhibit this line.
What are perikymata and what happens to them? Perikymata are lines on the surface of the enamel that demarcate the regular incremental lines (of Retzius) near the cervix of the tooth. The ridges are generally worn away with functional occlusion.
What is an enamel spindle? A defect in the enamel layer. It is trapped odontoblast process that crossed the DEJ to touch the pre-ameloblast.
What is an enamel spindle? areas where young enamel proteins were not completely transformed during maturation leaving slightly more matrix, hypomineralized parts of enamel rods & widened rod sheaths.
What is an enamel lamella? A defect (thin sheet of organic material) that crosses the entire thickness of the enamel layer.
What might cause the formation of alamella? Uncalcified enamel matrix and cellular debris from the enamel organ (for lamellae formed before eruption) and organic debris from the oral cavity filling a fracture that occurred after tooth eruption.
What are “weak spots” in the enamel layer of a tooth located? In the grooves and fissure of the occlusal surface of the crown.
Why are these weak spots present? Thinner enamal layers due to the strangulation of ameloblasts as they more away from the DEJ but crowd one another in these forming grooves and fissures.
What is gnarled enamel and where is it found? Highly twisted/irregular groups of enamel rods caused by wandering as groups of ameloblasts left a region of smaller surface area and migrated to a larger surface area - the cusp of the tooth.
What is the clinical implication of gnarled enamel? It makes restorations of the cusp difficult as you should not leave enamel unsupported by dentin or it fractures. This highly contorted enamel morphology increases this chance.
What is etching? Acid preferentially dissolving the rod head providing a roughened surface for adherence.
Where will etching result in an insignificant effort? In regions of prismless enamel (in 70% of all secondary teeth)
What is the difference between enamel spindles and enamel tufts? Enamel spindles have nothing to do with enamel as they are trapped odontoblast processes.
Created by: wiechartm