Dental Enamel Flashcards
Provide a basis for understanding their importance in disease progression (dental caries).
Describe the composition and structure of dental enamel, at both the light microscope and ultrastructure level, in relation to the function of the tissue
Enamel is thickest _ and thinnest at the _
over cusps and incisal edges, cervical margin
Over the cusps of unworn permanent teeth it is about ? thick and on lateral surfaces it is up to ? thick
2.5 mm, 1.3 mm
It differs from all other calcified tissues in the body by
lacking cells
The hydroxyapatite comprises about _% of the tissue by weight, the remainder consisting of
95, organic matrix and water
Mature enamel contains only _% by weight of organic matrix (mainly protein but not collagen). It may contribute to
1–2%, some of the mechanical properties of enamel, such as fracture toughness.
Water constitutes about % by weight of enamel. The presence of water is related to the porosity of the tissue.
2, the porosity of the tissue.
Hydroxyapatite crystallites are long, ribbon-like, measuring _ in width and _ in thickness. It has been suggested that each crystal may stretch from the
70 nm, 30 nm, amelodentinal junction (ADJ) to the tooth surface. Length unknown
mature enamel crystals are no longer perfectly hexagonal but rather exhibit an irregular outline because
they press against each other during the final part of their growth (enamel maturation).
The hydroxyapatite crystallites in enamel contain impurities such as _ and _. These impurities make these mineral phases
carbonate and magnesium, more soluble in acid.
In early developing enamel, enamel proteins account for 25–30% of composition
25–30%
immature enamel comprises several proteins, of which are the most abundant.
amelogenin (90%), ameloblastin (5%) and enamelin
is one of the enamel proteins that persists in mature enamel;
Tuftelin
Enamel crystallites are grouped together as _ and some form the _
prisms (rods), interprismatic enamel (interrod enamel).
The enamel prism or rod consists of several million crystallites packed into a long thin units approximately _ in diameter and up to _ in length:
5μm, 2.5 mm
In longitudinal sections of the tooth the prisms are approximately parallel/perpendicular to the ADJ
perpendicular
‘keyhole’ shape.
‘head’ and ‘tail’ regions, the tail of one prism lying between the heads of the adjacent prisms.
Interprismatic enamel composition is similar, but has a different optical effect, because
the crystals here deviate by 60 degrees from those in the prism cores
The boundaries of the prisms reflect
the sudden changes in crystallite orientation that give an optical effect different from that of the prism cores.
The level of organic material and water is likely to be higher/lower at the prism boundary regions because of _.
higher, the larger pores produced by the abutment (support) of hydroxyapatite crystallites at this junction
Describe crystal orientation changes in prisms
In the head of the prism the crystals run parallel to the long axis of the prism. In the tail, the crystals gradually diverge from this to become angled 60 degrees to the long axis, no clear division between head and tail of the same prism.
The difference in orientation of crystallites and the prismatic structure is due to
the secretion of enamel matrix and mineral by ameloblasts Tomes processes which have two secretory surfaces.
each prism (head) is formed by _ but _ contribute to each interprismatic region(tail).
a single ameloblast, four ameloblasts
how many prismatic structures are there
3
circular pattern
clear distinction between prisms and interprismatic enamel.
Offset keyholes are designed to
break up lines of shear within the enamel and dissipate the forces applied to the tissue.
‘gnarled’ enamel.
groups of prisms show regular changes in direction. Helps enamel over cusps and incisal edges better resist occlusal masticatory forces.
ADJ marks the site of
initial deposition of enamel and dentine.
ADJ concave + convex structure. convexities are on the _ and the corresponding concavities are _
enamel surface, on the dentine surface can be seen following removal of enamel by demineralisation.
Tooth surfaces which _ have more/less scalloping of the ADJ beneath them compared to _ which have more/less scalloping
face high shearing masticatory forces, such as cusps and incisal edges, more, smoother on lateral surfaces, less
Enamel spindles are
dark, club-shaped structures which penetrate up to 25μm into the enamel
Enamel spindles arise at _, particularly _
the ADJ, around the cusps of teeth where most crowding of odontoblasts would have occurred
Enamel tufts
look like tufts of grass, appearance represents areas between prisms where enamel protein has collected, presumed to be residual organic enamel matrix/proliferate 1/3 into enamel, contain ‘tuftelin’ enamel protein, highest protein content of mature enamel
Enamel lamellae are_. They form _
fine cracks in the surface of enamel that may be visible to the naked eye, during enamel development due to incomplete maturation of groups of prisms, in which case they would contain enamel proteins. Spaces between groups of prisms are another example of lamellae and may be caused by stress cracks
Hunter-Schreger bands
sinusoidal path with groups of prisms above and below them following paths in different directions.
areas where the prisms are section transversely are called _ and areas where they are longitudinal are called _.
diazones, parazones
The outer 20–70 μm of newly erupted permanent teeth is _. Here the enamel crystallites are all aligned _
nonprismatic, at right angles to the surface and parallel to each other.
Why is surface enamel nonprismatic
more highly mineralised than the rest of the enamel because of the absence of prism boundaries, not subject to abrasion, are less porous and less susceptible to acid
Why are there striations in enamel
The incremental lines seen in histological sections of mature enamel are the result of the rhythmic recurrent deposition of the enamel.
2 types of striations
There are two types of incremental line: short period (cross-striations. circadian rhythm) and long period (enamel striae, also known as the striae of Retzius).
describe striae of Retzius.
prominent lines that run obliquely across the enamel prisms to the surface. In transverse sections they form concentric rings. represent the successive positions of the enamel-forming front
striation intervals
there are generally 7 to 9 cross-striations between adjacent striae in enamel. The striae are therefore formed at about weekly intervals.
It has been suggested that there may be a rhythm with a 27-hour cycle in addition to a diurnal daily rhythm. The two rhythms would coincide approximately every 7 days, producing a fault in the developing enamel.
perikymata grooves
wavelike features that are normally found in the surface enamel of teeth, related to striations
Neonatal line
This is a particularly marked line in enamel is formed at birth and reflects the metabolic changes at birth.
Prisms appear to change both direction and thickness at the time of this event.
enamel over time
becomes less permeable with age as its microporosity reduces, becomes thinner
Why does an early carious legion look like a white spot
The early caries lesion in enamel appears as a ‘white spot’ due to subsurface mineral loss with partial dissolution of crystals so that there is an enhanced spacing between crystals.
Acid etching
allows micro-mechanical retention for adhesive bonding to tooth tissue. increase surface area, phosphoric acid is used
Microporosity of enamel
Ions and small molecules from the oral fluids can enter interprismatic enamel regions. enamel and dentine change colour as a result of the passage of the bleaching agents through the tooth
