(2) Anatomy and Histology of the Periodotium - Enamel (B) Flashcards
What section has to be taken to view enamel?
ground section
completely lost in demineralised section
Can immature enamel be studied by a demineralised section?
yes
What are the basic structural units of enamel?
Prisms (rods) / enamel prisms
What comprises each enamel prism?
several million hydroxyapatite crystals packed into a long thin rod 5-6um in diameter and up to 2.5mm in length.
What separates each prism?
inter-rod substance
How do you differentiate enamel prisms and inter-rod substances?
Crystals with different orientation (deviate by 40-60°)
Name structures A and B
A = interrod
B = rod
How do you identify a prism boundary?
a sudden change in crystallite orientation.
How would you describe a prism in a cross-section?
keyhole pattern (pattern III)
Where does the tail of a prism lie? (cross section)
The tail of one prism lies between the heads of the two adjacent prisms
What is responsible for the optical appearance at a prism boundary?
An abrupt change of crystal orientation at the prism boundary
Name feature D
head of prism shown by cross-section
Do the crystal heads run parallel with one another?
Yes
along the long axis of the prism
What cut are each of these images?
cross sections
Does the keyhole pattern give strength to enamel?
YES
How to we see the difference between the head and tale sections?
the different orientation of crystals
In the tail, the crystals diverge gradually to become angled at 65-70° to the long axis.
How would you describe the prism head and tail crystal orientations?
The change from head to tail is gradual in each prism, however a tail of one prism shows a sudden divergence from the head of an adjacent prism.
the crystal orientation
What cut of the enamel can you see the boundaries of prisms easily?
the longitudinal
the run parallel running back
What happens every 10-13 layers of prism?
Every 10-13 layers of prisms follow the same direction, but blocks above and below follow paths in different directions.
= Hunter-Schreger bands.
What is shown here
Hunter- Schreger bands.
What is the banding pattern of enamel called?
Hunter- Schreger bands.
What is the approx. width of Hunter- Schreger bands?
50um
Are Hunter- Schreger bands visible?
visible due to light reflection in different directions.
Why cant you usually see visible due to light reflection in different directions in enamel?
In the outer ¼ of enamel, all prisms run in the same direction and so there is no banding.
How do prisms appear over the cusps of enamel?
appear twisted around each other in a complex arrangement known as gnarled enamel
What is shown here?
Prisms over the cusps appear twisted around each other in a complex arrangement known as gnarled enamel
How tick is the outer surface layer of enamel in primary teeth?
20-100um
How thick is the outer surface layer of secondary enamel?
20-70um
What is the outer layer of enamel called?
aprismatic.
How are the crystallites arranged in aprismatic enamel?
The crystallites are aligned at right angles to the surface and parallel to each other.
Is the apirsmatic layer of enamel more or less mineralised to inner layers?
highly mineralised
Why is the outer layer of enamel more highly mineralised?
The surface layer is more highly mineralised than the rest of the enamel. This is attributed to the absence of prism boundaries where organic material is located.
How would you describe the way enamel is formed?
Incrementally
Describe how enamel is formed incrementally
Enamel is formed in increments: periods of activity alternating with periods of inactivity.
This results in incremental lines: short period (cross striations) and long period (enamel striae).
How do cross striations appear on enamel?
Cross striations appear as lines crossing the enamel prisms at right angles to their long axes.
They reflect a diurnal rhythm (daily increments of growth).
When do cross striations appear closer together?
Closer to each other near the enamel-dentine junction.
What is shown in the histological diagram?
stria of retzius
(enamel striae)
Enamel striae run obliquely across the prisms. They represent incremental lines and are known as the striae of Retzius.
Do striae of retzius reach the surface of the enamel?
no
How to stria of retzius appear in cross section of a tooth?
run circumferentially like the rings of a tree.
How many cross striation are there between enamel and adjacent stira?
7-10
This suggests weekly intervals.
When do the stria of retzius (cross section) run closest together?
cervically
How to enamel stria appear on the surface of teeth?
appear as fine grooves running circumferentially around the crown.
What are the stria on the surface of teeth referred to as?
The lines are termed Perikymata grooves, with perikymata ridges in between.
Name the feature
Perikymata grooves
How does surface enamel compare to subsurface enamel?
Physically and chemically, surface enamel differs from subsurface enamel.
Surface enamel is harder, less porous, less soluble and more radio-opaque.
Richer in trace elements.
Less carbonate.
Aprismatic, therefore highly mineralised.
What are the arrows pointing to?
The pits are within the perikymata.
formed on protected areas of enamel
What do the small pits on perikymata represent?
They mark the ends of ameloblasts (prism end markings.
1-1.5μm in depth.
What is the neonatal line?
prominent enamel stria
What are the arrows pointing to?
enamel caps
What is an enamel cap?
Enamel caps are small elevations 10-15μm across.
How does an enamel cap form?
they result from enamel deposition on top of debris late during tooth development.
What are the arrow pointing to?
focal holes
What are focal holes on the surface of enamel?
Focal holes are depressions on the surface.
loss of mineralised material on top of organic material
How do focal holes form?
Loss of enamel caps with the underlying material.
this happens through abrasion or attrition.
What is shown here? (surface enamel)
Enamel brochs are elevations on the enamel surface.
Where is the enamel broch usually found?
the cemento-enamel junction
What tooth do you usually see the enamel broch?
premolar 6s
What does an enamel broch look like?
extra piece of enamel with radiating crystalites
What type of microscopy is used here?
transmission
