Enamel Histology

Question 1

Which germ layer does enamel arise from?

Answer 1

Ectodermal in origin

Question 2

What happens to enamel in ground sections?

Answer 2

The hard, mineralized tissue remains intact but the soft connective tissues and epithelia are lost

Question 3

What is the ideal type of sample to use for visualize the soft tissues and organic matrices of mineralized tissue?

Answer 3

Decalcified sections

Question 4

What is the ideal type of sample to use for visualize the mineralized part of enamel tissue?

Answer 4

Ground sections

Question 5

What structures does the enamel cover?

Answer 5

The crown of the tooth (thickest over the cusps and incisal edges and thinnest at the cervical margin)

Question 6

What are the physical properties of enamel?

Answer 6

Enamel is the hardest biological tissue and is abrasian resistant

Enamel does not undergo repair or replacement

Enamel has low tensile strength and is brittle and needs dentin for the flexible support that is needed

The properties of enamel vary at different regions within tissue

Surface enamel is harder, denser and less porous than subsurface enamel.

Enamel is birefringent crystalline material, the crystals refracting light differently in different directions

Young enamel is white because of the low translucency. With age the enamel translucency increases and some of the colour of the underlying dentin is transmitted.

Question 7

What makes up the chemical composition of enamel?

Answer 7

Hydroxyapatite is the principle mineral component of enamel (88 - 90% of the tissue volume and 95 - 96% by weight

Fluoride may substitute for hydroxyl ions, conferring greater stability and resistance to
acidic dissolution.

Organic matrix makes up 1 - 2% of mature enamel. This varies between free amino acids to large protein complexes.

Question 8

What happens to fluoride levels throughout the thickness of enamel?

Answer 8

Fluoride levels decline from the outer
surface towards the dentin, perhaps
because the fluoride is acquired during
enamel maturation.

Question 9

How much of the composition of enamel is water?

Answer 9

Water makes up approximately 2% by weight of enamel.

Question 10

What do crystallites that make up mature enamel look like on cross section?

Answer 10

Regularly hexagonal in cross-section. The cores of crystallited differ slightly in composition from periphery being richer in Mg and CO3. Core is more soluble than periphery.

Question 11

What is water presence in enamel related to?

Answer 11

The porosity of the tissue

Question 12

What makes up the basic structural unit of enamel?

Answer 12

It is made up of enamel prisms or rods consisting of several million hydroxyapatite crystallites packed into a long thin rod 5 - 6 µm in diameter and up to 2.5 mm in length.

Question 13

Where do enamel prisms run?

Answer 13

They run from the enameldentin junction to the surface

Question 14

What do the boundaries of the crystalline prisms reflect?

Answer 14

Sudden changes in crystallite orientation that give an optical effect different from that of the prism core or body. At the boundaries, the crystallites deviate by 40 - 60 degrees from those inside the prism

Question 15

What are the enamel prism patterns commonly seen in enamel?

Answer 15

Pattern 1 enamel: Prisms are circular

Pattern 2 enamel: the enamel prisms are aligned in parallel rows

Pattern 3 enamel: The prisms are arranged in staggered rows such that the tail of a prism lies between 2 heads in the next row.

Question 16

How can the keyhole appearance of the prisms of enamel be visualized?

Answer 16

With controlled demineralization, many of the structural features seen in ground sections will be retained.

Question 17

What lies in the boundary between rod and interrod?

Answer 17

It is delimited by a narrow space containing organic material called the rod sheath

Question 18

What is the cross-sectional arrangement and shape of the prisms?

Answer 18

They are characteristically of keyhole arrangement with the tails pointing cervically and the heads occlusally.

In the tail the crystallites gradually diverged from this to become angles 65 - 70 degrees to the long axis.

Question 19

How are enamel prisms arranged in longitudinal section?

Answer 19

They follow a sinusoidal path with 10 - 13 layers of prisms following the same directions but blocks above and below follow paths in different directions.

Question 20

What is each bundle of enamel prisms that are moving in the same direction called?

Answer 20

Hunter-Schreger bands

Question 21

How wide are Hunter-Schreger bands?

Answer 21

50 µm wide

Question 22

What is the purpose of Hunter-Schreger bands and are they present throughout the thickness of enamel?

Answer 22

They make enamel resistant to fracture and when exposed on the surface leads to a micro-ridged grinding surface

In approximately the outer quarter of enamel the prisms run in the same direction and no Hunter-Schreger bands are present.

Question 23

What part of enamel is aprismatic?

Answer 23

The outer 20 - 100 µm of newly erupted deciduous teeth

The outer 20 - 79 µm of newly erupted permanent teeth

Question 24

How ia aprismatic enamel different in composition?

Answer 24

The surface layer is more highly mineralised than the rest of the enamel due to the absence of prism boundaries where most organic material is located.

Question 25

How is aprismatic enamel formed?

Answer 25

It is formed in the absence of Tomes’ processes on the ameloblasts in the final stages of enamel deposition.

Question 26

What are incremental lines?

Answer 26

Enamel is formed incrementally, periods of activity alternating with periods of quiescence the resulting lines are called incremental lines

Question 27

What are cross-striations?

Answer 27

Short period incremental lines (they reflect diurnal rhythm)

Question 28

What are enamel striae?

Answer 28

Long period incremental lines.

When sections of enamel are cut along the longitudinal axis and the crown is viewed, there are structural lines that run obliquely across the prisms from near the enamel-dentin junction surface. These are called striae of retzius.

Question 29

Do enamel striae show up on the outer surface?

Answer 29

No the manner they are deposited leads to them being covered by flat surfaces.

Only way to see them is if there is enamel loss.

Question 30

How many cross-striations are present between adjacent striae in human teeth? How far apart are these cross-striations?

Answer 30

7 - 10 usually, usually they are about 4 µm apart. In the middle portion of enamel they are about 25 - 30 µm apart.

In cervical enamel they are closer to each other due to slower formation of enamel and so are only about 2µm apart.

Question 31

What causes formation of accentuated striae?

Answer 31

Metabolic disturbances during the time of mineralization.

Question 32

What are perikymata grooves?

Answer 32

Over the whole of the lateral enamel, enamel striae reach the surface in a series of fine grooves running circumferentially around the crown and are separated by ridges that are close together near the cervical margin (~15 - 20 µm)

Question 33

What are possible uses of incremental lines?

Answer 33

They can help with assessment of time taken to form the crown of the tooth and to age the material.

Question 34

What is the neonatal line?

Answer 34

A particularly marked stria that is formed at birth. This reflects the neonatal line and reflects metabolic changes at birth.

Question 35

How does surface enamel differ from subsurface enamel?

Answer 35

Surface enamel is harder, less porous, less soluble and more radio opaque than subsurface enamel.

Outer enamel is aprismatic in most areas and thus more highly mineralized and resistant to caries.

Small cracks are frequently found in surface enamel.

Question 36

What does the enamel-dentin junction look like?

Answer 36

Has a scalloped pattern in areas where shearing forces would be high (beneath cusps and incisal edges)

Smooth on the lateral surfaces. The convexities are on the enamel surface and the concavities on the dentinal surface.

A number of features are also present on this area such as enamel spindles, tufts, and lamellae.

Question 37

What are enamel spindles?

Answer 37

Narrow, round, sometimes club-shaped tubules. They are not aligned with the prisms and are thought to be the result of some odontoblast processes that, during the early stages of enamel development insinuated themselves between the ameloblasts.

Question 38

Where are enamel spindles most commonly found?

Answer 38

Enamel spindles are most common beneath cusps where most crowding of odontoblasts would have occured.

They are best seen in longitudinal sections

Question 39

What are enamel tufts?

Answer 39

Enamel tuft is the term given to junctional structures in the inner third of the enamel that in ground sections resembles tufts of grass.

They are hypomineralized and recur at approximately 100 µm intervals along the junction.

Question 40

How are enamel tufts best visualized?

Answer 40

Owing to their alignment they are best visualized in transverse sections of enamel.

Question 41

What are enamel lamellae?

Answer 41

They are sheet-link apparent structural faults that run through the entire thickness of the enamel.

Question 42

What do enamel lamellae look like and how are they best visualized?

Answer 42

They are hypomineralised and narrower, longer and less common than enamel tufts but, like tufts, are best visualized in transverse sections of enamel.

Question 43

How do enamel lamellae arise?

Answer 43

Developmentally due to incomplete maturation of groups of prisms or after eruption as cracks during function.

Question 44

What are enamel pores?

Answer 44

Water-filled spaces between crystallites.

Question 45

How porous is enamel?

Answer 45

Approximately 3 - 5% by volume

Question 46

What do enamel pores look like?

Answer 46

Within the prisms most pores exist as very narrow gaps between closely packed crystallites but some, though they are still small, appear elongated and tubelike.

Question 47

What happens to enamel with age?

Answer 47

Enamel wears away slowly with age, dependent on diet and masticatory habits.

Gets darker in colour due to reduced translucency of the tooth (secondary dentine shows through) and due to coatings and stains.

Question 48

How common are developmental defects in enamel?

Answer 48

Affects approximately 68 - 95% of the population

Defects may be environmental or genetic in origin.

Question 49

How does hypoplasia manifest on the surface of enamel?

Answer 49

As pits and grooves on the enamel surface.

Question 50

How does hypomineralisation manifest itself on teeth?

Answer 50

The surface is generally intact but opaque rather than translucent. This is related to a delay in the removal of amelogenins during maturation

Question 51

Why is understanding enamel mineralisation and structure important for operative dentistry?

Answer 51

Many of the structural features of enamel are acutely relevant in restorative dentistry. The understanding of the initiation and progress of dental caries has been
based on a knowledge of enamel composition and
morphology and has led to a much more conservative
approach by utilising the phenomenon of
remineralisation and reducing the need for the removal
of sound tissue.
• Different acids at different concentrations can produce a
variety of patterns of partial prism dissolution to provide
a roughened surface suitable for adherence of
restorative materials (acid conditioning).