Unit 3b - Developement of Crown

Question 1

Describe amelogenesis.

Answer 1

• requires presence of dentin
• is a product of the enamel organ
• IEE cells elongate and stimulate odontoblasts to line up at future DEJ, change polarity and lay down predentin
• basement membrane disintegrates
• the cusp tips and incisal edges are formed first

Question 2

What is the Tome’s process

Answer 2

• a structural process of each ameloblast responsible for secreting enamel matrix
• the ameloblasts move outward toward the surface from the DEJ leaving behind a matrix of enamel rods and interrod substance
• enamel mineralizes upon formation

Question 3

What are incremental lines?

Answer 3

• about 4microns of enamel and dentin are produced intermittently
• between phases of formation, there is a slight pause in production
• this leaves small incremental lines in both dentin and enamel tissues that can be observed histologically

Question 4

What are cross striations

Answer 4

-exist as light and dark cross striations
-are at angles to other lines showing in a histological section of enamel
-ameloblasts seem to take a bigger break in formation less often and therefore these lines are not as equal in distance apart
-these are called Striae of Retzius
-situated in an oblique pattern toward the enamel surface from the DEJ
-similar to the lines of Owen in dentin
-

Question 5

What are the neonatal linesÉ

Answer 5

A pronounced line of Retzius that corresponds to the birth of the individual

• thus it demarcates the enamel formed prenatally and after birth
• product of ameloblasts
• once formed, no changes in structure
• other accentuated lines = stress, illness etc

Question 6

What is perikymata?

Answer 6

• step-like pattern occurring when rows of ameloblasts are finished secreting enamel matrix (on surface of enamel)
• run in a horizontal plane across the facial surface of the tooth crown and are usually worn by attrition and mastication

Question 7

Describe the mineralization phase of enamel

Answer 7

• enamel mineralization is ‘almost’ immediate
• NO enameloid - minerals are laid down during formation
• minerals need to grow in size (maturation)
• suggestions that about 25% is laid down and about 70% is due to growth
• note that enamel and dentin formation may be well under way at the cusp and yet at the cervical line or cervical loop area, odontoblasts have not yet differentiated and the innermost cells of the enamel organ are still at the IEE stage

Question 8

What is the composition of enamel?

Answer 8

• 4% organic - enamelins (enamel proteins) and ground substance
• 96% inorganic - hydroxyapatite crystals: calcium and phosphate salts
• once crystals are laid down, they mature/grow to pack very tight within enamel and rod structures
• these are susceptible to dissolution (demineralization)

Question 9

Describe enamel rods/prisms

Answer 9

• tightly packed mass of hydroxyapatite crystals
• highly organized
• shaped like keyholes
• each rod extends from DEJ to surface
• inner 1/2 wavy rods

Question 10

Describe the rod sheath

Answer 10

• surrounds each enamel rod
• contains most of organic matrix of enamel
• glue-like substance cements the rods together is called interrod substance
• less mineralized

Question 11

Describe Hunter Schreger Bands

Answer 11

• closest to the DEJ the course of the rods are wavy (inner 1/3) while nearer the surface the rods straighten out
• altenating dark and light bands are created as a result of the pattern of groups of enamel rods travelling together from the DEJ to the surface of the enamel

Question 12

What are enamel lamellae

Answer 12

• like a geographical fault in enamel
• defects in calcification *development & post eruption
• possibly due to injury during development *age related - wear and tear
• may contribute to caries pattern
• micro lamellae - between rods or groups of rods
• extend for varying deptsh from teh surface of the enamel and consist of linear, longitudinally oriented defects filled with enamel protein or organic debris from the oral cavity

Question 13

What are enamel spindles?

Answer 13

• odontoblastic process extends across DEJ ‘trapped’ in mineralized enamel
• short singular structures

Question 14

What are enamel tufts?

Answer 14

• unmineralized ends of enamel rods/or areas between rods in early development
• at DEJ and a short way into the enamel
• described as tufts/small brush like structures on histological slides
• not pattern *follows rod groups in various levels

Question 15

Describe the DEJ

Answer 15

• shape is scalloped - more at incisal/occ and less at CEJ
• function - increases surface area; possibly for adhesion?
• less mineralized area *increases spread of caries

Question 16

Describe gnarled enamel

Answer 16

• structure of rods near cusp tips
• results from twisting and tumultuous pathway of rods in this area
• rods are intertwined at the cusp tip
• groups of outer enamel rods all run nearly perpendicular to the surface of the enamel, whereas inner groups of enamel rods alternate
• some appear in cross section while adjacent groups appear longitudinal

Question 17

How does enamel repair?

Answer 17

• ameloblasts are lost shortly after crown enamel completion
• once formed it cannot be repaired
• imperfections remain *identify some
• mineral cannot be withdrawn or removed for body’s use *preg fallacy!
• remineralization with fluorid ion, saliva (calcium and phosphate)
• calcium hydroxyapatite

Question 18

What is the clinical importance of enamel?

Answer 18

• high % mineral makes it resistant to attrition - some occurs naturally
• curvature of enamel rods and interlocking structure resist breakage
• pattern of mineralization or lack of mineralization influence path of caries
• ie Lamellae and Striae of Retzius
• enamel structures that may influence permeability of enamel/the pattern of caries progression…….name them

Question 19

What are developmental lobes?

Answer 19

• each tooth develops from 4 or more growth centers called developmental lobes
• these grow and coalesce and their junctions are marked by developmental grooves
• this formation of enamel and dentin are occurring in 3, 4 or 5 focal areas first and later join to form the shape of individual crowns

Question 20

How does age change enamel?

Answer 20

• attrition - wear facets
• color change = darker * picks up stain over time
• less caries with age - surface enamel becomes less permeable; flatter occlusal (less pits and fissures); diet chagnes; fluoride uptake in outer layers make it more resistant
• newly erupted enamel surface is less mineralized till 1-2 years post eruption = maturation of mineral and uptake of fluoride & minerals from oral environment

Question 21

What are the stages of ameloblast?

Answer 21

• IEE
• Preameloblasts
• ameloblasts - complete formation
• lose Tome’s process
• form a Primary Enamel Cuticle (the last secretion of this cell) - thought to be a protective function
• the cells then revert to epithelial cell (becomes part of REE - coming)

Question 22

What is the primary enamel cuticle

Answer 22

The last function of the ameloblast is said to be the production of this primary enamel cuticle.

• once the enamel thickness has reached its final thickness, enamel production ends, the ameloblast loses its Tom’s process and secretes a smooth prismless coating over the enamel surface
• the ameloblasts then revert to an inner enamel epithelial cell

Question 23

Describe the formation of the REE (Reduced Enamel Epithelium).

Answer 23

• when enamel matrix formation is completed, the Stellate reticulum narrows, becoming indiscernible
• this leaves ameloblasts in contact with the OEE
• once this occurs the final primary enamel cuticle is formed and ameloblasts top functioning (no more enamel)
• the cells flatten out and blen in with the OEE becoming the REE
• this surrounds the crown until the tooth erupts, it become SSE 3-4 days after eruption and serves as the initial junctional epithelium
• produced after the completion of enamel apposition when the enamel organ undergoes compression of its many layers on the enamel surface

Question 24

What is the Nasmyth’s Membrane?

Answer 24

-the REE, the basal lamina of the REE adjacent to the enamel surface and the Primary Enamel cuticle together referred to as Nasmyth’s Membrane

Question 25

What are some abnormalities in crown formation?

Answer 25

• dentinogenesis imperfecta
• amelogenesis imperfecta
• abnormalities related to morphodifferentiation - extra cusp, microdontia, dens in dente
• hypoplasia
• hypomineralization
• fluorisis

Question 26

What are some cysts related to the REE and/or tooth germ?

Answer 26

Dentigerous cyst
Primordial cyst (primordium - tooth germ)

Question 27

What is dentinogenesis imperfecta?

Answer 27

• dominant inherited disorder
• faulty formation of connective tissues
• dentin is softer than normal
• poor DEJ so enamel breaks or chips away
• tx usually includes crowns to preserve crown structure

Question 28

What is amelogenesis imperfecta?

Answer 28

• group of inherited disorders/affecting enamel formation
• affects both dentitions
• defects in one of the 3 stages of development:
• hypoplasia
• calcification
• maturation