19. Histology of Tooth Crown

Question 1

Describe how dentine is formed involving mineralising front, odontoblasts and fibroblasts. What biochemical processes occur at mineralising front?

Answer 1

• initially, odontoblasts produce ECM called predentine containing collagen and dentine structural proteins (inc. those involved in regulating dentine mineralisation like dentine phosphoprotein)
• mineralisation of predentine is an active process regulated by odontoblasts - through active calcium ion transport, secretion of alkaline phosphatase etc
• mineralising front can appear fairly uniform when mineralisation process is slow (linear). When it’s faster, results in appearance of globular structures in predentine (globular)
• mineralisation foci grow in size and fuse with others

Question 2

How do Contour lines of Owen form?

Answer 2

• stress lines in dentine
• appear when secondary curvature of dentinal tubules is accentuated in region of dentine
• increased curvature occurs due to stress factor acting on odontoblasts that were located at this region of dentine in tooth development
• can be caused by birth, illness, malnutrition etc and affects enamel and dentine formation

Question 3

Which 2 tissues are seperated by mineralising front?

Answer 3

predentine and dentine

Question 4

How far do odontoblast processes reach into dentinal tubules?

Answer 4

• depends on age and dental health
• in healthy dentine, reach all the way to EDJ
• peritubular dentine formed due to aging or mild attrition occludes dental tubules causing recession of odontoblast processes

Question 5

Name outermost layer of coronal dentine

Answer 5

mantle dentine

Question 6

Structural properties and how it forms - mantle dentine

Answer 6

• formed by newly differentiated odontoblasts via secretion of matrix vesicles
• lacks major dentine structural protein dentin phosphoprotein - highly acidic protein that attracts calcium ions - so reduced mineralisation (around 5%)
• in addition to type 1 collagen fibres, lots of type 3 (von Korff fibres - orientated perpendicular to EDJ) that intermingle with proteins of basal lamina of ameloblasts to increase stability at EDJ
• dentinal tubules are strongly branched - this unique structure is to prevent spread of cracks in enamel to dentine

Question 7

Name main types of nerve fibres found in body of dental pulp and functions

Answer 7

• mainly afferent sensory pain-response fibres
• autonomic fibres for vasoconstriction and vasodilation too

Question 8

Why is enamel not visible on an unerupted tooth?

Answer 8

• removed by decalcification process required to prepare histological sections
• remaining enamel proteins collapse and are removed too
• gap is called ‘enamel space’

Question 9

What changes in enamel organ occur to produce reduced enamel epithelium?

Answer 9

• at the end of enamel maturation, REE is formed by reduction of enamel organ
• ameloblast cells reduce in size and SR, SI and OEE condense into thin cell layer

Question 10

Describe function of reduced enamel epithelium

Answer 10

• protects unerupted crown surface from resorption by osteoclasts and from deposition of cementum
• functions during eruption by fusing with oral epithelium where it forms junctional epithelium of dento-gingival junction

Question 11

Name short and long period incremental lines in enamel and dentine and how they form

Answer 11

• short period reflect daily activity of ameloblasts and odontoblasts
• in enamel, called cross striations and von Ebner in dentine
• appear as small alternating dark and light
• long period lines show unknown biological rhythm that is species-specific. Andresen lines in dentine are 16-20 microm apart and Striae of Retzius in enamel are 25-35

Question 12

Why are growth lines accentuated in certain areas of tooth and what are these called?

Answer 12

• stress lines in dentine are called Contour Lines of Owen and in enamel Wilson lines
• neonatal line of enamel is an example of Wilson

Question 13

How does primary and secondary curvature of dentinal tubules form?

Answer 13

• primary refers to sigmoidal route of dentinal tubules. caused by change in position of odontoblasts in pulp chamber as pushed apically by own dentine production
• secondary shows the meandering appearance of individual dentinal tubules - caused by natural variation in position of odontoblast process before it’s trapped in dentinal tubule
• under stress, odontoblast process becomes bent which can manifest as a Contour line of Owen

Question 14

Where would you find interglobular dentine? How does it form and what’s it’s mineralisation status?

Answer 14

• found in upper third of coronal dentine - region of fastest dentine formation
• formed by incomplete fusion of calcospherites, thus hypomineralised

Question 15

Where would you find Granular Layer of Tomes? How does it form and what’s it’s mineralisation status?

Answer 15

• found in peripheral root dentine
• unclear if it represents branching and backward looping of odontoblasts
• or if it is formed by incomplete fusion of calcospherites (analogous to interglobular)
• hypomineralised

Question 16

Where would you find peritubular dentine? How does it form and what’s it’s mineralisation status?

Answer 16

• forms with increasing age or response to mild attrition
• associated with recession of odontoblast processes from dentine tubules
• results in narrowing or complete closure of dentinal tubules
• more mineralised/hypermineralised than intertubular

Question 17

What is enamel called in cusp region and why does it have this complex appearance and what’s it’s function?

Answer 17

• as crown develops and becomes larger, cohorts of ameloblasts displaced apically by own enamel production - results in change in prism direction
• in gnarled enamel, prism orientation becomes extremely angular/prism decussation as ameloblasts adapt to rapidly exapanding enamel surface
• prism decussation is thought to stop propagation of cracks into deeper enamel and tooth cusps can withstand strong masticatory force

Question 18

Appearance and function of EDJ

Answer 18

• scalloped
• reflects convex shape of distal ends of groups of ameloblasts against which first dentine is formed
• increases SA and stabilises EDJ - e.g below tooth cusps where shearing force is high

Question 19

Explain what enamel spindles are and how they’re formed
How do they relate to adjacent prism orientation?

Answer 19

• caused by odontoblast process that has intercalated between 2 ameloblasts during early stage tooth development - before hard tissue forms
• once enamel is formed, odontoblast process becomes trapped in enamel and presents as histological structure called enamel spindle
• orientation of enamel spindle doesn’t follow prism direction as Tomes’ process produce enamel prism at oblique angle relative to position of ameloblasts

Question 20

What does the neonatal line represent?

Answer 20

position of mineralising front in dentine or distal ends of ameloblasts at time of birth

Question 21

In which teeth would there be a neonatal line?

Answer 21

• all primary
• first permanent molars

Question 22

Using position of neonatal line, how could you determine if a tooth was a 1st or 2nd primary molar?

Answer 22

• first is more developed at birth with more formed hard tissue so neonatal line is further away from EDJ
• vice versa in 2nd

Question 23

Explain appearance of EDJ in primary teeth and why it looks diff in perm

Answer 23

• primary teeth form faster so less scalloping of EDJ

Question 24

What is the difference in prism direction in cervical enamel of primary and perm teeth? Why do we need to know?

Answer 24

• enamel prisms in cervical of prim are inclined occlusally but apically in perm
• enamel prisms shouldn’t be undercut during restorative work as can fracture and release restoration

Question 25

How do enamel lamellae form?
Give their features and clinical significance

Answer 25

• extend from EDJ to enamel surface
• developmental defects in enamel formation and contain organic material (trapped enamel organ comps or connective tissue) hence hypomineralised
• cracks in enamel can form along these or along damaged enamel in older people or after braces removal
• most cracks present in enamel of ground sections due to ground sectioning procedure. true enamel cracks don’t contain organic material
• could potentially increase susceptibility to caries as bacteria can enter teeth through these cracks

Question 26

How do enamel tufts form?

Answer 26

• project from EDJ for short distance and branched
• hypomineralised as have residual enamel proteins like tuftelin that weren’t removed in enamel muturation
• form as a consequence of abrupt changes in direction of groups of enamel prisms that arise from different regions of scalloped EDJ