19. Histology of Tooth Crown Flashcards
Describe how dentine is formed involving mineralising front, odontoblasts and fibroblasts. What biochemical processes occur at mineralising front?
- 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
How do Contour lines of Owen form?
- 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
Which 2 tissues are seperated by mineralising front?
predentine and dentine
How far do odontoblast processes reach into dentinal tubules?
- 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
Name outermost layer of coronal dentine
mantle dentine
Structural properties and how it forms - mantle dentine
- 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
Name main types of nerve fibres found in body of dental pulp and functions
- mainly afferent sensory pain-response fibres
- autonomic fibres for vasoconstriction and vasodilation too
Why is enamel not visible on an unerupted tooth?
- removed by decalcification process required to prepare histological sections
- remaining enamel proteins collapse and are removed too
- gap is called ‘enamel space’
What changes in enamel organ occur to produce reduced enamel epithelium?
- 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
Describe function of reduced enamel epithelium
- 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
Name short and long period incremental lines in enamel and dentine and how they form
- 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
Why are growth lines accentuated in certain areas of tooth and what are these called?
- stress lines in dentine are called Contour Lines of Owen and in enamel Wilson lines
- neonatal line of enamel is an example of Wilson
How does primary and secondary curvature of dentinal tubules form?
- 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
Where would you find interglobular dentine? How does it form and what’s it’s mineralisation status?
- found in upper third of coronal dentine - region of fastest dentine formation
- formed by incomplete fusion of calcospherites, thus hypomineralised
Where would you find Granular Layer of Tomes? How does it form and what’s it’s mineralisation status?
- 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
Where would you find peritubular dentine? How does it form and what’s it’s mineralisation status?
- 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
What is enamel called in cusp region and why does it have this complex appearance and what’s it’s function?
- 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
Appearance and function of EDJ
- 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
Explain what enamel spindles are and how they’re formed
How do they relate to adjacent prism orientation?
- 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
What does the neonatal line represent?
position of mineralising front in dentine or distal ends of ameloblasts at time of birth
In which teeth would there be a neonatal line?
- all primary
- first permanent molars
Using position of neonatal line, how could you determine if a tooth was a 1st or 2nd primary molar?
- first is more developed at birth with more formed hard tissue so neonatal line is further away from EDJ
- vice versa in 2nd
Explain appearance of EDJ in primary teeth and why it looks diff in perm
- primary teeth form faster so less scalloping of EDJ
What is the difference in prism direction in cervical enamel of primary and perm teeth? Why do we need to know?
- 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
How do enamel lamellae form?
Give their features and clinical significance
- 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
How do enamel tufts form?
- 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
