HISTOPATHOLOGY OF CARIES

Enamel is composed of tightly packed hydroxyapatite crystals, which are organized in long columnar rods (enamel rods), but during caries progression certain histological changes are seen in enamel .
The following 4 histological zones of an enamel lesion clearly explains the development of enamel caries:

zone 1: translucent zone
zone 2: dark zone
zone 3: body zone
zone 4: surface zone



zone 1: translucent zone
ü deepest zone representing the advancing front of enamel
caries.
ü in this zone , pores or voids form along the enamel prism(rod)
boundaries (due to easy H+ ion penetration)
ü it appears structureless when perfused with quinolone solution (having refractive index comparable to that of enamel) and seen with polarized light (hence translucent)


zone 2: dark zone
ü next deepest zone
ü presence of many tiny pores block light transmission. These smaller air or vapor-filled pores make the regiion opaque.
ü loss fo crystalline structure suggesting the process of demineralization and remineralization in this zone.



zone 3: body zone
ü in demineralization phase, it is the largest portion of the lesion.
ü(whereas in remineralization phase, zone2/dark zone is the largest portion which increases in the expense of the “body zone”)
ü largest pores seen (pore volume 5 to 25%)
ü presence of bacteria if pores large enough to permit their entry
ü striae of Retzius well marked
(striae of Retzius is the primary point of entry of carious lesion into rod/prism cores of enamel)



zone 4: surface zone
ü relatively unaffected by caries (only partial demineralization)
ü Because surface of enamel is relatively immune to caries (due to hypermineralization- because of saliva contact , and
higher surface F-content)
ü also pore volume is lower than the body of lesion.



After the involving the enamel, the carious lesion progresses to the dentinal structure.


Caries advancement in dentin proceeds through 3 stages-
1) demineralization of dentin (by weak organic acids)
2) degeneration and
dissolution of organic material of dentin , mainly collagen
fibers (type I)
3) bacterial invasion after the loss of structural integrity caused
due to 1) and 2).


During the development of dentinal caries, clinically 5 different zones of progression can be seen (a/c to Sturdevant )
zone 1: normal dentin
zone 2: subtransparent dentin
zone 3: transparent dentin
zone 4: turbid dentin
zone 5: infected dentin



Histologically , 5 zones of early dentinal caries progression can be seen (listed pulpally to occlusally):
zone 1: zone of fatty degeneration of Tomes’ fibers
zone 2: zone of dentinal sclerosis
zone 3: zone of decalcification of dentin
zone 4: zone of microbial invasion
zone 5: zone of decomposed dentin



zone 1: zone of fatty degeneration of Tomes’ fibers
ü the most advancing front of dentinal caries
ü characterized by the presence of a layer of fat globules ; hence
stains red with the stain, sudan red.
ü significance: 1) fat layer leads to impermiability of the dentinal
tubules (DT) – trying to prevent further invasion of
carious lesion
2) favors sclerosis of dentin in zone 2.



zone 2: zone of dentinal sclerosis
ü layer of sclerotic dentin which appears white in transmitted light
ü calcification of DT as a rxn of vital pulp and vital dentin to carious invasion , so as to prevent further penetration of microorganisms.
ü formation of this zone is minimal in rapidly progressing caries, and prominent in slow caries.



zone 3: zone of decalcification of dentin
ü this zone lies above the zone of sclerotic dentin
ü initial decalcification of only the walls of the DT
ü presence of PIONEER BACTERIA- first of the microorganisms penetrating DT before there is any clinical evidence of caries.
ü bacteria present in individual DT are in pure form (i.e. either completely cocci or completely bacilli; not in mixed form)



zone 4: zone of microbial invasion
ü in a layer above zone 3.
ü characterized by the presence of microorganisms
ü in early stage of caries- acidogenic microorganisms
üin deeper layer- proteolytic microorganisms replace acidogenic bacteria
ü supports the hypothesis that initiation (by acidogenic bacteria) and progression ( by proteolytic microorganisms ) are 2 distinct processes in caries development.

During initiation phase- in the early stage when caries is not deep , acidogenic bacteria predominant which utilizes carbohydrate for their metabolism
Later in progression phase – as the caries goes deeper , less and less of carbohydrate substrate available , hence acidogenic bacteria are replaced by proteolytic microorganisms which uses dentinal protein for their metabolism.



zone 5: zone of decomposed dentin
üMost superficial zone of early dentinal caries.
ü no recognizable structure in decomposed dentin
ü collagen and minerals seem to be absent
ü great number bacteria dispersed in this decomposed granular matter.