Zone I: Zone of fatty degeneration of odontoblast process
Zone 2: Zone of dentinal sclerosis characterized by deposition of cal- cium salts in dentinal tubules
Zone 3: Zone of decalcification of dentin, a narrow zone, preceding bacterial invasion
Zone 4: Zone of bacterial invasion of decalcified but intact dentin Zone 5: Zone of decomposed dentin

Early dentinal caries

Fatty degeneration of odontob/ast process

>Disposition of fat globules – precedes early sclerotic changes  >Special stains – Sudan red
>Significance-
1.Fat contributes to impermeability 

2.Predisposing factor for dental sclerosis

Sclerotic dentin

Decalcification of dentinal tubules

>Above dentinal sclerosis – zone of decalcification

>Occurs in advance of bacterial invasion of DT 

>Pioneer bacteria
>The initial decalcification – only the walls of DT 

>Study of tubules- pure form of microorganisms

Zone of microbial invasion

>Proteolytic organisms – predominantly in deeper layers Acidogenic microorganisms – more in early caries
>Supporting the hypothesis that initiation and progression are two distinct processes and must be differentiated

Advanced dentinal caries

>Decalcification of the walls of DT – confluence

>Thickening of sheath of Neumann – along its course • Increase in the diameter of DT – microorganisms

>Focal coalescence of adjacent tubules and ovoid area of destruction- liquefaction foci
>Acidogenic organisms – initial decalcification

REFERENCE- Shafers textbook of oral pathology 8th edition

Four zones are clearly distinguishable, starting from the inner advancing front of the lesion. These are the (1) translucent zone, (2) dark zone, (3) body of the lesion and (4) surface layer.

Zones of enamel caries Translucent zone {TZ)

First recognizable zone of alteration

Advancing front of the lesion

Half the lesions demonstrate TZ, not always present

Seen in longitudinal ground sections in clearing (quinoline – RI – 1.62)

TZ appears structureless

Pore volume – I% (compared to 0.1 % of sound enamel)

Dark zone

Body of the lesion

Between unaffected, surface and dark zone
Area of greatest demineralization
Pore volume – 5% in periphery and 25% in centre
Quinoline imbibition – body appears transparent
Water imbibition – positive birefringence compared to sound enamel Striae of Retzius – prominent

Surface zone

Quantitative studies – partial demineralization of 1-10% • Pore volume – less than 5% of the spaces

Negative birefringence – water imbibition

Positive birefringence – porous subsurface
All the four zones of enamel caries cannot be seen with same immersion medium.

REFERENCE – Shafers textbook of oral pathology 8th edition