1. Incremental lines of von Ebner:[Imbrication lines]
Represent the rhythmic process of dentinogenesis.
Run at right angles to the tubules
5-day rhythm of deposition separated by 20 μm
Best seen in longitudinal sections

2. Contour lines of Owen
Originally referred to the result of a coincidence of secondary curvatures of the dentinal tubules
Now applied also to accentuated lines caused by deficiencies in mineralization
The neonatal line is a prominent example.

3. Granular layer of Tomes
Seen only in ground sections under transmitted light
Granular area beneath the dentin-cementum junction, increases apically

4. They are true spaces related to the looping of the dentinal tubules
Cannot be seen in H and E sections or on EM
Recently related to a special arrangement of collagen and non collagenous proteins

5. Reactions to stimuli Dentin being vital, responds to external stimuli.
The reactions are
Dentin sensitivity
Sclerotic dentin
Dead tract formation
Tertiary dentin

6. Dentin sensitivity
Defined as a short or transient sharp pain of a rapid onset that arises from exposed dentin
Due to operative procedures, thermal change, dehydration or hypertonic solutions
Provides no evolutionary benefit

7. Theories of pain transmission
Direct neural stimulation: The dentin contains nerve endings that respond when it is stimulated.
Transduction theory: The odontoblasts serve as receptors and are coupled to nerves in the pulp
Hydrodynamic theory: The tubular nature of dentin permits fluid movement to occur within the tubule when a stimulus is applied, a movement registered by pulpal free nerve endings close to the odontoblasts.

8. Direct neural stimulation:
Some nerves occur within some tubules in the inner dentin but that dentin sensitivity does not depend solely, if at all, on the stimulation of such nerve endings.

9. Transduction theory Suggests that the odontoblast acts as a receptor
The second possible mechanism to explain dentin sensitivity considers the odontoblast to be a receptor cell.
Rejected because:
the membrane potential of odontoblasts measured in vitro is too low to permit transduction and that local anesthetics and protein precipitants do not abolish sensitivity also militated against this concept.

10. HYDRODYNAMIC THEORY
The third mechanism proposed to explain dentin sensitivity involves movement of fluid through the dentinal tubules.
The fluid movement through the tubule distorts the local pulpal environment and is sensed by the free nerve endings in the plexus of Raschkow.
When the cavity is dried with air or cotton wool, a greater loss of fluid is induced, leading to more movement and more pain.
The increased sensitivity at the dentinoenamel junction is explained by the profuse branching of the tubules in this region.

12. Dentinal tubules are composed of odontoblastic processes and dentinal fluid. The dehydration of dentin by air blasts causes outward fluid movement and stimulates the mechanoreceptor of the odontoblast, resulting in dentinal sensitivity.
Suggests that the receptors at the base of odontoblasts are stimulated directly or indirectly by fluid movement through the tubules.

13. Dentinal sclerosis [Transparent dentin]
Occlusion of the dentinal tubules by calcified material resulting in a glassy transparent appearance
The amount of sclerotic dentin increases with age and is most common in the apical third of the root and in the crown midway between the dentinoenamel junction and the surface of the pulp.

14. Usually a protective response to injury, but can occur without a stimulus.
Increases with age, reduces the permeability and prolongs the vitality of the tooth.
Occlusion of the tubules may occur in several other ways:
deposition of mineral within the tubule without any dentin formation ,
a diffuse mineralization that occurs with a viable odontoblast process still present, and
mineralization of the process itself and tubular contents, including intratubular collagen fibrils.

15. Dead tracts Named by Fish 1930 [Opaque dentin or metamorphosed dentin]
They are tubules devoid of a vital odontoblastic process, due to cell death
Caused by injurious stimuli, esp., in larger tubules
Appears dark in transmitted light and white in reflected light due to presence of air.
Not penetrated by the mounting medium

17. Age changes in dentin Increase in sclerotic dentin.
Increase in the number of dead tracts.
Increase in formation of reparative and reactive dentin.
Vitality of dentin

18. Clinical considerations
Inherited dentinal defects
Dentinogenesis imperfecta
Dentin dysplasia
Infective processes
Dental caries
Operative procedures
Exposure
Bonding mechanisms

19. Dentinogenesis Imperfecta