resides within a dentinal tubule, which is like a
capillary tube with a diameter that is much smaller than
that of an erythrocyte. Microtubules and microfilaments
are the principal components of the process, providing
infrastructure for transportation from the cell body to
the remote cell process.
In addition to a role in forming dentine, odontoblasts
may be involved in sensory transduction.
The presence of tight, adhering and gap junctions may
imply that these cells communicate with each other;
and if one is affected, many others are also affected.
Gap junctions exist between and among odontoblasts
and nerve fibres,
and they provide a pathway of low
electrical resistance between and among the odonto-blasts and nerve fibres.
The hydrodynamic effects of
fluid displacement within the dentinal tubules or the
odontoblasts may activate mechanoreceptors of sensory
nerve axons.
The odontoblast itself may be capable of
mechanotransduction by stretch-activated ion channels
in the cell membrane.
Odontoblasts are also implicated
in the regulation of pulp blood flow and in the
development of pulp inflammation. The enzyme
NADPH-diaphorase involved in the production of
nitric oxide, a potent vasodilator, is present in the
odontoblasts.
Their capacity to synthesize the
inflammatory mediator PGI2
has been demonstrated
and this may excite nerves in the vicinity resulting in a
brief hyperalgesia.
Although there is abundant information on the
structural aspects of odontoblasts, very little is known
about the dynamic aspect of these cells, especially in the
mature pulp. Forming and maintaining dentine involves
active transportation of calcium ions, collagen
precursors or components of the extracellular matrix
from the pulp proper to the long process,
an activity
that presumably requires energy and hence oxygen. An
in vitro respiratory study using the direct method of
Warburg has demonstrated significantly higher oxygen
uptake in the peripheral regions of bovine molar pulps,
indicating that odontoblasts may have a high oxidative
metabolism.
A study with an in vitro culture system
has demonstrated that a large amount of oxygen is
essential for maintaining proper functions of odonto-blasts.
Using oxygen-sensitive micro-electrodes, it has
been shown that odontoblast cells consume a relatively
high amount of oxygen in the rat incisor pulp in vivo .
The average oxygen consumption rate of the odonto-blasts
obtained from that study is 3.2mL/O2/min/100g
tissue, which is comparable with that of the brain.
Furthermore, a transmission electron microscope study
has shown that odontoblasts are the cells most sensitive
to ischaemia.
Odontoblasts in the pulp horn of rat
molars with experimentally-induced hypoxia retain
tritiated misonidazole, a marker preferentially labeling
cells with hypoxia.
The earliest signs of pulp reaction to insults (such as
dental caries) are morphological changes and an over-all
reduction in the number and size of odontoblast cell
bodies.
The disruption in the underlying odontoblast
cell layer occurs even before the appearance of
inflammatory changes in the pulp.
An electromicro-scopic study on the ultrastructural changes of ischaemic
pulps induced experimentally by extraction has shown
that distinct cellular changes, such as chromatin
clumping, irregular nuclear membrane and swollen
mitochondria, appear in odontoblasts as early as one
hour after extraction.
Although no explanation has
been offered for the vulnerability of odontoblasts to
insults, it can be speculated that the lack of oxygen due
to circulatory disturbance during pulp inflammation
may be the main contributing factor.