It has also been speculated that the odontoblast (and its process) might act as a transducer to convert noxious stimuli into nerve impulses. This concept is based on the notion that odontoblasts make gap junctions (electronic synapses) with adjacent nerves and that the flow of ions through these junctions, in response to changes in the odontoblasts, could lead to depolarization of somatosensory neurons. Patch-clamp recordings made on segments of plasma membrane from isolated odontoblasts have demonstrated potassium and chloride channels and a resting membrane potential of about –40 to –50 mV.151 It has been suggested that mechanosensitive ion channels could lead to changes in ion conductance across the plasma membrane in response to hydrodynamic forces exerted on the odontoblastic process.152 Although gap junctions between odontoblasts and nerves have been reported, there is still no proof that odontoblasts communicate electrically with nerve endings.
A subpopulation of cells cultured from human dental pulp have voltage-gated sodium channels and other properties associated with neuronal satellite cells.153 Whether these pulp cells originate from the odontoblastic layer and whether or not they have a role in pulpal somatosensation remains to be established. Additional discussion of dentinal and pulpal sensory mechanisms is contained in chapter 10.
It has also been suggested that dentinal nerves may have an effector function on odontoblasts. Indirect evidence for an effector activity includes the fact that many dentinal nerves contain the neuropeptide, calcitonin gene–related peptide (CGRP).154 Evidence of protein secretion from dentinal nerves has also been reported.142 Recent studies of the presence of several exocytosis regulatory proteins (synapsin and synaptogamin) in dentinal nerve endings adds more support that dentinal nerves have an effector function in addition to their somatosensory afferent actions.155
Many of the nerve endings in the odontoblastic layer and predentin contain substance P and CGRP.144,156–158 These neuropeptides may be involved in vascular dilation and neurogenic inflammation.159–160 Indirect evidence supports the idea that the release of neuropeptides from dental sensory nerve fibers is important in the recruitment of immunocompetent cells to the dental pulp.161 Experimental studies also suggest that these neuropeptides may promote dentinogenesis.162,163
The potential for development of neurogenic inflammation in pulp is supported by the demonstration that coronal and apical pulp contain CGRP-positive nerves in association with blood vessels and within the connective tissue stroma.159,160,164 Some CGRP-positive nerve fibers are found in the subodontoblastic layer. Sprouting of CGRP-positive nerve endings occurs following dental injury.165,166 The release of CGRP increases vascular permeability of pulpal blood vessels.167 Recent studies have demonstrated excitatory amino acid receptors in bovine dental pulp.168 Activation of excitatory amino acid receptors leads to the release of CGRP in pulp.
Blood vessels enter the tooth through the apical foramen and course coronally in the midregion of the radicular pulp. The largest arteries have a muscular coat of three to six layers of smooth-muscle cells. The outer adventitia is rather inconspicuous, because it blends gradually with the pulpal connective tissue. The endothelial cells that line the arteries often appear to bulge into the lumen. The accompanying veins have one to two layers of smooth-muscle cells and a wider lumen. As the major vessels course through the radicular pulp, they give off peripheral branches that arborize to form a rich capillary network associated with the odontoblastic layer.169,170 The greatest degree of branching occurs in the coronal pulp, especially below the cusps.
The endothelial lining is of the continuous type, except for the fenestrated capillaries that are adjacent to the predentin. The cell-to-cell junctions of the endothelium are characterized by adherens junctions and overlapping cell processes. Numerous pynocytotic vesicles are present on the luminal and abluminal surfaces of the endothelium. The luminal surface of capillaries and venules contain many cytoplasmic processes. Spindle-shaped pericytes contain moderately high numbers of cytoplasmic filaments and are spaced apart as single cells in close contact with the basal lamina of the endothelial cells. The pericytes serve as stem cells capable of multipotential differentiation. Recent studies have shown that a population of dendritic cells is associated with the major pulpal blood vessels.171,172 These cells are specialized for phagocytosis, processing, and presentation of antigens.
Blood is supplied to the odontoblastic layer by capillaries that are in close apposition to the odontoblast cell bodies and the predentin (Figs 2-11a to 2-11c).173–175 Individual capillaries penetrate the intercellular spaces between the odontoblasts to form the predentinal capillary plexus. The predentinal capillary plexus reaches a peak of development coincident to the most active phase of dentin formation. Fenestrated endothelial linings have been reported in capillaries located close to the predentin.169 The close proximity of thin-walled capillaries to the odontoblasts and the predentin suggests that there is a high requirement for oxygen, ions, and metabolites during the rapid phase of dentin formation. When dentin formation is completed, the predentinal capillary plexus is no longer present. At this stage, nutrients reach the odontoblasts from the subodontoblastic plexus.
Reparative dentinogenesis is preceded by angiogenesis. Several angiogenic growth factors (platelet-derived growth factor, vascular endothelial growth factor, and fibroblast growth factor) have been isolated from dentin matrix.176 It has been proposed that angiogenic growth factors are released during dentin degradation, thereby stimulating the development of new blood vessels in the zone of injury.176
Figs 2-11a to 2-11c Blood supply to the pulp. (Adapted from Ohshima and Yoshida169 with permission from Springer-Verlag.) (a) Electron micrograph illustrating the presence of capillaries (CL) in the odontoblastic (OB) layer. (Original magnification × 3,000.) (b) Left inset area in Fig 2-11a at higher magnification. Capillaries (CL) are generally of the fenestrated type near the predentin (arrowheads). (Original magnification × 8,000.) (c) Right inset area in Fig 2-11a at higher magnification. Capillaries (CL) are generally of the continuous type nearer to the pulp. (Original magnification × 8,000.)
Cells and Extracellular Matrix of the Dental Pulp
The dental pulp is a connective tissue derived from the proliferation and differentiation of the cells of the dental papilla. In its developmental stage, the dental pulp contains a relatively high content of glycosaminoglycans and sparsely distributed, fine collagen fibrils (types I and III).177,178 Initially, the network of collagen fibers is composed mostly of argyrophilic reticular fibers rich in type III collagen. As the pulp matures, the synthetic capacity of pulpal fibroblasts increases, and more collagen bundles of type I collagen are formed. Despite the increased amount of type I collagen, the mature pulp continues to have an unusually high content of type III collagen.179 Although the numbers of collagen fibers continue to increase with age, the pulp maintains