Numerous coated pits and coated vesicles indicative of membrane retrieval and receptor-mediated endocytosis are conspicuous elements of the odontoblastic process.106
A sheath, rich in glycosaminoglycans, separates the process from the surface of the peritubular dentin (Figs 2-8 and 2-9).107,108 This sheath is similar to the lamina limitans found around osteocyte cell processes.109 Thin cytoplasmic side branches, arising from the main odontoblastic process, pierce through the sheath and extend toward adjacent odontoblastic processes.110–112 These small branches of the odontoblastic process contain only fine filaments.
Although it has been suggested that odontoblastic processes might communicate via side branches, there is no evidence that gap junctions are formed between adjacent side branches. If adjacent odontoblastic processes made gap junctional contacts via smaller side branches, their organization would be comparable to the canalicular cell processes of bone, whereby osteocytes intercommunicate through gap junctions.
Fig 2-8 Components of tubular dentin. The odontoblastic process (OP) occupies the dentinal tubule space (DTS) and is surrounded by the lamina limitans (LL). (PTD) Peritubular dentin; (ITD) intertubular dentin.
Fig 2-9 Odontoblastic process (OP) from middle region of the dentin viewed in longitudinal section. The lamina limitans (LL) is closely applied to the plasma membrane of the process. Demineralized peritubular matrix (PTM) abuts the LL. A secretory granule (SG) is present in the process. (Original magnification × 54,000.)
Many investigators who have tried to determine the true length of the odontoblastic process by transmission electron microscopy have concluded that the process does not extend beyond the middle of the dentin.101,103,104,113 In contrast, most investigators who have examined fractured dentin surfaces by scanning electron microscopy have reported that the odontoblastic process extends out to the dentinoenamel junction.110,114–117 Dramatic scanning electron micrographs were obtained of dentin pretreated with hydrochloric acid to remove the mineral phase and with collagenase to remove the collagen fibrils of the organic matrix.117 A remarkable system of branching tubular structures, stretching from the predentin to the dentinoenamel junction, was revealed when these methods were used. The tubular structures were erroneously identified as odontoblastic processes.
The contrasting results obtained by routine transmission electron microscopy and scanning electron microscopy were resolved when it was shown that the structures presumed to represent odontoblastic processes could be removed by digestion with hyaluronidase.107,108,118 This was taken as proof that the tubular structures, erroneously identified as odontoblastic processes, were in fact organic sheaths (lamina limitans) located between the odontoblastic process and the peritubular dentin. The true length of the odontoblastic process in mature dentin remained to be established.
Another approach to this problem was the use of fluorescein-labeled antitubulin as a method for identifying odontoblastic processes. In those studies, it was found that antitubulin was localized along the entire length of the dentinal tubule, suggesting, once again, that the odontoblastic process extended all the way to the dentinoenamel junction.119 An alternate explanation for the positive fluorescence might be that, following retraction or degradation of the odontoblastic process, tubulin might bind to the lamina limitans or to the walls of the dentinal tubules in sufficient amounts to give positive staining.
Confocal microscopic studies of odontoblasts infused with a fluorescent dye have shown that the odontoblastic process in fully developed teeth does not extend to the outer dentin.120,121 The longest processes were found in coronal dentin.
Another explanation put forth to explain the difficulty encountered in establishing the true length of the odontoblastic process is that the process might contract toward the cell body in response to noxious stimuli, such as the fixatives used to preserve cells prior to routine electron microscopy. This idea received support in studies of teeth that had been frozen rapidly to immobilize cytoplasmic structures prior to chemical tissue fixation.122 When this approach was used, structures resembling odontoblastic processes were observed in dentin near the dentinoenamel junction. Additional research is needed to explore the interesting possibility that the odontoblastic process contracts in response to stimuli in its immediate environment.
Dentinal tubules
Dentinal tubules extend from the mantle dentin to the predentin, across the full thickness of circumpulpal dentin (see Fig 2-1). The distal end of the dentinal tubule branches extensively into fine secondary tubules that permeate the mantle dentin.123,124 Small side branches extend from tubule to tubule in the circumpulpal dentin.111,124 Dentinal tubules are wider toward the pulp and generally more concentrated in the region of the pulp horns.102,125
Some dentinal tubules appear to be obliterated by nonmineralized collagen fibrils, while others are blocked with mineral.8,126,127 The physiology of dentin permeability is to a great degree a function of the patency of the tubules.108,128,129 Dentinal tubules contain serum proteins, including fibrinogen, albumin, and immunoglobulins.130–132 These proteins are carried into the tubules in dentinal fluid, where they may become trapped in the lamina limitans or bound to the mineral phase of dentin. The flow of dentinal fluid increases following dental operative procedures and during pulpal inflammation.133
Carious dentin contains higher levels of immunoglobulins. It has been suggested that the high affinity of antibody binding to hydroxyapatite may serve as a protective reservoir of antibacterial immunoglobulin.134
The first description of tubular “canals” in dentin was made by Leeuwenhoek in the 17th century. He fabricated simple microscopes capable of magnifications of no more than × 100. With these rudimentary instruments, he amused his contemporaries by demonstrating microscopic canals in dentin, and animalcules (bacteria) in saliva. More than 300 years later, with microscopes capable of achieving magnifications of more than × 1,000,000, there are still new frontiers to explore in the structure of dentin.
Formation of Intertubular and Peritubular Dentin
The greater bulk of the mineralized circumpulpal dentin is called intertubular dentin (see Fig 2-1). It is formed by the mineralization of predentin. The matrix of intertubular dentin is rich in type I collagen fibrils. The uniform size and the arrangement of the collagen fibrils are best viewed in scanning electron micrographs (see Figs 2-6a and 2-6b).43 Hydroxyapatite crystals, about 40 nm in length, are formed in and around the collagen fibrils of the intertubular dentin.
A second and minor component of circumpulpal dentin, the peritubular dentin, develops around the odontoblastic process (see Fig 2-1). The organic matrix of peritubular dentin is rich in glycosaminoglycans and relatively free of collagen fibrils. Bone sialoprotein and osteonectin have been localized in peritubular dentin. The hydroxyapatite