Pathway 13-2 Cells of the Pulp PDF

514 PART II Advanced Science Topics Cells of the Pulp ODONTOBLAST Because odontoblasts are responsible for dentinogenesis, both...

514 PART II Advanced Science Topics Cells of the Pulp ODONTOBLAST Because odontoblasts are responsible for dentinogenesis, both during tooth development and aging, the odontoblast is the most characteristic and specialized cell of the dentin- pulp complex. During dentinogenesis, the odontoblasts form dentin and the dentinal tubules, and their presence within the tubules makes dentin a living responsive tissue. ​  A  13 Structure and Functions of the Dentin-Pulp Complex 515 Dentinogenesis, osteogenesis, and cementogenesis are in through canaliculi, just as osteocytes and cementocytes are many respects quite similar, and odontoblasts, osteoblasts, linked through the canaliculi in bone and cementum. This and cementoblasts have many characteristics in common. provides a pathway for intercellular communication and Each of these cells produces a matrix composed of collagen circulation of fluid and metabolites through the mineral- fibrils, noncollagenous proteins, and proteoglycans that are ized matrix. capable of undergoing mineralization. The ultrastructural The ultrastructural features of the odontoblast have been characteristics of odontoblasts, osteoblasts, and cemento- the subject of numerous investigations. The cell body of the blasts are likewise similar in that each exhibits a highly or- active odontoblast has a large nucleus that may contain up dered rough endoplasmic reticulum (RER), a prominent to four nucleoli (Fig. 13.4). The nucleus is situated at the Golgi complex, secretory granules, and numerous mito- basal end of the cell and is contained within a nuclear en- chondria. In addition, these cells are rich in RNA, and their velope. A well-developed Golgi complex, centrally located in nuclei contain one or more prominent nucleoli. These are the supranuclear cytoplasm, consists of an assembly of the general characteristics of protein-secreting cells. smooth-walled vesicles and cisternae. Numerous mito- The most significant differences among odontoblasts, chondria are evenly distributed throughout the cell body. osteoblasts, and cementoblasts are their morphologic char- RER is particularly prominent, consisting of closely stacked acteristics and the anatomic relationship between the cells cisternae forming parallel arrays that are dispersed dif- and the mineralized structures they produce. Whereas os- fusely within the cytoplasm. Numerous ribosomes closely teoblasts and cementoblasts are polygonal to cuboidal in associated with the membranes of the cisternae mark the form, the fully developed odontoblast of the coronal pulp is sites of protein synthesis. Within the lumen of the cister- a tall columnar cell.63,245 In bone and cementum, some of nae, filamentous material (probably representing newly the osteoblasts and cementoblasts become entrapped in the synthesized protein) can be observed. matrix as osteocytes or cementocytes, respectively. The The odontoblast appears to synthesize mainly type I col- odontoblast, on the other hand, leaves behind a cellular lagen,212,408 although small amounts of type V collagen process to form the dentinal tubule, and the cell body re- have been found in the extracellular matrix (ECM). In addi- sides outside the mineralized tissue. Lateral branches be- tion to proteoglycans39,85,125 and collagen,212,224 the odonto- tween the major odontoblast processes interconnect180,258 blast secretes dentin sialoprotein41 and phosphophoryn,41,70 Predentin Microtubule Secretory vesicle Lysosome Mitochondria Centriole Golgi complex RER Cilia Nucleus Nucleolus Fig. 13.4 ​Diagram of a fully differentiated odontoblast. RER, Rough endoplasmic reticulum. 516 PART II Advanced Science Topics a highly phosphorylated phosphoprotein involved in extra- follow a course that is parallel with the long axis of the cell cellular mineralization.41,78 Phosphophoryn is unique to and impart the impression of rigidity. Although their dentin and is not found in any other mesenchymal cell precise role is unknown, theories as to their functional sig- types.78 The odontoblast also secretes both acid phosphatase nificance suggest that they may be involved in cytoplasmic and alkaline phosphatase. The latter enzyme is closely linked extension, transport of materials, or the provision of a to mineralization, but the precise role of alkaline phospha- structural framework. Occasionally, mitochondria can be tase in dentinogenesis is not completely understood. Acid found in the process where it passes through the predentin. phosphatase, a lysosomal enzyme, may be involved in digest- The plasma membrane of the odontoblast process closely ing resorbed material from the predentin matrix.89 approximates the wall of the dentinal tubule. Localized con- In contrast to the active odontoblast, the resting or inac- strictions in the process occasionally produce relatively large tive odontoblast has a decreased number of organelles and spaces between the tubule wall and the process. Such spaces may become progressively shorter.63,245 These changes can may contain collagen fibrils and fine granular material that begin with the completion of root development and erup- presumably represents ground substance (see also The Pulpal tion when dentin production shifts from primary to second- Interstitium and Ground Substance in this chapter). The peri- ary dentin. tubular dentin matrix within the tubule is lined by an elec- Direct actions of odontoblasts on dental nerves and vice tron-dense limiting membrane called the lamina limitans.265, 361,405 versa have been proposed based on the excitability of odon- A narrow space separates the limiting membrane from toblasts, the differential expression of receptors for neuro- the plasma membrane of the odontoblast process, except for peptides on odontoblasts (Fig. 13.5), the demonstration of the areas where the process is constricted. the thermosensitive transient receptor potential (TRP) ion In restoring a tooth, the removal of enamel and dentin channels, and the finding that all nine voltage-gated so- often disrupts odontoblasts.42,46,74,81,206,262 It would be of dium channels are variably expressed on odontoblasts in considerable clinical importance to establish the extent of developing, mature, and aging rat teeth.53,72,84,109,236,237 In the odontoblast processes in human teeth, because with addition, a possible function of odontoblast in immune this knowledge the clinician would be in a better position to regulation has been proposed by the finding of innate im- estimate the impact of the restorative procedure on the un- mune components in the odontoblast layer.387 Hence the derlying odontoblasts. However, the extent to which the odontoblasts should be capable of recognizing and differen- process penetrates into dentin has been a matter of consid- tially responding to bacterial components, thereby serving erable controversy. It has long been thought that the pro- immune and pulp-dentine barrier functions. cess is present throughout the full thickness of dentin. Although ultrastructural studies using transmission elec- tron microscopy have described the process as being limited ODONTOBLAST PROCESS to the inner third of the dentin,49,113,361,405 it should be A dentinal tubule forms around each of the major odonto- noted that this could possibly be the result of shrinkage oc- blastic processes. The odontoblast process occupies most of curring during fixation and dehydration. Other studies em- the space within the tubule and coordinates the formation ploying scanning electron microscopy have described the of peritubular dentin. process extending further into the tubule, often as far as the Microtubules and microfilaments are the principal ultra- dentoenamel junction (DEJ),132,181,343,419 but it has been structural components of the odontoblast process and its suggested that what has been observed in scanning elec- lateral branches.112,160 Microtubules extend from the cell tron micrographs is actually the lamina limitans.361,362,405 body out into the process.112 These straight structures In an attempt to resolve this issue, monoclonal antibod- ies directed against microtubules were used to demonstrate tubulin in the microtubules of the process. Immunoreactiv- ity was observed throughout the dentinal tubule, suggest- ing that the process extends throughout the thickness of dentin.343 However, a study employing confocal microscopy found that odontoblast processes in rat molars do not ex- Dentin Odontoblast tend to the outer dentin or DEJ, except during the early Odontoblast process stages of tooth development.49 It is likely that the walls of Nucleus tubules contain many proteins originally derived from odontoblasts that no longer remain at that site. Because dentin matrix has no turnover, these antigens remain fixed in place. From a clinical perspective, it is important to remember that these processes in the tubules represent appendages from living odontoblasts in the pulp, which explains why the dentin must be considered a vital tissue, the destruction of which will affect the pulp. The odontoblast is considered to be a fixed postmitotic cell Fig. 13.5 ​Confocal microscopic image showing an odontoblast with its process expressing neurokinin 2 receptor. Neurokinin 2 has affinity to all in that once it has fully differentiated, it apparently cannot the neuropeptides of the neurokinin family. The dotted line represents undergo further cell division. If this is indeed the case, the life the border of the predentin.  (From Fristad I, Vandevska-Radunovic V, Fjeld span of the odontoblast coincides with the life span of the K, et al: NK1, NK2, NK3 and CGRP1 receptors identified in rat oral soft tissues, viable pulp. However, its metabolic activity can be dynami- and in bone and dental hard tissue cells, Cell Tissue Res 311:383–391, 2003.) cally altered (described under the heading Pulpal Repair).  13 Structure and Functions of the Dentin-Pulp Complex 517 RELATIONSHIP OF ODONTOBLAST STRUCTURE PULP FIBROBLAST TO SECRETORY FUNCTION Fibroblasts are the most numerous cells of the pulp. They Studies using radiolabeled chemicals have shed light on the appear to be tissue-specific cells capable of giving rise to functional significance of the cytoplasmic organelles of the cells that are committed to differentiation (e.g., odonto- active odontoblast.407,408 In experimental animals, intra- blast-like cells) if given the proper signal. These cells syn- peritoneal injection of a collagen precursor (e.g., 3H-proline) thesize type I and III collagen, as well as proteoglycans and was followed by autoradiographic labeling of the odonto- GAGs. Thus they produce and maintain the matrix proteins blasts and predentin matrix (Fig. 13.6).408 Rapid incorpora- of the ECM. Because they are also able to phagocytose and tion of the isotope in the RER soon lead to labeling of the digest collagen, fibroblasts are responsible for collagen Golgi complex in the area where the procollagen was packed turnover in the pulp. and concentrated into secretory vesicles. Radiolabeled vesi- Although distributed throughout the pulp, fibroblasts cles passed along their migration pathway until they reach are particularly abundant in the cell-rich zone. The early the base of the odontoblast process. Here they fused with the differentiating fibroblasts are polygonal and appear to be cell membrane and released their tropocollagen molecules widely separated and evenly distributed within the ground into the predentin matrix by the process of exocytosis. substance. Cell-to-cell contacts are established between the It is now known that collagen fibrils precipitate from a so- multiple processes that extend out from each of the cells. lution of secreted tropocollagen and that the aggregation of Many of these contacts take the form of gap junctions that fibrils occurs on the outer surface of the odontoblast plasma provide for electronic coupling or chemical signaling from membrane. Fibrils are released into the predentin and in- one cell to another. In terms of ultrastructure, the organ- crease in thickness as they approach the mineralized matrix. elles of the immature fibroblasts are generally in a rudi- Whereas fibrils at the base of the odontoblast process are ap- mentary stage of development, with an inconspicuous proximately 15 nm in diameter, fibrils in the region of the Golgi complex, numerous free ribosomes, and sparse RER. calcification front have attained a diameter of about 50 nm. As they mature, the cells become stellate in form, and the Similar tracer studies407 have elucidated the pathway of Golgi complex enlarges, the RER proliferates, secretory ves- synthesis, transport, and secretion of the predentin proteo- icles appear, and the fibroblasts take on the characteristic glycans. The protein moiety of these molecules is synthe- appearance of protein-secreting cells. In addition, collagen sized by the RER of the odontoblast, whereas sulfation and fibrils accumulate along the outer surface of the cell body. addition of the glycosaminoglycan (GAG) moieties to the With an increase in the number of blood vessels, nerves, protein molecules take place in the Golgi complex. Secre- and collagen fibers, there is a relative decrease in the num- tory vesicles then transport the proteoglycans to the base of ber of fibroblasts in the pulp. the odontoblast process, where they are secreted into the Many fibroblasts of the pulp are characterized by being predentin matrix. Proteoglycans, principally chondroitin relatively undifferentiated. A more modern term for these sulfate, accumulate near the calcification front. The role of undifferentiated cells is MSC. MSC derived from different the proteoglycans is speculative, but mounting evidence tissues appear to be tissue specific and are responsible for suggests that they act as inhibitors of calcification by bind- maintenance of specific tissue functions. However, MSC ing calcium. It appears that just before calcification, the share some common characteristics, as they are able to dif- proteoglycans are removed, probably by lysosomal enzymes ferentiate into other cell lineages when given the proper secreted by the odontoblasts.86 signals. Like MSC from bone and adipose tissues, pulpal MSC proliferate well in culture (Fig. 13.7), and when stimu- lated they have neurogenic, adipogenic, chondrogenic, and Fig. 13.6 ​Autoradiograph demonstrating odontoblasts and predentin in a developing rat molar 1 hour after intraperitoneal injection of Fig. 13.7 ​Morphology of cultured pulpal cells. Many of the cells will 3 H-proline. have mesenchymal stem cell characteristics. 518 PART II Advanced Science Topics osteogenic differentiation potential.282 Many pulpal cells extravasated red blood cells, dead cells, and foreign bodies seem to remain in a relatively undifferentiated modality, from the tissue. Ingested material is destroyed by the action compared with fibroblasts of most other connective tis- of lysosomal enzymes. Another subset of macrophages sues.138 This perception has been supported by the observa- participates in immune reactions by processing antigen and tion of large numbers of reticulin-like fibers in the pulp. presenting it to memory T cells.287 The processed antigen is Reticulin fibers have an affinity for silver stains and are bound to class II major histocompatibility complex (MHC) similar to the argyrophilic fibers of the pulp. However, in a molecules on the macrophage, where it can interact with careful review, it appears that actual reticulin fibers may specific receptors present on naive or memory T cells not be present in the pulp; instead the previously described (Fig. 13.8).135 Such interactions are essential for T-cell– fibers are actually argyrophilic collagen fibers.15 The fibers dependent immunity. Similar to fibroblasts, macrophages apparently acquire a GAG sheath, and it is this sheath that take an active part in the signaling pathways in the pulp. is impregnated by silver stains. In the young pulp, the non- When activated by the appropriate inflammatory stimuli, argyrophilic collagen fibers are sparse, but they progres- macrophages are capable of producing a large variety of sively increase in number as the pulp ages. soluble factors, including interleukin (IL) 1, tumor necrosis Many experimental models have been developed to study factor (TNF), growth factors, and other cytokines. One wound healing in the pulp, particularly dentinal bridge study showed that a subset of macrophages express lym- formation after pulp exposure or pulpotomy. One study100 phatic markers, indicating a link between macrophages demonstrated that mitotic activity preceding the differenti- and lymphatic function and development.19 ation of replacement odontoblasts appears to occur primar- ily among perivascular fibroblasts. DENDRITIC CELL Pulpal fibroblasts seem to take active part in signaling pathways in the dental pulp. For example, fibroblast growth Dendritic cells are accessory cells of the immune system. and synthesis are stimulated by neuropeptides; in turn, fi- Similar cells are found in the epidermis and mucous mem- broblasts produce nerve growth factor (NGF) and proin- branes, where they are called Langerhans cells.175,286 Den- flammatory cytokines during inflammation.32,415,420 NGF dritic cells are primarily found in lymphoid tissues, but they plays an important role not only in tooth development but are also widely distributed in connective tissues, including also in regulating neuronal and possibly odontoblast re- the pulp (Fig. 13.9).325 These cells are termed antigen- sponses to injury via activation of similar neurotrophin re- presenting cells and are characterized by dendritic cytoplas- ceptors expressed on both cell types (see also Plasticity of mic processes and the presence of class II MHC complexes Intradental Nerve Fibers in this chapter).415 on their cell surface. In the normal pulp they are mostly located in the periphery of the coronal pulp close to the predentin, but they migrate centrally in the pulp after anti- MACROPHAGE genic challenge.428 They are known to play a central role in Macrophages are monocytes that have left the bloodstream, the induction of T-cell–dependent immunity. Like antigen- entered the tissues, and differentiated into various subpop- presenting macrophages, dendritic cells engulf protein an- ulations. The different subpopulations can be studied by tigens and then present an assembly of peptide fragments their antigenic properties in immunohistochemical studies. of the antigens and MHC class II molecules. It is this assem- Many are found in close proximity to blood vessels. A major bly that T cells can recognize. Then the assembly binds to a subpopulation of macrophages is active in endocytosis and T-cell receptor and T-cell activation occurs (see Fig. 13.8). phagocytosis. Because of their mobility and phagocytic Fig. 13.10 shows a cell-to-cell contact between a dendritic- activity, they are able to act as scavengers, removing like cell and a lymphocyte. T cell Helper T cell receptor MHC class II (self) Activation Antigen peptide Antigen-presenting cell (not self) Antigen Fig. 13.8 Function of major histocompatibility complex (MHC) class II molecule-expressing cells. They act as antigen-presenting cells that are essential for the induction of helper T-cell-dependent immune responses.  13 Structure and Functions of the Dentin-Pulp Complex 519 D OB Fig. 13.9 ​Class II antigen-expressing dendritic cells in the pulp and dentin border zone in normal human pulp, as demonstrated by immunocytochem- istry. D, Dentin; OB, odontoblastic layer. Fig. 13.10 ​Immunoelectron micrograph of a cell resembling a dendritic cell and a lymphocyte. They show cell-to-cell contact. anticoagulant, and histamine, an important inflammatory LYMPHOCYTE mediator, as well as many other chemical factors. Hahn and colleagues135 reported finding T lymphocytes in normal pulps from human teeth. T8 (suppressor) lympho- cytes were the predominant T-lymphocyte subset present in these samples. Lymphocytes have also been observed in the pulps of impacted teeth.205 The presence of macrophages, dendritic cells, and T lymphocytes indicates that the pulp is well equipped with cells required for the initiation of im- mune responses.175,325 B lymphocytes are scarcely found in the normal uninflamed pulp. MAST CELL Mast cells are widely distributed in connective tissues, where they occur in small groups in relation to blood ves- sels. Mast cells are seldom found in the normal pulp tissue, although they are routinely found in chronically inflamed pulps.325 The mast cell has been the subject of considerable attention because of its dramatic role in inflammatory reactions. The granules of mast cells contain heparin, an