Cohen's Pathways of the Pulp - 12th Edition - PDF

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 i...

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.