Morphologic Zones of the Pulp PDF

Morphologic Zones of the Pulp ODONTOBLAST LAYER The outermost stratum of cells of the healthy pulp is the THE PULP-DENTIN COMPLEX...

Morphologic Zones of the Pulp ODONTOBLAST LAYER The outermost stratum of cells of the healthy pulp is the THE PULP-DENTIN COMPLEX odontoblast layer (Figs. 13.1 and 13.2). This layer is located The dental pulp and dentin function as a unit and the odon- immediately subjacent to the predentin. The odontoblast toblasts represent a crucial element in this system. The odon- processes, however, pass on through the predentin into the toblasts are located in the periphery of the pulp tissue, with inner part of dentin. Consequently, the odontoblast layer is extensions into the inner part of dentin. Dentin would not actually composed of the cell bodies of odontoblasts. In ad- exist unless produced by odontoblasts, and the dental pulp is dition, capillaries, nerve fibers, and dendritic cells may be dependent upon the protection provided by the dentin and found among the odontoblasts. enamel. Likewise, integrated dynamics of the pulp-dentin In the coronal portion of a young pulp that is actively complex imply that impacts on dentin may affect the pulpal secreting collagen, the odontoblasts assume a tall colum- components and that disturbances in the dental pulp will in nar form.63 The odontoblasts vary in height; consequently, turn affect the quantity and quality of the dentin produced. their nuclei are not all at the same level and are aligned in a staggered array, often described as a palisade appearance. * This organization makes the layers appear to be three to five The authors acknowledge the outstanding work of Drs. Henry Trowbridge, Syngcuk Kim, Hideaki Suda, David H. Pashley, and Fredrick cells in thickness even though there is only one actual layer R. Liewehr in previous editions of this text. The present chapter is built of odontoblasts. Between adjacent odontoblasts there are on their foundational work. small intercellular spaces approximately 30 to 40 nm in 512  13 Structure and Functions of the Dentin-Pulp Complex 513 ​ ​ width. Odontoblast cell bodies are connected by tight and in the crown of the tooth, the odontoblast cell bodies are gap junctional complexes.29 63,162 Gap junctions are formed less crowded and are able to spread out laterally.245 During by connexin proteins115 that permit cell-to-cell passage of maturation and aging, there is a continued ongoing crowd- signal molecules. ing in the odontoblast layer, particularly in the coronal The odontoblast layer in the coronal pulp contains more pulp, due to narrowing of the pulp space. Apoptosis of cells per unit area than in the radicular pulp.245 Whereas odontoblasts seems to adjust for this limited space during the odontoblasts of the mature coronal pulp are usually development.257 columnar, those in the midportion of the radicular pulp are There is a series of specialized cell-to-cell junctions (i.e., more cuboidal.63 Near the apical foramen, the odontoblasts junctional complexes), including desmosomes (i.e., zonula appear as a squamous layer of flattened cells. Because fewer adherens), gap junctions (i.e., nexuses), and tight junctions dentinal tubules per unit area are present in the root than (i.e., zonula occludens) that connect adjacent odontoblasts. 514 PART II Advanced Science Topics Spot desmosomes located in the apical part of odontoblast more central region of the pulp (see Fig. 13.1). It is much cell bodies mechanically join odontoblasts together. The nu- more prominent in the coronal pulp than in the radicular merous gap junctions provide permeable pathways through pulp. Besides fibroblasts, the cell-rich zone may include a vari- which signal molecules can pass between cells (Fig. 13.3) to able number of immune cells like macrophages and dendritic synchronize secretory activity that produces relatively uni- cells but also undifferentiated mesenchymal stem cells (MSCs). form predentin layers (see Fig. 13.2). These junctions are On the basis of evidence obtained in rat molar teeth, it has most numerous during the formation of primary dentin. been suggested128 that the cell-rich zone forms as a result of Gap junctions and desmosomes have also been observed peripheral migration of cells populating the central regions joining odontoblasts to the processes of fibroblasts in the of the pulp, commencing at about the time of tooth erup- subodontoblastic area. Tight junctions are found mainly in tion. Migration of immunocompetent cells out of and into the apical part of odontoblasts in young teeth. These struc- the cell-rich zone has been demonstrated as a result of anti- tures consist of linear ridges and grooves that close off the genic challenge.428 Although cell division within the intercellular space. However, tracer studies suggest direct cell-rich zone is rare in normal pulps, the death of the odon- passage of small elements from subodontoblastic capillaries toblasts triggers a great increase in the rate of mitosis. Be- to predentin and dentin between the odontoblasts.359 It ap- cause odontoblasts are postmitotic cells, irreversibly injured pears that tight junctions determine the permeability of the odontoblasts are replaced by cells that migrate from the cell- odontoblast layer when dentin is covered by enamel or ce- rich zone onto the inner surface of the dentin.100 This mi- mentum by restricting the passage of molecules, ions, and totic activity is probably the first step in the formation of a fluid between the extracellular compartments of the pulp new odontoblast layer.74,261,263,345 Studies implicate stem and predentin.29 During cavity preparation, these junctions cells as a source for these replacement odontoblasts.353 are disrupted, thereby increasing dentin permeability.380,381 PULP PROPER CELL-POOR ZONE The pulp proper is the central mass of the pulp (see Fig. 13.1). Immediately subjacent to the odontoblast layer in the coro- It consists of loose connective tissue and contains the larger nal pulp, there is often a narrow zone approximately 40 mm blood vessels and nerves. The most prominent cell in this zone in width that is relatively free of cells (see Fig. 13.1) and is the fibroblast. hence is called the cell-free layer of Weil. It is traversed by blood capillaries, unmyelinated nerve fibers, and the slen- der cytoplasmic processes of fibroblasts (see Fig. 13.2). The presence or absence of the cell-poor zone depends on the functional status of the pulp.63 It may not be apparent in young pulps, where dentin forms rapidly, or in older pulps, where reparative dentin is being produced. CELL-RICH ZONE In the subodontoblastic area, there is a stratum containing a relatively high proportion of fibroblasts compared with the M ​