| Matrix metalloproteinases (MMPs) and their specific tissue inhibitors (TIMPs) in mature human odontoblasts and pulp tissue | ||
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Dentin, the most voluminous mineralized connective tissue of the tooth, forms the hard tissue portion of the dentin-pulp complex, whereas the dental pulp is the living, soft connective tissue that retains the vitality of dentin (Linde & Goldberg 1993, Torneck 1994). Dentin contains multiple closely packed dentinal tubules in which the dentinal fluid and the cytoplasmic processes of the cells that have formed the dentin, the odontoblasts, are located (Torneck 1994). These most distinctive cells of the dentin-pulp complex lie along the predentin border forming the peripheral boundary of the dental pulp (see Fig. 1). Under the odontoblasts there is a cell-free zone (the zone of Weil), and beneath that a layer having high cell density (a cell-rich zone). The pulp core, in which the cell density is again reduced, consists of connective tissue, major blood vessels and nerves. The unity of dentin-pulp is responsible for dentin formation and protection of the tooth (Ten Cate 1994a).
Mature odontoblasts, which are located around the pulp chamber as a single cell layer, result from the differentiation of mesenchymal cells of the dental papilla during tooth development (Couve 1986, Ten Cate 1994c). Fully differentiated odontoblasts have withdrawn from the cell-cycle and are, thus, postmitotic cells in nature. Functionally active odontoblasts are polarized, having long cell bodies, which contain a well-developed granular endoplasmic reticulum, many mitochondria, a Golgi apparatus, a nucleus and several secretory vesicles (Linde & Goldberg 1993, Torneck 1994). At the distal end of the cell body, close to the predentin, odontoblasts are attached to each other by intercellular junctions, which enable communication between the cells (Ushiyama 1989, Bishop & Yoshida 1992). Peripheral to the intercellular junctions odontoblast cell processes arise and insert into dentinal tubules, crossing through the predentin zone to the mineralized dentin (Linde & Goldberg 1993). The cell processes lack major organelles involved in protein synthesis, but contain an abundance of longitudinally arranged microfilaments and microtubules. In addition, numerous vesicles reflecting both endo- and exocytosis traffic exist in the processes (Torneck 1994).
The main task of the odontoblasts is to synthesize and secrete collagens and several non-collagenous proteins of which the dentin organic matrix is formed. In addition, odontoblasts secrete signalling molecules, mainly of TGF-β superfamily (Cassidy et al. 1997), which are significant for cellular functionality (Bessho et al. 1991). Odontoblasts control dentin matrix mineralization at least by determining the nature of the extracellular matrix and by controlling the influx of mineral ions (Ten Cate 1985). After completion of primary dentin formation, odontoblasts transit into a resting state and their cell body structure transforms to a smaller and flattened type, with cellular structures changing their conformation or even disappearing (Couve 1986, Ten Cate 1994c). However, odontoblasts remain functional and still secrete and synthesize physiological secondary dentin, but at much slower rate. It is suggested that upon various stimuli, resting odontoblasts are capable of up-regulating their secretory activity and responding by synthesizing tertiary dentin (Ten Cate 1994c).
Pulp organic matrix principally consists of type I and III collagens in a ratio of 55:45. In addition, ground materials such as glycosaminoglycans, glycoproteins and water comprise a supporting medium for cells and also for transportation of nutrients and other metabolites between the cells and vasculature (Torneck 1994).
Fibroblasts are the most abundant cell population of the pulp tissue, especially in the cell-rich zone (Torneck 1994). Their function is to form and maintain the pulp compartment. Other cell types of the pulp tissue are undifferentiated mesenchymal cells, macrophages and other immunocompetent cells. Also, vascular, nervous and lymphatic structures with their respective cell types are present. As a response to specific stimuli, such as caries or attrition, mesenchymal stem cells are believed to have the potential to differentiate into either odontoblast-like cells or fibroblasts (Torneck 1994). Odontoblast-like cells are responsible for the synthesis and secretion of reparative dentin under the circumstances where original odontoblasts are destroyed due to severe dental injury (D’Souza et al. 1995). In the pulpal steady-state condition macrophages are involved in the elimination of dead cells, whereas during inflammation macrophages remove bacteria and interact with other inflammatory cells to protect the pulp (Torneck 1994). Pulp tissue, however, is not responsible for the formation of physiological dentin of the tooth (Linde & Goldberg 1993).