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 »  Home  »  Endodontic Articles 3  »  The dentinogenic effect of mineral trioxide aggregate (MTA) in short-term capping experiments
The dentinogenic effect of mineral trioxide aggregate (MTA) in short-term capping experiments
Discussion - References.



Discussion.
Many clinical and experimental studies have shown that mature dental pulp cells possess the ability to differentiate into a specific cell lineage forming tubular dentine in the absence of normal developmental conditions, i.e. dental epithelium and basement membrane (Baume 1980). This phenomenon takes place stereotypically, as an intrinsic defensive mechanism, in the repairing of pulp environment (Kakehashi et al. 1965, Yamamura 1985, Fitzgerald et al. 1990). The dentinogenic potential of dental ectomesenchymal cells is progressively expressed as a part of the natural healing process in the mature teeth (Baume 1980). It has been well recognized that dentinogenic activity can be observed in exposed pulps without any exogenous application (Kakehashi et al. 1965, Cox et al. 1987, Tsuji et al. 1987, Inoue & Shimono 1992). The superficial zone of extracellular matrix, which is stereotypically formed at the wound surface of the repairing connective tissues, is physiologically followed in exposed dental pulp by hard tissue deposition. A layer of pulp-specific formative cells (odontoblast-like cells) producing reparative dentine is finally formed as a sign of normalized pulp function. As odontoblast-like cells are recognized (in morphological terms), the elongated pulpal cells with increased cytoplasm/nucleus ratio and polarized nuclei, which form tubular matrix in a polar predentinelike pattern (Baume 1980, Lesot et al. 1994, Tziafas 1997).

Intergrown crystalline depositions onto the surface of MTA
Figure 7. SEM micrograph 2 weeks after application of MTA on mechanically exposed pulp of dog. Intergrown crystalline depositions onto the surface of MTA (x2000).

Two-layered bridge formation consisting from osteodentine
Figure 8. Light microscopy micrograph 3 weeks after application of MTA on mechanically exposed pulp of dog. Two-layered bridge formation consisting from osteodentine (arrows) and reparative dentine matrix (arrowheads) formation along the pulpal side of MTA (Hematoxylin-eosin, x40).

During the natural wound healing process in dental pulp, odontoblast-like cell differentiation and reparative dentine formation occurred in association with osteodentine or fibrodentine hard tissue formation (Baume 1980, Ruch 1985). This primitive type of pulp biomatrix seems to control the differentiation of pulpal cells into odontoblast-like cells and initiation of reparative dentine formation, substituting the dental epithelium and basement membrane to provide the necessary molecular influences (Ruch 1985, Lesot et al. 1994, Tziafas 1997). This mechanism controlling initiation of reparative dentinogenesis has been repeatedly confirmed after pulp capping with calcium hydroxide-based materials. Schröder (1985) reviewed the reparative process following capping of human teeth with calcium hydroxide: initially the cells under the wound surface proliferate, migrate and elaborate new collagen along the superficial necrotic zone or the pulpal surface of capping material. The necrotic zone and the new collagen layer attract mineral salts, becoming calcified matrices (fibrodentine). Then a layer of odontoblast-like cells is formed in association with the fibrodentine and reparative dentine is secreted. Many data from capping experiments suggest that initiation of reparative dentine formation might not be attributed to any specific dentinogenic effect of calcium hydroxide, although its effect in controlling infection and stimulating the wound healing process might not be excluded.

Light microscopy micrograph 3 weeks after application of MTA on mechanically exposed pulp
Figure 9. Light microscopy micrograph 3 weeks after application of MTA on mechanically exposed pulp of dog.
a) Two layered bridge formation consisting from osteodentin (arrow) and reparative dentin matrix (arrowheads) formation along the pulpal side of MTA (Hematoxylin-eosin, x40);
b) Higher magnification of 9a. Tubular matrix following osteodentin (arrow) associated with elongated formative cells (Hematoxylin-eosin, x200).

Elongated polarized cells associated with osteotypic matrix
Figure 10. Light microscopy micrograph 3 weeks after application of MTA on mechanically exposed pulp of dog. Elongated polarized cells (arrowheads) associated with osteotypic matrix (arrows) formed in direct contact with the pulpal side of MTA (Hematoxylin-eosin, x200).

The present experiments demonstrated that pulp capping with MTA induces cytological and functional changes in pulpal cells, resulting in formation of fibrodentine and reparative dentine at the surface of mechanically exposed dental pulp. These observations are in agreement with those described previously in cultures of osteoblasts on MTA (Koh et al. 1997, Koh et al. 1998, Mitchell et al. 1999) or after application of MTA in capping situations (Pitt Ford et al. 1996). MTA offered a biologically active substrate for pulpal cells, able to regulate dentinogenic events. The pulp cell responses were studied here in short-term healing intervals to evaluate whether MTA induces directly reparative dentinogenesis, or the stereotypic intermediate formation of osteodentine/fibrodentine precedes any dentinogenic event. The initial effect of MTA on the surface of mechanically exposed pulp is the formation of a superficial layer of crystalline structures onto the pulpal surface of the capping material. Columnar cells undergoing nuclear and cytoplasmic polarization and showing a well developed cytoplasmic organization are further arranged along the crystalline structures. This short-term reaction clearly indicates stimulation of the biosynthetic activity of pulpal cells by the capping procedure but it could not be characterized as direct induction of reparative dentine formation. Initiation of reparative dentinogenesis can be identified in morphological studies only by the palisade appearance of elongated and polarized odontoblast- like cell layer able to secrete tubular matrix in a polar predentin-like pattern, although further biochemical and immuno-histochemical data are needed to characterize completely the nature and specificity of this phenomenon.
A new matrix of atubular form with cellular inclusions was observed beneath the capping material after 2 weeks. Elongated or cuboidal formative cells were found along this matrix. The TEM study showed collagen fibres, which had been densely packed in direct contact with the superficial crystalline layer. Formative cells associated with this matrix exhibited distinct organization throughout the whole cytoplasm, but no clear polarization. The SEM study of the pulpal side of this matrix showed an atypical surface structure and amorphous crystal precipitation. The data suggest a fibrodentinal nature of the newly synthesized matrix formed along the MTA–pulp interface (Baume 1980, Tziafas 1997).
Reparative dentinogenesis was clearly observed 3 weeks after capping of exposed pulps with MTA, in association with the firm fibrodentinal matrix. Odontoblastlike cells elaborating tubular matrix in a predentine-like pattern were seen in all cases. From the developmental point of view, these data confirm the similar mechanism for initiation of reparative dentinogenesis in capping with MTA and Ca(OH) 2 -based materials: in both agents capping of mechanically exposed pulps showed that fibrodentine matrix formation preceded any expression of the odontoblast-like cell phenotype (Schröder 1985, Fitzgerald et al. 1990). It seems that initiation of reparative dentine formation as a part of the natural wound healing process in MTA-treated pulps cannot be attributed to any specific dentino-inductive effect of the capping material. Nevertheless, the formation of an appropriate pulp environment due to the alkalinic properties of MTA favouring expression of the dentinogenic potential of pulpal cells, as has been suggested for Ca(OH) 2 -treated pulps (Torneck et al. 1983, Cvek et al. 1987), might be considered as a critical influence. The regulatory effect of MTA in production of osteocalcin or alkaline phosphatase, or interleukin-6 and -8 (Koh et al. 1997, Koh et al. 1998) might be further related to the stimulation of dentinogenic activity in the dental pulp treated with MTA. In addition, the important role of the fibronectin-rich zone, which is formed onto the crystalline structures (Seux et al. 1991, Tziafas et al. 1995, Yoshiba et al. 1996) observed along the pulpal side of MTA and a possible effect of the alkalinic environment in dissolution of growth factors from the surrounding dentine, as has been suggested for Ca(OH) 2 (Lesot et al. 1994), may not be excluded.

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