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PTHrP, parathyroid-hormone-related protein; TGF-beta, transforming growth factor-beta; EGF, epidermal growth factor; RANKL, receptor activator of NF-kappaB ligand; OPG, osteoprotegerin; PDL, periodontal ligament; TRAP, tartrate-resistant acid phosphatase; PKA, protein kinase A; PKC, protein kinase C; MAP, mitogen-activated protein; ERK, extracellular signal-regulated kinase; cAMP, cyclic Adenosine 3'5'-Monophosphate.

INTRODUCTIONDegradation of dental resins in human saliva, that lead to decrease in mechanical strength and surface hardness, have been reported in in vitro studies 1,2） . The results of these studies suggested that such degradation arose from the chemical degradation of methacrylate polymers due to enzyme-catalyzed hydrolysis reaction of ester bond. However, methods that can adequately investigate the enzymatic degradation mechanism involved in the breakdown of methacrylate polymers are not available. Therefore, the chemical degradation of methacrylate monomers has been a subject of research to clarify the hydrolytic degradation behavior of ester linkages in polymer networks 3-5） . Several studies have been conducted to reveal the degradation behavior of bisphenol A diglycidyl dimethacrylate （BisGMA） and triethylenglycol dimethacrylate （TEGDMA） monomers in phosphate buffer solution containing cholesterol esterasewhich simulates salivary enzyme solution 6,7） . The hydrolyzed products of these monomers were detected in the simulated solution by enzymatic attack. The results of these studies revealed that a rapid hydrolysis reaction with the enzyme occurred within several days.It is known that esterase, a salivary enzyme, displays structure-dependent hydrolytic activity in the hydrolysis of esters such as fatty acid ester 8） . However, there is a lack of information regarding the relationship between the chemical structure of methacrylate monomers and the hydrolysis by salivary enzymes. It is expected that the development of a new methacrylate monomer with higher resistance to hydrolysis by salivary enzymes would contribute to improving the durability of composite resins in vivo.Recently, Jaffer et al. 9） measured the chemical stability of dimethacrylate monomers in human saliva and revealed that the degradation of BisGMA was faster than that of urethane methacrylate oligomer. The findings of this study should serve to expedite research activities that focus on how the structural dependency of methacrylate monomers influence their resistance to hydrolysis. In particular, BisGMA is the most common dimethacrylate monomer of composite resin materials 10,11） . Therefore, improvement in the chemical stability of BisGMA in human saliva would result in enhanced biostability of composite resin restorations.Jaffer et al. 9） had demonstrated that the human saliva contains sufficient esterase activity to catalyze the degradation of monomers.The aim of this study, therefore, was to assess the chemical stability of methacrylate monomers in human saliva to reveal the relationship between enzymatic degradation resistance and the chemical structure of monomers. Further, to examine the stability of the experimental monomers under clinical conditions, degradation test was carried out in human whole saliva. Six types of model monomethacrylate monomer and two BisGMAderivative monomers were synthesized, and their degradation as a function of time was investigated by using high-performance liquid chromatography （HPLC） . Deg...

Intrusive trauma was experimentally applied to the tooth germ at different developmental stages in the rat first molar. The tooth germ at the earliest (postnatal day 1, initiating stage of enamel matrix formation) and the latest (postnatal day 10, calcifying stage of preformed enamel matrix) developmental stages studied showed localized enamel hypoplasia as a direct sequela of trauma. The tooth germs in which enamel matrix was rapidly thickening (postnatal days 3, 5, 7) and had not yet started to calcify showed the most intense and extensive injuries to the formation and structural organization of both enamel and dentin. As for indirect effects secondary to trauma, tooth germ dislocation was observed chiefly in tooth germs at the same developmental stages, frequently resulting in ankylosis. The present experimental model may be helpful for clarifying the histogenesis of traumatic changes in the developing tooth germ.

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