Age-dependent changes of the mandible bone throughout the lifespan in female F344/N rat.

Age-dependent changes of the mandible bone in female F344/N rats, aged 22-1196 days, were analyzed using physiological bone properties and morphology. Bone weight, bone area, bone mineral components, and bone mineral density were assessed using dual-energy X-ray absorptiometry. The bone weight, bone area, bone mineral components, and bone mineral density increased rapidly until approximately 150 days of age, increased gradually thereafter, and then stabilized or decreased after 910 days of age. The ratio of bone mineral components to bone weight (bone mineral ratio) increased rapidly until approximately 43 days of age and stabilized thereafter. Size of the mandible, which was measured at 13 points on mandible surface, increased with age, and the rate of change showed a similar pattern to the other parameters. From a principal component analysis on morphometric measurements, principal component 1 (size factor) increased proportionally with age, whereas principal component 2 (shape factor) decreased until approximately 88 days of age and then increased after 365 days of age. As a result, the scatter plots for principal component 1 and principal component 2 were V-shaped, which indicates that the mandible developed in size, with deformation at younger ages, and recovered its original shape later in life. Our results revealed the occurrence of inflection points at approximately 43, 88, 150, 365, and 910 days of age. Some of these ages corresponded to transition points revealed by the age-dependent changes of the occlusal mandibular condyle and tooth wear in the same rat.

Effects of aging and reproductive history on bone parameters of common marmoset (Callithrix jacchus) mandibles.

To understand effects of aging and reproductive history in the bones of common marmosets (Callithrix jacchus), mandibles from 79 males and 66 females were analyzed. Dry bone specimen was prepared from dissected mandible, and analyzed using a dual-energy x-ray absorptiometry (DXA) measurement system in terms of bone weight (BnW), bone area (AREA), bone mineral content (BMC), bone mineral density (BMD, ratio of BMC to AREA) and bone mineral ratio (BMR, ratio of BMC to BnW). The mandible bones became porous and thicker with age. The age-related changes in BnW, AREA and BMC showed inflection points at around 1.5-2 Y and 13-15 Y. The period before 1.5-2 Y corresponds to the growth phase, the period between the inflection points is the aging phase, followed by senescence after the second inflection point. BMD increased until 1.5-2 Y and gradually decreased thereafter in males, with a more dramatic decrease in females, probably because of pregnancy and lactation. BMR was stable after reaching its peak by 1 Y, unlike the other parameters we analyzed. BMD of parous female tended to be lower than that of nulliparous female aged 2-5 Y. This study identified some of the particular effects of aging and reproductive history on characteristics of mandible bones in common marmoset.

Occlusal tooth wear in female F344/N rats with aging.

OBJECTIVES: This study was conducted to ascertain whether laboratory rats are an adequate animal model for aging oral cavity research, especially on occlusal tooth wear (OTW), which progresses with aging and causes abnormal occlusions. Mastication has been reported to relate to cognition in the elderly. Thus, it is important to care for the oral cavity, especially in the frail elderly, for the maintenance of all-round quality of life. Adequate and appropriate animal models are essential for basic and clinical research on the oral cavity. METHODS: Dried maxilla and mandible specimens from 98 young, aging or aged female F344/N rats were used. RESULTS: The levels of OTW of all molars were monitored with aging. The molar tooth began to wear at 1-month old (M) and progressed rapidly till 12M. Subsequently, OTW progressed slowly till 30M, and then rapidly again after 35M. CONCLUSIONS: This study showed that progress of OTW is well correlated with the entire life span of the rat, and suggested that the rat aged over 12M would be an adequate animal model for research on OTW in middle-aged and elderly people.

Natural dental caries in molars of osteogenic disorder Shionogi rats.

Osteogenic disorder Shionogi (ODS) rats are genetically defective in ascorbic acid biosynthesis. They exhibit a gait abnormality due to dysfunctional bone formation and display various dental abnormalities. Conditions of the oral cavity and tooth quality both influence the development of dental caries. This study was designed to determine the characteristics of dental caries in ODS/ ShiJclod/od rats. Caries were scored and compared among ODS/ShiJclod/od, ODS/ShiJcl+/+, and Jcl:Wistar retired breeders. Among male rats, the caries scores of the ODS/ShiJclod/od and ODS/ShiJcl+/+ groups were similar to each other but greater than those in Jcl:Wistar rats, whereas among female rats, caries scores in ODS/ShiJclod/od animals were equivalent to or somewhat greater than those in ODS/ShiJcl+/+ rats, whose scores were markedly greater than those of Jcl:Wistar rats. The results suggest that ODS/ShiJcl rats were more susceptible to dental caries than were Jcl:Wistar rats. Under the conditions of the study, caries scores between ODS/ ShiJclod/od and ODS/ShiJcl+/+ rats differed only among parous females.

Aging change of mandibular condyle in female F344/N rat.

To ascertain whether laboratory rats represent an adequate animal model for aging oral cavity research, this study focused on the morphology of the mandibular condyle. Aging changes of cartilaginous conformation and shape of the mandibular condyles were analyzed in female F344/N rats. In the condylar cartilage, articular, proliferative cell and hypertrophic cell layers were observed in 1-month-old (M) rat, whereas flattened cell layer was notable at 2M. A mature cell layer was observed in the condylar cartilage of rats at 7.7 M and over. Deranged cartilaginous layers and thinning articular layer were observed in 30.9 M rat. The sagittal length of the condyle decreased, whereas the frontal one increased with aging and/or age. There were three phases in the transition patterns of the size of the condyle, which seemed to correspond to the respective growing, aging and senescence phases in the rat. The results suggested that degenerative change of condylar cartilage in rat was similar to that in human, whereas change of the shape of the condyle was different between rat and human, caused by a different pattern of mastication.

Occlusal tooth wear in male F344/N rats with aging.

With the aim of clarifying the aging properties of an animal model, the progress of occlusal tooth wear (OTW) of molars in male F344/N rats was monitored. Dried maxilla and mandible specimens from 61 male F344/N rats, aged 7 to >30 months, were used. The levels of OTW of all molars were monitored with aging. The cuspis dentis of molar teeth were worn out by 7 months (M) of age, and the occlusal surface became flat. As for each molar tooth (M(1-3), numbered in accordance with its position), OTW of M(1) was more severe in the lower than in the upper jaw, whereas M(3) was more severe in the upper than the lower jaw. OTW of M(2) in both the upper and the lower jaws progressed rapidly after 27M. OTW in male F344/N rats progressed faster than in females. However, when compensated for life span, both genders had similar profiles in OTW progress with aging. This study suggested that male rats were more convenient than females as a model for gerodontological research because of the earlier course of OTW progress.

Morphometric comparison of the rat mandible in F344 substrains, F344/Du and F344/N.

The shape of the mandible was compared by morphometric methods to ascertain the genetic differences between two substrains of F344 rats, F344/DuCrlCrlj and F344/NSlc. Since these two substrains are clearly different in survival and the incidence of age associated disorders; thus, some genetic differences are suggested to be present between them. Although previous microsatellite analysis did not detect any differences between the two F344 substrains, the present study clearly detected interesting differences in the mandible morphology. At 2 months of age, the F344/Du mandible was characterized by a larger size, especially in length, than the F344/N mandible. The shape of the mandible seemed to be more variable in F344/N. This clear substrain difference suggests the importance of the substrain recognition in F344 rats, especially in experimental usage.

Comparison of rat mandible bone characteristics in F344 substrains, F344/Du and F344/N.

The characteristics of the mandible bone were compared through DXA methods between two major substrains of F344 rats, F344/DuCrlCrlj and F344/NSlc at around 60 days of age. Since these two substrains are clearly different in survival and mandible morphology, some genetic differences are supposed to exist. In contrast to a previous microsatellite analysis, clear and significant differences were detected in the body and mandible weights, the mandible bone mineral contents (BMC), bone area (AREA), bone mineral density (BMD) and bone mineral ratio (BMR), between F344/DuCrlCrlj and F344/NSlc, with the mandible molar teeth intact in the bone. Thus, care is needed in the experimental use of these substrains, as results may differ between them. The newly proposed parameter, BMR, may especially contribute to the comparison of bone characteristics among species.