Age-dependent changes of the periodontal ligament in rats

Even after the end of the natural tooth eruption, there is a continuous renewal of the periodontal collagenous fiber system, depending on functional demands. The aim of this study was to analyse the age-dependent changes and regional differences of the collagen renewal rate of the periodontal ligament in healthy rats. The study was performed by autoradiography of the molars of rats aged 1, 8, and 18 months, where collagen was labelled by intravenously applied 3H-proline. After an 8-hour incorporation period, the animals were killed. For comparative examinations, molar roots were subdivided into cervical, middle, and apical thirds. Structural and quantitative analyses were performed by light microscopy and autoradiography, using an image-analysing computer-assisted operating unit that determined the 3H-proline-labelled collagen by photometry based on extinction measurement. With increasing age of the animals, the number of silver grains (3H-proline-blackened collagen) was reduced and the quantitative evaluation indicated a reduction of 3H-proline in the periodontal ligament. The lowest level of 3H-proline activities was observed in the middle, and the highest level in the apical root third, independent of age. All preparations revealed condensations of silver grains, which were located in the region of the periodontal ligament adjacent to the alveolar bone, but did not reveal any preferred position with regard to the dental topography. With progressive age, the uptake of 3H-proline in the periodontal ligament was reduced by about 20 to 30%, a result that corresponds to a decrease in collagenous fiber production. Collagen was mainly formed in the apical and cervical root third, starting from the alveolar bone side, presumably in response to functional strain.