Metabolic turnover of collagen in the mouse molar periodontal ligament during tooth eruption.

Metabolic turnover of collagen in the developing molar periodontal ligament was studied in 10, 12 and 16 days old mice by autoradiography after the administration of [3H]proline. Grain counts, in number per unit area, were made over three different zones (apical, middle and cervical) of the sections and activity time curves were drawn. Calculated regressions (log10 grain density V time) were fitted and analyses of variance to test the linearity of the regressions were carried out using the computer. Slopes +/- S.E. of the regressions, and half-lives were estimated. This study has demonstrated that collagen turnover occurred throughout the whole thickness of the ligament, from bone to cementum. Therefore, there was no evidence of a metabolically active 'intermediate plexus' in the periodontal ligament. However, the turnover rates in the apical zones of the three groups studied were significantly higher than those in the middle zones, which in turn were higher than those in the cervical zones, suggesting a differential rate of collagen turnover within the periodontal ligament. This whole sequence was significantly higher in the 12 days old compared with the 10 and 16 days old groups. Estimated half-lives confirmed this pattern of rate of collagen turnover. The molar periodontal ligament behaves and functions as a 'stimulated system' during the period 10 to 16 days, with a statistically significant rate of collagen turnover at the twelfth day, which is also the time of tooth eruption in the mouse. Thus, collagen turnover is an essential and accompanying process of tooth eruption.

Fibroblast cell population kinetics in the mouse molar periodontal ligament and tooth eruption.

Fibroblast cell population kinetics in the developing molar periodontal ligament was investigated in 10, 12, 16 and 20 days old mice by autoradiography after the administration of [3H]thymidine. Labelled mitoses, in number per unit area, were counted over the apical zones of the sections and percentage labelled mitoses (PLM) curves were drawn. Median values for some phases durations were read off at the 0.5 level of mitotic labelling. In determining other kinetic parameters the periodontal fibroblast population was considered separately as (1) a steady state system, (2) an exponentially growing system. An attempt was made to estimate mean values for these parameters using the Gilbert Computer programme. The programme generated the original data points and the fitted curve in graphical and numerical form together with the mean values and standard errors. The fact that the Gilbert programme assumes a stationary population in its theoretical PLM curve analysis was used to establish the kinetic type of the periodontal fibroblast population. The present study has demonstrated that in the growing periodontal ligament where cell specialisation and migration occurs steady state system is the kinetic model applicable. Failure of the computed PLM curves to fit the data adequately confirmed the fibroblast migration (apico-occlusal migration) from the apical zone to other zones. A definite cytokinetic basis for the periodontal ligament fibroblast proliferation and migration was established. Accordingly, fibroblast proliferation and migration takes place by a reduction in the DNA synthesis time (T8), cell generation tie (Tc) and possibly by a similar reduction in other parameters. Maximum reduction in these parameters is associated with the peak proliferative and migratory activity in the 12 days old group which is also the time that tooth eruption takes place in the mouse. Thus fibroblast proliferation and migration are major causative factors responsible for tooth eruption. Based on these results, a mechanism for tooth eruption is proposed.

Fibroblast cell proliferation in the mouse molar periodontal ligament.

Cytogenesis of the developing molar periodontal ligament was studied by determining labelling indices in 4 groups of young mice. Autoradiographs were made using paraffin sections of demineralised specimens from 10 days, 12 days, 16 days and 20 days old mice allowed to survive a varying period (1--96 hours) after the administration of [3H]thymidine. Labelled fibroblasts in number per unit area were counted over three different zones (apical, middle and cervical) of the sections and labelling index distribution curves were drawn. The present study has demonstrated the existence of a progenitor area with proliferating fibroblasts in the apical zone and a migration of these cells into other zones (middle and cervical). However, it has also shown that a measurable proliferative rate is present at all levels of the molar periodontal ligament in all ages of animals investigated. Such an observation if compatible with fibroblasts migrating from the apical zone in an occlusal direction and maintaining a decreasing degree of proliferative activity as they progress. Consequently, a tentative apico-occlusal migratory model for the periodontal ligament fibroblast proliferation and migration is proposed. Accordingly, fibroblast proliferation takes place in the apical progenitor zone, and cells migrate from the apical zone to middle and cervical zones, but maintaining a decreasing degree of proliferative activity as they migrate. Peak proliferative and migratory activity is seen in the 12 days group which is also the time that tooth eruption takes place in the mouse. Thus, a close relationship exists between fibroblast cell proliferation, migration and tooth eruption.

Effects of rehabilitation on the jaws and teeth of protein-deficient and calorie-deficient pigs.

The growth of the jaws and teeth was examined in pigs which had been fed protein-deficient and calorie-deficient diets for the first year of life and thereafter allowed unlimited food. After 2 years of rehabilitation, jaw size had almost reached that of well-fed controls, although in the mandible the labial alveolar process remained abnormally proclined and in both jaws the crowns of third permanent molars were smaller than in control animals. Calorie-deficient pigs did not recover as well as did protein-deficient animals. Pigs fed a calorie-deficient diet for the first 2 years of life before rehabilitation also made a good recovery and were only slightly smaller in most dimensions than those undernourished for 1 year and then rehabilitated. Despite the excellent recovery in jaw dimensions, some malposition of teeth persisted after rehabilitation, especially in animals which had been undernourished for 2 years.

Metrical analysis of growth changes in the jaws and teeth of normal, protein deficient and calorie deficient pigs.

Weanling pigs were separated into three groups: control animals were allowed unlimited food; protein deficient animals were allowed unlimited carbohydrate or fat but restricted in protein intake; calorie deficient animals were restricted in total food intake. The skulls and teeth of animals killed at one year and at two years of age were measured from radiographs, and from the results were derived the following conclusions: (1) Pigs fed on protein and on calorie deficient diets all showed considerable retardation of the growth of jaws and teeth, consistently more severe in the calorie deficient than in the protein deficient animals. (2) The shape of the mandible was altered by both types of malnutrition, the height of the ascending ramus being reduced to a greater extent than either its anteroposterior width or the length of the body of the mandible. The labial alveolar process became more proclined with increasingly severe malnutrition. (3) The upper and lower jaws were retarded to a similar extent in both groups of experimentally malnourished animals. (4) Retardation of tooth growth was less severe than that of jaw growth. (5) Contralateral asymmetry of molar crown size in the protein deficient animals was initally quite marked, but when the teeth were approaching their final size the asymmetry was no greater than in control animals.