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.