In vitro synchronization of embryonic mouse incisor preodontoblasts and preameloblasts: repercussions on terminal differentiation

Preodontoblasts divide asynchronously and their terminal differentiation oc curs gradually. Experimental data suggested that the expression of competen ce by preodontoblasts to respond to specific epigenetic signals, triggering their overt differentiation, requires a minimal number of cell cycles. The intrinsic timing mechanism could imply division counting and preodontoblas ts of juxtaposed cell generations might sequentially withdraw from the last physiological cycle. To test such an hypothesis, embryonic mouse lower inc isors were cultured in vitro and treated sequentially with nocodazole in or der to induce a transitory synchronization of the dividing preodontoblasts and preameloblasts. This synchronization led to a disorganization of the ph ysiological gradual terminal differentiation of the odontoblasts, giving ri se to three distinct domains comprising respectively: 1) odontoblasts with altered polarization and predentin secretion; 2) odontoblasts demonstrating equivalent polarization and predentin deposition; and 3) preodontoblasts-o dontoblasts involved in gradual terminal differentiation. These results str ongly suggest that the gradient of odontoblast functional differentiation r esults from sequential withdrawal from asynchronous cell cycles of competen t cells able to overtly differentiate.