Using the proliferating cell nuclear antigen (PCNA) immunostaining, we previously identified, after pulp exposure, three zones of proliferating cells in the rat molar pulp. Zones I and II were in the crown near the pulp. Zone III was near the apex revealing a recruitment of mitotic cells at distance from the lesion. To gain further insight into the spatio-temporal evolution of proliferating pulp cells of zone III, we performed a longitudinal study of PCNA staining in rat molar mesial root at 3, 8, and 15 d after pulp exposure associated to implantation of unloaded or amelogenin loaded agarose beads. At day 3 after implantation, PCNA-positive cells were located in the central part of the radicular pulp. At day 8, PCNA-labeled cells were aligned in the lateral part of the pulp beneath the odontoblast/sub-odontoblast layer. At day 15, PCNA labeling became undetectable in the root and was located in the coronal pulp. These results suggest that after pulp exposure, PCNA-positive cells may migrate from the central part of the radicular pulp to the sub-odontoblast cell layer and then from the apical root to the crown. Electron microscopy and immunostaining analysis showed that pulpal cells were linked by desmosome-like and gap-junctions. Extracellular matrix was composed of thin collagen fibrils associated with glycosaminoglycans favoring cell mobility. These data suggest that the syncytium-like structure formed by pulp radicular cells may be a pre-request for plithotaxis, a collective cell migration process. This emergent mechanism may govern pulp healing and regeneration after dental lesion.
Keywords: Apical niche; cell sliding; cytoskeletal structures; extracellular matrix; plithotaxis; pulp progenitors.