Characterization of stem and progenitor cells in the dental pulp of erupted and unerupted teeth and analysis of their progression into odontoblasts using GFP reporter transgenic mice

Odontoblast differentiation is a sequential process that involves several intermediate steps characterized by the formation of pre-odontoblasts, polarized odontoblasts, secretory/functional odontoblasts and, finally, terminally differentiated odontoblasts secreting mineralized dentin. ^ Dentin production is continuous throughout the life of a tooth and the dentin-pulp complex has a regenerative potential that leads to the formation of tertiary dentin. Strong stimuli that lead to destruction of existing odontoblasts is followed by formation of reparative dentin secreted by a new generation of odontoblast cells. Candidate populations giving rise to the new generation of odontoblasts are many and include progenitors and putative mesenchymal stem cells (MSC) in dental pulp. This study focused on the characterization of progenitor and stem cells in dental pulps from mouse teeth. ^ Mouse dentition is specific in that it contains molars (form root and erupt) and continuously growing incisors (root never forms). Continuous growth of incisors indicates the presence of cell populations capable of incessant differentiation into odontoblasts and self-renewal to maintain incisor growth throughout the life of the animal. In this study we have used molars and incisors at different stages of development: erupted and unerupted (still forming). ^ Our study provides evidence that dental pulp of the erupted molars contains a small population of multipotent cells, whereas the dental pulp of the unerupted molars is enriched in osteo-dentinogenic progenitors engaged in the formation of coronal and radicular odontoblasts. We also show that continuous generation of odontoblasts in continuously growing mouse incisors is supported by the progenitor population and not multipotent MSC in the dental pulp. ^ We have also used primary dental pulp cultures from pOBCol3.6GFP, pOBCol2.3GFP and DMP1-GFP transgenic mouse lines to gain better insight into the intermediate steps of odontoblast differentiation. Our studies showed that these transgenes are activated at different stages of odontoblast differentiation. The pOBCol3.6GFP transgene is the earliest marker, activated in cells at early stages of polarization. The pOBCol2.3GFP transgene is activated at a later stage of polarization just before or at the time of formation of secretory odontoblast. The DMP1-GFP transgene is activated in early secretory/functional odontoblasts just before the formation of mineralized matrix.