The Epithelial-Mesenchymal Transition During Tooth Root Development

ABSTRACT

Hertwig's epithelial root sheath (HERS) consists of bilayered cells derived from the inner and outer dental epithelia and plays important roles in tooth root formation as well as in the maintenance and regeneration of periodontal tissues. With regards to the fate of HERS, and although previous reports have suggested that this entails the formation of epithelial rests of Malassez, apoptosis or an epithelialmesenchymal transformation (EMT), it is unclear what changes occur in the epithelial cells in this structure. This study examined whether HERS cells undergo EMT using a keratin-14 (K14) creROSA 26 transgenic reporter mouse. The K14 transgene is expressed by many epithelial tissues, including the oral epithelium and the enamel organ. A distinct K14 expression pattern was found in the continuous HERS bi-layer and the epithelial diaphragm were visualized by detecting the beta-galactosidase (lacZ) activity in 1 week postnatal mice. The 2 and 4 week old mice showed a fragmented HERS with cell aggregation along the root surface. However, some of the lacZ-positive dissociated cells along the root surface were not positive for pan-cytokeratin. These results suggest that the K14 transgene is a valuable marker of HERS. In addition, the current data suggest that some of the HERS cells may lose their epithelial properties after fragmentation and subsequently undergo EMT.