Effect of Difference of Proliferative Ability on Fragmentation of Hertwig＇s Epithelial Root Sheath

Objective To verify a novel hypothesis that it is the difference of proliferative ability between Hertwig＇s epithelial root sheath （HERS） and the neighboring mesenchyme that leads to fragmentation of HERS. Methods Developmental processes of rat mandibular first molars from postnatal （PN） 10 to 25 days were observed. HERS was traced by immunohistochemistry for keratin 14. The activity of proliferation was examined by immunohistochemistry and immunofluorescence for proliferating cell nuclear antigen （PCNA） at PN 10 and 25 days respectively. Results At PN 10, HERS grew to its maximum length and maintained integrity. Subsequently, along with tooth root elongation, HERS was gradually fragmented. Either at PN 10 or 25 days, there were more PNCA-positive cells in the neighboring mesenchyme than in the HERS. Conclusion During rat tooth root development, the neighboring mesenchyme possesses higher proliferative ability than HERS, which results in the fragmentation of HERS.